# User's guide

## Introduction:  Welcome to EmPy!

[EmPy](http://www.alcyone.com/software/empy/) is a powerful, robust and mature
templating system for inserting Python code in template text.  EmPy
takes a source document, processes it, and produces output.  This is
accomplished via expansions, which are signals to the EmPy system
where to act and are indicated with markup.  Markup is set off by a
customizable prefix (by default the at sign, `@`).  EmPy can expand
arbitrary Python expressions, statements and control structures in
this way, as well as a variety of additional special forms.  The
remaining textual data is sent to the output, allowing Python to be
used in effect as a markup language.

EmPy also supports hooks, which can intercept and modify the behavior
of a running interpreter; diversions, which allow recording and
playback; filters, which can alter output and can be chained together.
The system is highly configurable via command line options,
configuration files, and environment variables.  EmPy documents can
also be imported as modules, and an extensive API is also available
for embedding EmPy functionality in your own Python programs.

EmPy also has a supplemental library for additional non-essential
features (`emlib`), a documentation building library used to create
this documentation (`emdoc`), and an extensive help system (`emhelp`)
which can be queried from the command line with the main executable
`em.py` (`-h/--help`, `-H/--topics=TOPICS`).  The base EmPy
interpreter can function with only the `em.py`/`em` file/module
available.

EmPy can be used in a variety of roles, including as a templating
system, a text processing system (preprocessing and/or
postprocessing), a simple macro processor, a frontend for a content
management system, annotating documents, for literate programming, as
a souped-up text encoding converter, a text beautifier (with macros
and filters), and many other purposes.


### Markup overview

Expressions are embedded in text with the `@(...)` notation;
variations include conditional expressions with `@(...?...!...)`  and
the ability to handle thrown exceptions with `@(...$...)`.  As a
shortcut, simple variables and expressions can be abbreviated as
`@variable`, `@object.attribute`, `@sequence[index]`,
`@function(arguments...)`, and combinations.  Functions can be called
with expanded markup as arguments using `@function{markup}{...}`.
Full-fledged statements are embedded with `@{...}`.  Control flow in
terms of conditional or repeated expansion is available with
`@[...]`.  A `@` followed by any whitespace character (including a
newline) expands to nothing, allowing string concatenations and line
continuations.  Line comments are indicated with `@#...` including
the trailing newline.  `@*...*` allows inline comments.  Output can
be disabled and re-enabled with `@-...` and `@+...`, including the
trailing newlines.  Escapes are indicated with `@\...`; diacritics
with `@^...`; icons with `@|...`; and emoji with `@:...:`.
`@%...`, `@%!...`, `@%%...%%` and `@%%!...%%` indicate
"significators," which are distinctive forms of variable assignment
intended to specify document metadata in a format easy to parse
externally.  In-place expressions are specified with `@$...$...$`.
Context name and line number changes can be made with `@?...` and
`@!...`, respectively.  A set of markups (`@((...))`, `@[[...]]`,
`@{{...}}`, `@<...>`) are customizable by the user and can be used
for any desired purpose.  `` @`...` `` allows literal escaping of any
EmPy markup.  Output can be toggled on and off with `@+` and `@-`,
respectively.  And finally, a `@@` sequence (the prefix repeated
once) expands to a single literal at sign.

The prefix defaults to `@` but can be changed with
the command line option `-p/--prefix=CHAR` (_environment variable:_ `EMPY_PREFIX`, _configuration variable:_ `prefix`).

### Getting the software

The current version of EmPy is 4.2.1.

The official URL for this Web site is <http://www.alcyone.com/software/empy/>.

The latest version of the software is available in a tarball here:  
<http://www.alcyone.com/software/empy/empy-latest.tar.gz>.

The software can be installed through PIP via this shell command:

% python3 -m pip install empy

For information about upgrading from 3._x_ to 4._x_, see  
<http://www.alcyone.com/software/empy/ANNOUNCE.html#changes>.

### Requirements

EmPy works with any modern version of Python.  Python version 3._x_ is
expected to be the default and all source file references to the
Python interpreter (_e.g._, the bangpath of the .py scripts) use
`python3`.  EmPy also has legacy support for versions of Python going
back all the way to 2.4, with special emphasis on 2.7 regardless of
its end-of-life status.  It has no dependency requirements on any
third-party modules and can run directly off of a stock Python
interpreter.

EmPy will run on any operating system with a full-featured Python
interpreter; this includes, but is probably not limited to, the
operating systems Linux, Windows, Windows Subsystem for Linux 2 (WSL2)
and macOS (Darwin).  Using EmPy requires knowledge of the [Python
language](https://www.python.org/).

EmPy is compatible with many different Python implementations,
interpreter variants, packaging systems, and enhanced shells:

{#supported-versions-table}
| Variant | Supported versions | Description |
| --- | --- | --- |
| [CPython](https://www.python.org/) | 2.4 and up | Standard implementation in C |
| [PyPy](https://www.pypy.org/) | 2.7 and up | Implementation with just-in-time compiler |
| [Stackless Python](https://github.com/stackless-dev/stackless/wiki/) | 2.4 and up | Implementation supporting microthreading |
| [IronPython](https://ironpython.net/) | 2.7 and up | Implementation for .NET CLR and Mono |
| [Jython](https://www.jython.org/) | 2.5 to 2.7 (and up?) | Implementation for JVM |
| [ActiveState Python](https://www.activestate.com/products/python/) | 2.7 and up | Secure supply chain open source solution |
| [eGenix PyRun](https://www.egenix.com/products/python/PyRun/) | 2.5 and up | One-file, no-installation CPython environment |
| [WinPython](https://winpython.github.io/) | 3.0 and up | Portable Scientific Python for Windows |
| [PortablePython](https://portablepython.com/) | 2.7 and up | Minimalistic Python distribution for Windows |
| [IDLE](https://docs.python.org/3/library/idle.html) | all | Python's Integrated Development and Learning Environment |
| [IPython](https://ipython.org/) | all | Powerful interactive shell; kernel for [Jupyter](https://jupyter.org/) |

EmPy is also compatible with scaled-down implementations of Python,
provided they support the set of standard modules that EmPy requires,
namely:

- `codecs
`
- `copy
`
- `getopt
`
- `os
`
- `platform
`
- `re
`
- `sys
`
- `unicodedata
`

Only a few .py module file(s) are needed to use EmPy; they can be
installed system-wide through a distribution package, via PIP, or just
dropped into any desired directory in the `PYTHONPATH` (as a module)
and/or `PATH` (as an executable).  A minimal installation need only
install the em.py file, either as an importable module or an
executable, or both, depending on the user's needs.

EmPy also has optional support for several [third-party emoji
modules](#third-party-emoji-modules); see [](#emoji-markup) for
details.

The testing system included (the test.sh script and the tests and
suites directories) is intended to run on Unix-like systems with a
Bourne-like shell (_e.g._, sh, bash, zsh, etc.).  EmPy is routinely
tested with all supported versions of all available interpreters.

If you find an incompatibility with your Python interpreter or
operating system, [let me know](#reporting-bugs).


### License

This software is licensed under
[BSD (3-Clause)](https://opensource.org/licenses/bsd-3-clause/).


## Getting started

This section serves as a quick introduction to the EmPy system.  For
more details, see the sections below.

:::{hint}

As an introduction to the terminology, the following names are used
throughout:

{#terminology-table}
| Name | Description |
| --- | --- |
| `EmPy` | The name of the software |
| `em.py` | The name of the executable and main source file |
| `em` | The name of the main module |
| `empy` | The name of the [pseudomodule](#pseudomodule-and-interpreter), as well as the PyPI package |
| `.em` | The conventional filename extension for EmPy documents |

:::


### Starting EmPy

After installing EmPy (see [](#getting-the-software)), EmPy is invoked
by running the EmPy **executable**, `em.py`, on the command line
(standalone mode).  If it is invoked without arguments, it will accept
input from `sys.stdin`.  Unless otherwise specified, the output is
sent to `sys.stdout`.  You can use this as an interactive session to
familiarize yourself with EmPy when starting out:

% em.py
... accepts input from stdin with results written to stdout ...

If an EmPy document is specified (which by convention has the
extension .em, though this is not enforced), then that document is
used as input:

% em.py document.em
... document.em is processed with results written to stdout ...

:::{note}

In some distribution packages, the EmPy interpreter may be named
`empy` rather than `em.py`.  In the [official release
tarballs](#getting-the-software), and throughout this documentation,
it is `em.py`.  This is to distinguish it from the pseudomodule
`empy`.

:::

:::{warning}

If your document filename begins with a `-`, it will be interpreted as
a command line argument and cause command line option processing
errors.  Either precede it with a relative path (_e.g._, `em.py
./-weirdname.em`) or the GNU-style `--` option which indicates there
are no further options (_e.g._, `em.py -- -weirdname.em`).

:::

Any number of command line arguments (beginning with `-` or `--`) can
precede the document name.  The command line argument `--` indicates
that there are no more options.  For instance, this command writes its
output to document.out:

% em.py -o document.out document.em
... document.em is processed with results written to document.out ...

Many options are available to change the behavior of the EmPy system.
This command will open the input file as UTF-8, write the output file
as Latin-1, show raw errors if they occur, and delete the output file
if an error occurs:

% em.py --input-encoding=utf-8 --output-encoding=latin-1 -r -d -o document.out document.em
... you get the idea ...

EmPy documents can also take arguments, which are an arbitrary
sequence of strings that follow after the document, and are analogous
to the Python interpreter arguments `sys.argv`:

% em.py document.em run test
... empy.argv is ['document.em', 'run', 'test'] ...

:::{tip}

When installed as a module, EmPy can also be invoked as

% python3 -m em
...

:::

:::{tip}
  
You can create executable EmPy scripts by making their file executable
and starting them with a bangpath:
  
```shell
#!/usr/bin/env em.py

... EmPy code here ...
```

By default, bangpaths are treated as EmPy comments unless
`--no-ignore-bangpaths` (_configuration variable:_ `ignoreBangpaths = False`) is specified.

:::

:::{tip}

If you wish to run EmPy under Python 2._x_ for some reason on a system
that also has Python 3 installed, explicitly invoke the Python 2
interpreter when running it (`python2 em.py ...`).  If you wish to
make this more streamlined, edit the first line ("bangpath") of the
em.py executable and change it to read `#!/usr/bin/env python2` (or
whatever your Python 2._x_ interpreter is named).

:::

:::{seealso}

See the [](#command-line-options) section for a list of command line
options that EmPy supports.

:::


### The prefix and markup expansion

EmPy **markup** is indicated with a configurable **prefix**, which is
by default the at sign (`@`).  The character (Unicode code point)
following the prefix indicates what type of markup it is.  There are a
wide variety of markups available, from comments to expression
evaluation to statement execution, and from prefixes, literals and
escapes to diacritics, icons and emojis.  Converting markup into text
to be rendered as output is referred to as **expansion**.  Here is a
long EmPy code sample illustrating some of the more essential markups
in EmPy, though there are several not shown here:

:::{admonition} Example 1: Markup sample
_Source_: ⌨️

``````
Comments:
The line below will not render.
@# This is a line comment, up to and including the newline.
If a line comment appears in the middle of a line, @# this is a comment!
the line will be continued.
Inline comments can be @*placed inline* (this phrase did not render, 
but note the double space due to the spaces before and after it).
@**
  * Or it can span multiple lines.
  **@
Whitespace markup consumes the following space.
So two@ words become one word.
And this @
is a line continuation.
@* Inline comments can be used as a line comment. *@
Note the use of the trailing prefix to consume the final newline; this 
is a common idiom.

Literals:
Double the prefix to render it: @@.
String literals can be used to render escaped Python strings: @
@"A is also \N{LATIN CAPITAL LETTER A}".
Escape markup can render arbitrary characters:
These are all Latin capital letter A: @
A, @\B{1000001}, @\q1001, @\o101, @\x41, @\u0041, @\U00000041, @\N{LATIN CAPITAL LETTER A}.
Backquotes can be used to escape EmPy markup.
This is not evaluated: @`@(!@#$%^&*()`.

Expressions:
Python expressions can be evaluated like this: 1 + 2 = @(1 + 2).
Expressions can be arbitrary complex: @
This is running in Python @('.'.join(str(x) for x in __import__('sys').version_info[:3])).
Expressions can contain builtin ternary operators:
Seven is an @(7 % 2 == 0 ? 'even' ! 'odd') number.
They can even handle exceptions: @
Division by zero is @(1/0 $ 'illegal').

Statements:
@{
print("Hello, world!")
x = 123
}@
x is now @(x), which can be simplified to @x.
Statements can execute arbitrarily complex Python code,
including defining functions and classes.

Back to expressions, they can be simplified:
@{
# Define some variables.
class Person:

    def __init__(self, name):
        self.name = name

a = [4, 5, 6]
p = Person('Fred')
}@
x is @x.
a[1] is @a[1].
The name of p is @p.name.
You can even call functions this way:
p's name when shouted is @p.name.upper().
Note that the parser does not try to evaluate end-of-sentence punctuation.

Control structures:
Iterate over some numbers and classify them, but stop after 5:
@[for n in range(-1, 10)]@
@[  if n > 5]@
And done.
@[    break]@
@[  end if]@
@n is @
@[  if n < 0]@
negative@
@[  elif n == 0]@
zero@
@[  elif n % 2 == 0]@
even@
@[  else # odd]@
odd@
@[  end if]@
.
@[end for]@
Note the use of indentation inside control markup and end-of-line
whitespace markup (a prefix with trailing whitespace is consumed) to
make things more clear (though this is optional).

You can even define your own EmPy functions:
@[def officer(name, species, rank, role)]@
@# The definition is EmPy, not Python!
@name (@species, @rank, @role)@
@[end def]@
Some of the bridge crew of the USS Enterprise (NCC-1701):
- @officer("James T. Kirk", "Human", "captain", "commanding officer")
- @officer("Spock", "Vulcan-Human hybrid", "commander", "science officer")
- @officer("Montgomery Scott", "Human", "commander", "chief engineer")
- @officer("Nyota Uhura", "Human", "lieutenant commander", "communications officer")
- @officer("Hikaru Sulu", "Human", "commander", "astrosciences/helmsman")

Diacritics: Libert@^e', @^e'galit@^e', fraternit@^e'!
Icons for curly quotes: @|"(these are curly quotes.@|")
This is an emoji: @:pile of poo:.  (Of course I would choose that one.)
``````


_Output_: 🖥️

``````
Comments:
The line below will not render.
If a line comment appears in the middle of a line, the line will be continued.
Inline comments can be  (this phrase did not render, 
but note the double space due to the spaces before and after it).
Whitespace markup consumes the following space.
So twowords become one word.
And this is a line continuation.
Note the use of the trailing prefix to consume the final newline; this 
is a common idiom.

Literals:
Double the prefix to render it: @.
String literals can be used to render escaped Python strings: A is also A.
Escape markup can render arbitrary characters:
These are all Latin capital letter A: A, A, A, A, A, A, A, A.
Backquotes can be used to escape EmPy markup.
This is not evaluated: @(!@#$%^&*().

Expressions:
Python expressions can be evaluated like this: 1 + 2 = 3.
Expressions can be arbitrary complex: This is running in Python 3.10.12.
Expressions can contain builtin ternary operators:
Seven is an odd number.
They can even handle exceptions: Division by zero is illegal.

Statements:
Hello, world!
x is now 123, which can be simplified to 123.
Statements can execute arbitrarily complex Python code,
including defining functions and classes.

Back to expressions, they can be simplified:
x is 123.
a[1] is 5.
The name of p is Fred.
You can even call functions this way:
p's name when shouted is FRED.
Note that the parser does not try to evaluate end-of-sentence punctuation.

Control structures:
Iterate over some numbers and classify them, but stop after 5:
-1 is negative.
0 is zero.
1 is odd.
2 is even.
3 is odd.
4 is even.
5 is odd.
And done.
Note the use of indentation inside control markup and end-of-line
whitespace markup (a prefix with trailing whitespace is consumed) to
make things more clear (though this is optional).

You can even define your own EmPy functions:
Some of the bridge crew of the USS Enterprise (NCC-1701):
- James T. Kirk (Human, captain, commanding officer)
- Spock (Vulcan-Human hybrid, commander, science officer)
- Montgomery Scott (Human, commander, chief engineer)
- Nyota Uhura (Human, lieutenant commander, communications officer)
- Hikaru Sulu (Human, commander, astrosciences/helmsman)

Diacritics: Liberté, égalité, fraternité!
Icons for curly quotes: “these are curly quotes.”
This is an emoji: 💩.  (Of course I would choose that one.)
``````


:::

:::{tip}

If you wish to change the prefix, use `-p/--prefix=CHAR` (_environment variable:_ `EMPY_PREFIX`, _configuration variable:_ `prefix`).

:::

:::{seealso}

See the [](#markup) section for detailed specifications on all
support EmPy markup.

:::


### Pseudomodule and interpreter

The **interpreter** instance is available to a running EmPy system
through the globals; by default, it is named
`empy`.  When it is referenced this way, it
is called a **pseudomodule** (since it acts like a module although it
is not actually a module you can import):

:::{admonition} Example 2: Pseudomodule sample
_Source_: ⌨️

``````
This version of EmPy is @empy.version.
The prefix in this interpreter is @empy.getPrefix() @
and the pseudomodule name is @empy.config.pseudomoduleName.
Do an explicit write: @empy.write("Hello, world!").
The context is currently @empy.getContext().
Adding a new global in a weird way: @
@empy.updateGlobals({'q': 789})@
Now q is @q!
You can do explicit expansions: @empy.expand("1 + 1 = @(1 + 1)").
q is @(empy.defined('q') ? 'defined' ! 'undefined').
``````


_Output_: 🖥️

``````
This version of EmPy is 4.2.1.
The prefix in this interpreter is @ and the pseudomodule name is empy.
Do an explicit write: Hello, world!.
The context is currently <example 2 "Pseudomodule sample">:5:26.
Adding a new global in a weird way: Now q is 789!
You can do explicit expansions: 1 + 1 = 2.
q is defined.
``````


:::

:::{seealso}

See the [Pseudomodule/interpreter](#pseudomodule-interpreter) section
for details on the pseudomodule/interpreter.

:::

### Diversions, filters & hooks

**Diversions** can defer and replay output at a desired time:

:::{admonition} Example 3: Diversions sample
_Source_: ⌨️

``````
This text is output normally.
@empy.startDiversion('A')@
(This text was diverted!)
@empy.stopDiverting()@
This text is back to being output normally.
Now playing the diversion:
@empy.playDiversion('A')@
And now back to normal output.
``````


_Output_: 🖥️

``````
This text is output normally.
This text is back to being output normally.
Now playing the diversion:
(This text was diverted!)
And now back to normal output.
``````


:::

**Filters** can modify output before sending it to the final stream:

:::{admonition} Example 4: Filters sample
_Source_: ⌨️

``````
@{
# For access to the filter classes.
import emlib
}@
This text is normal.
@empy.appendFilter(emlib.FunctionFilter(lambda x: x.upper()))@
This text is in all uppercase!
@empy.appendFilter(emlib.FunctionFilter(lambda x: '[' + x + ']'))@
Now it's also surrounded by brackets!
(Note the brackets are around output as it is sent, 
not at the beginning and end of each line.)
@empy.resetFilter()@
Now it's back to normal.
``````


_Output_: 🖥️

``````
This text is normal.
THIS TEXT IS IN ALL UPPERCASE!
[NOW IT'S ALSO SURROUNDED BY BRACKETS!
(NOTE THE BRACKETS ARE AROUND OUTPUT AS IT IS SENT, 
NOT AT THE BEGINNING AND END OF EACH LINE.)
]Now it's back to normal.
``````


:::

**Hooks** can intercept and even alter the behavior of a running
system:

:::{admonition} Example 5: Hooks sample
_Source_: ⌨️

``````
@# Modify the backquote markup to prepend and append backquotes
@# (say, for a document rendering system, cough cough).
@{
import emlib

class BackquoteHook(emlib.Hook):

    def __init__(self, interp):
        self.interp = interp
    
    def preBackquote(self, literal):
        self.interp.write('`' + literal + '`')
        return True # return true to skip the standard behavior

empy.addHook(BackquoteHook(empy))
}@
Now backquote markup will render with backquotes: @
@`this is now in backquotes`!
``````


_Output_: 🖥️

``````
Now backquote markup will render with backquotes: `this is now in backquotes`!
``````


:::

:::{seealso}

See the [](#diversions), [](#filters), or the [](#hooks) sections for
more information.

:::

### Embedding

EmPy is modular and can be **embedded** in your Python programmers,
rather than running it standalone.  In Python, simply import the `em`
module and create an `Interpreter`:

```python
import sys

import em

config = em.Configuration(...)
output = sys.stdout
with em.Interpreter(config=config, output=output) as interp:
    ... do things with interp ...
```

An exception which occurs during processing will be handled by the
interpreter's error handler.

For one-off uses, you can use the global, standalone `expand` function:

```python
import em

result = em.expand(source)
```

When calling this function, an ephemeral interpreter is dynamically
created, used, and shutdown to perform the expansion.  If an exception
occurs during this processing, it will be raised to the caller, rather
than handled by the ephemeral interpreter.

:::

:::{important}

When you create an interpreter, you must call its `shutdown` method
when you are done.  This is required to remove the proxy on
`sys.stdout` that EmPy requires for proper operation and restore your
Python environment to the state it was before creating the
interpreter.  This can be accomplished by creating the interpreter in
a `with` statement -- interpreters are also context managers -- or by
creating it and shutting it down in a `try`/`finally` statement.

This is not needed when calling the `expand` global function; it
creates and shuts down an ephemeral interpreter automatically.

:::

:::{seealso}

See the [](#embedding-empy) section for more details on embedding EmPy
in your Python programs.

:::


### Getting help

For basic help, use the `-h/--help` option:

% em.py -h # or: --help
Welcome to EmPy version 4.2.1.

USAGE:
./em.py [<options>] [<filename, or `-` for stdin> [<argument>...]]
  - Options begin with `-` or `--`
  - Specify a filename (and arguments) to process that document as input
  - Specify `-` (and arguments) to process stdin with standard buffering
  - Specify no filename to enter interactive mode with line buffering
  - Specify `--` to stop processing options

...

For more help, repeat the `-h/--help` option (up to three times
for the full help).  For help on a particular topic, use the
`-H/--topics=TOPICS` option, where `TOPICS` is a comma-separated list of
topics.  The list of available topics can be shown by using the topic
`topics`:

% em.py -H topics # or: --topics=topics
Welcome to EmPy version 4.2.1.

TOPICS:
Need more help?  Add more -h options (-hh, -hhh) for more help.  Use -H <topic>
for help on a specific topic, or specify a comma-separated list of topics.  Try
`default` (-h) for default help, `more` (-hh) for more common topics, `all`
(-hhh) for all help topics, or `topics` for this list.  Use -V for version
information, -W for version and system information, or -Z for all debug
details.  Available topics:
  usage        Basic command line usage
  options      Command line options
  simple       Simple (one-letter) command line options
  markup       Markup syntax
  escapes      Escape sequences
  environ      Environment variables
  pseudo       Pseudomodule attributes and functions
  constructor  Keyword arguments for the Interpreter constructor
  variables    Configuration variable attributes
  methods      Configuration methods
  hooks        Hook methods
  named        Named escapes
  diacritics   Diacritic combiners
  icons        Icons
  hints        Usage hints
  topics       This list of topics

:::{tip}

Repeating the help option once (`-hh`) is the same as requesting the
`more` topic (`-H more`).  Repeating it three times (`-hhh`) is the
same as requesting the `all` topic (`-H all`).

:::

:::{warning}

The builtin help system requires the presence of the `emhelp` module.
If you have a minimal EmPy installation, this module may not be
available.  You can get it from the [release
tarball](#getting-the-software).

:::

:::{seealso}

See the rest of this document for details and specifications on all
the markup and features, and see the [Help topics section](HELP.md)
for the output of all the builtin help topics.

:::


## Markup

EmPy markup always begins with the EmPy prefix, which defaults to
`@`.  The character (Unicode code point) following the prefix
indicates what type of markup it is, and the different types of markup
are parsed differently.

It is legal to set the EmPy prefix to `None`; then, no markup will be
parsed or expanded and EmPy will merely process filters and encoding
conversions.  This can be done from the command line with the
`--no-prefix` option, or by indicating a prefix that is an
empty string (`''`) or the word `none`.

Using a non-default prefix that is also the first character of an
existing markup will swap that markup character with the default.  For
example, setting the prefix to `$` would otherwise collide with the
in-place token (`@$...$...$` with a default prefix).  On startup
it will be adjusted so that with a `$` prefix the in-place markup can
be accessed as `$@...@...@`.

The following subsections list the types of markup supported by EmPy
and in which version they were introduced, organized by category.
`NL` represents a newline and `WS` represents any whitespace.

:::{important}

All of the following code snippets and examples below assume that the
prefix is the default, `@`.  It can be changed with
`-p/--prefix=CHAR` (_environment variable:_ `EMPY_PREFIX`, _configuration variable:_ `prefix`).

:::

{#markup-table}
| Markup | Syntax | Description | Ver. |
| --- | --- | --- | --- |
| [Line comment](#line-comment-markup) | `@#... NL` | Consumes text up to and including newline | 1.0 |
| [Inline comment](#inline-comment-markup) | `@*...*` | Consumes text up to and including the final asterisk(s) | 4.0 |
| [Whitespace](#whitespace-markup) | `@ WS` | Consumes the following whitespace character | 1.0 |
| [Switch disable](#output-disable-markup) | `@-... NL` | Disables output | 4.2 |
| [Switch enable](#output-enable-markup) | `@+... NL` | (Re-)enables output | 4.2 |
| [Prefix](#prefix-markup) | `@@` | Produces the prefix character | 1.0 |
| [String](#string-markup) | `@'...'`, `@"..."`, `@'''...'''`, `@"""..."""` | Produces a string from a literal | 3.1.1 |
| [Backquote](#backquote-markup) | `` @`...` `` | Quotes contained markup up to final backquote(s) | 4.0 |
| [Escape](#escape-markup) | `@\...` | Render an escape character | 1.5 |
| [Named escape](#named-escape-markup) | `@\^{...}` | Render an escape control character by name | 4.0 |
| [Expression](#expression-markup) | `@(...)` | Evaluates an expression | 1.0 |
| [Simple expression](#simple-expression-markup) | `@variable`, `@object.attribute`, `@array[index]`, `@function(args...)`, etc. | Evaluates a simple expression | 1.0 |
| [Functional expression](#functional-expression-markup) | `@function{markup}{...}` | Evaluates a functional expression | 4.0 |
| [Extended expression](#extended-expression-markup) | `@(...?...!...$...)` | Expression evaluation with if-else-except | 1.3 |
| [In-place expression](#in-place-expression-markup) | `@$...$...$` | Copies and evaluates an expression | 1.4 |
| [Statement](#statement-markup) | `@{...}` | Executes a statement or statements | 1.0 |
| [Control](#control-markups) | `@[...]` | Control structures | 3.0 |
| [If control](#if-control-markup) | `@[if C1]`…`@[elif C2]`…`@[else]`…`@[end if]` | Branching control structure | 3.0 |
| [Break control](#break-and-continue-control-markup) | `@[break]` | Break out of repeating control structure | 3.0 |
| [Continue control](#break-and-continue-control-markup) | `@[continue]` | Continue with next iteration of repeating control structure | 3.0 |
| [For control](#for-control-markup) | `@[for N in E]`…`@[else]`…`@[end for]` | Iterating control structure | 3.0 |
| [While control](#while-control-markup) | `@[while E]`…`@[else]`…`@[end while]` | Looping control structure | 3.0 |
| [Dowhile control](#dowhile-control-markup) | `@[dowhile E]`…`@[else]`…`@[end dowhile]` | Looping control structure always entered once (`do`/`while` structure analogous to C, C++) | 4.0 |
| [Try control](#try-control-markup) | `@[try]`…`@[except E1 as N1]`…`@[else]`…`@[finally]`…`@[end try]` | Exception handling, guarding | 3.0 |
| [With control](#with-control-markup) | `@[with E as N]`…`@[end with]` | Handle a context manager | 4.0 |
| [Match control](#match-control-markup) | `@[match E]@[case C1]`…`@[else]`…`@[end match]` | Structural pattern matching | 4.1
| [Defined control](#defined-control-markup) | `@[defined N]`…`@[else]`…`@[end defined]` | Branch on whether a variable is defined | 4.0 |
| [Def control](#def-control-markup) | `@[def F(...)]`…`@[end def]` | Define an EmPy function | 3.0 |
| [Diacritic](#diacritic-markup) | `@^...` | Render and normalize diacritic combiner(s) | 4.0 |
| [Icon](#icon-markup) | `@\|...` | Render a customizable icon | 4.0 |
| [Emoji](#emoji-markup) | `@:...:` | Render a customizable emoji | 4.0 |
| [Significator](#significator-markup) | `@%... NL`, `@%!... NL`, `@%%...%% NL`, `@%%!...%% NL` | Declare a significator (metadata assignment) | 1.2 |
| [Context name](#context-name-markup) | `@?... NL` | Set the context filename | 3.0.2 |
| [Context line](#context-line-markup) | `@!... NL` | Set the context line | 3.0.2 |
| [Extension](#extension-markup) | `@((...))`, `@[[...]]`, `@{{...}}`, `@<...>`, … | Fully-customizable markups with no set definition | 4.1 |

:::{seealso}

The list of supported markup is available in the `markup` help topic
and is summarized [here](HELP.md#markup-summary).

:::


### Comment markup

Comment markup consumes its contents and performs no output.  A few
variants of comment markup are available.


#### Line comment markup: `@#... NL`

**Line comment markup** consists of a starting `@#` and consumes up
until (and including) the following newline.  Note that if the markup
appears in the middle of a line, that line will be continued since it
consumes the ending newline.

:::{admonition} Example 6: Line comments
_Source_: ⌨️

``````
@# This is a comment.  It will not render in the output.
@# Even would-be EmPy markup is consumed by a comment: @(!@#$%^&*()
Welcome to EmPy!
Here's some text @# This will consume the rest of the line
on the same line.
``````


_Output_: 🖥️

``````
Welcome to EmPy!
Here's some text on the same line.
``````


:::

:::{versionadded} 1.0

Line comment markup was introduced in EmPy version 1.0.

:::

#### Inline comment markup: `@*...*`

**Inline comment markup** (`@*...*`) is a form of comment markup
that can appear anywhere in text and can even span multiple lines.  It
consumes everything up to and including the final asterisk(s).

:::{admonition} Example 7: Inline comments, basic
_Source_: ⌨️

``````
This is text.  @* This is a comment in the text. *  This is continuing text.
(Note the extra spaces around where the comment was.)
@* A trailing whitespace markup consumes the whole line. *@
There is no extraneous blank line here.
``````


_Output_: 🖥️

``````
This is text.    This is continuing text.
(Note the extra spaces around where the comment was.)
There is no extraneous blank line here.
``````


:::

Multiple asterisks can be used as long as they are matched with the
end of the markup.  This allows asterisks to appear in the comment,
provided there are fewer asterisks than the delimiters:

:::{admonition} Example 8: Inline comments, advanced
_Source_: ⌨️

``````
@** Here's an asterisk inside the comment: * **@
@*** There can * be any number of asterisks ** as
     long as it's * less than ** the delimiters. ***@
@**
  * This is a multiline inline comment.
  **@
@*************************************
 * This comment thinks it's so cool. *
 *************************************@
So many comments!
``````


_Output_: 🖥️

``````
So many comments!
``````


:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @*...* `` followed by
a newline becomes `` @*...*@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 4.0

Inline comment markup was introduced in EmPy version 4.0.

:::


#### Whitespace markup: `@ WS`

While not quite a comment, **whitespace markup** is sufficiently
common and useful that it warrants introduction early on.  The
interpreter prefix followed by any whitespace character, including a
newline, is consumed.  This allows a way to concatenate two strings,
create a line continuation, or create a line separator:

:::{admonition} Example 9: Whitespace, basic
_Source_: ⌨️

``````
This was two@ words.  Now it is one.
Note that this consumes the newline @
so that this is on the same line.
@
Note there is no blank line above.
``````


_Output_: 🖥️

``````
This was twowords.  Now it is one.
Note that this consumes the newline so that this is on the same line.
Note there is no blank line above.
``````


:::

::::{tip}

{#idiom}
A trailing prefix after markup which has beginning and ending
delimiters -- for instance, inline comment (`@*...*`), expression
(`@(...)`), statement (`@{...}`) and control (`@[...]`) -- is
idiomatic for suppressing the newline when there is nothing at the end
of the line after the markup.  The trailing prefix will consume the
final newline, eliminating unwanted newlines.

For example, using a statement markup (see below) on a whole line will
result in a seemingly spurious newline:

:::{admonition} Example 10: Whitespace, idiom
_Source_: ⌨️

``````
Statement markup:
@{x = 123}
Note there's an extra newline above from the EmPy code after the
statement markup.  The markup itself doesn't print anything; it's from
the trailing newline after the markup.

To suppress the extra newline:
@{x = 456}@
The trailing prefix above consumes the trailing newline, eliminating it.
``````


_Output_: 🖥️

``````
Statement markup:

Note there's an extra newline above from the EmPy code after the
statement markup.  The markup itself doesn't print anything; it's from
the trailing newline after the markup.

To suppress the extra newline:
The trailing prefix above consumes the trailing newline, eliminating it.
``````


:::

::::

:::{versionadded} 1.0

Whitespace markup was introduced in EmPy version 1.0.

:::

#### Switch markup

Output to the underlying file can be switched on and off with output
**switch markup**.  This can be useful, for instance, when sourcing a
file that is a "header" or "module" which defines various objects but
its output is irrelevant; you can disable output at the beginning of
the file and enable it at the end to ensure that any output is
ignored.  By default, output is enabled.

Both markups consume everything up to and including the next newline,
effectively acting as a comment analogous to `@#`.

The markup is very simple.

:::{versionadded} 4.2

Switch markups were introduced in EmPy version 4.2.

:::


##### Switch disable markup: `@-... NL`

A line starting with `@-` is **switch disable markup** and will
disable output.  The entire line is consumed.

:::{admonition} Example 11: Output disable
_Source_: ⌨️

``````
This will be rendered to the output.
@-
This will not.
@- The rest of this line is consumed as a comment.
The output for this line is also disabled.
``````


_Output_: 🖥️

``````
This will be rendered to the output.
``````


:::

##### Switch enable markup: `@+... NL`

A line starting with `@+` is **switch enable markup** and will
(re-)enable output.  The entire line is consumed.

For example:

:::{admonition} Example 12: Output disable and enable
_Source_: ⌨️

``````
This will be rendered to the output.
@-
This will not.
@+
But this will.
@- Note that you can use the rest of the line as a comment.
This will not be rendered.
@+ This, too, will act as a comment.
This will also be rendered.
``````


_Output_: 🖥️

``````
This will be rendered to the output.
But this will.
This will also be rendered.
``````


:::

:::{note}

[](#diversions) attempt to write their contents to the output stream
only when they are played (or replayed), not when they are created.
Thus, you can create diversions when output is disabled just fine:

:::{admonition} Example 13: Output switches: diversions
_Source_: ⌨️

``````
@-
@# Output is disabled; now create a diversion.
@empy.startDiversion('test')@
This text is diverted.
@empy.stopDiverting()@
@# Replaying the diversion when output is disabled will print nothing.
@empy.replayDiversion('test')@
@+
@# But now that it is enabled, it will print normally.
@empy.playDiversion('test')@
``````


_Output_: 🖥️

``````
This text is diverted.
``````


:::

:::

### Literal markups

**Literal markups** are a category of markup that evaluate to some form of
themselves.

#### Prefix markup: `@@`

To render the prefix character literally in the output, duplicate it
as **prefix markup**.  For the default, `@`, it will be `@@`:

:::{admonition} Example 14: Prefix literals
_Source_: ⌨️

``````
This becomes a single at sign: @@.
``````


_Output_: 🖥️

``````
This becomes a single at sign: @.
``````


:::

:::{tip}

The prefix markup is not indicated by the prefix followed by an at
sign, but rather the prefix repeated once.  So if the prefix has been
changed to `$`, the prefix markup is `$$`, not `$@`.

:::

:::{versionadded} 1.0

Prefix markup was introduced in EmPy version 1.0.

:::


#### String markup: `@'...'`, `@"..."`, `@'''...'''`, `@"""..."""`

The interpreter prefix followed by a Python string literal (_e.g._,
`@'...'`) is **string markup**.  It evaluates the Python string
literal and expands it.  All variants of string literals with single
and double quotes, as well as triple quoted string literals (with both
variants) are supported.  This can be useful when you want to use
Python string escapes (not EmPy escapes) in a compact form:

:::{admonition} Example 15: String
_Source_: ⌨️

``````
This is a string: @'A single-quoted string'.
This is also a string: @"A double-quoted string".
This is another string: @'''A triple single-quoted string'''.
This is yet another string: @"""A triple double-quoted string""".
This is a multiline string: @"""Triple quotes containing newlines
will be preserved."""
This is a string using escapes: @
@'Welcome to \U0001d53c\U0001d55e\u2119\U0001d56a!'.
``````


_Output_: 🖥️

``````
This is a string: A single-quoted string.
This is also a string: A double-quoted string.
This is another string: A triple single-quoted string.
This is yet another string: A triple double-quoted string.
This is a multiline string: Triple quotes containing newlines
will be preserved.
This is a string using escapes: Welcome to 𝔼𝕞ℙ𝕪!.
``````


:::

:::{versionadded} 3.1.1

String markup was introduced in EmPy version 3.1.1.

:::

#### Backquote markup: `` @`...` ``

**Backquote markup** (`` @`...` ``) can be used to escape any text,
including EmPy markup.  Multiple opening backquotes can be used as
long as they are matched by an equal number in order to allow quoting
text which itself has backquotes in it:

:::{admonition} Example 16: Backquote
_Source_: ⌨️

``````
This is literal text: @`some text`.
This is a prefix: @`@`.
This would be expanded if it were not backquoted: @`@(1 + 1)`.
This would be an error if expanded: @`@(!@#$%^&*())`.
This contains backquotes: @```here's one: ` and here's two: `` ```.
``````


_Output_: 🖥️

``````
This is literal text: some text.
This is a prefix: @.
This would be expanded if it were not backquoted: @(1 + 1).
This would be an error if expanded: @(!@#$%^&*()).
This contains backquotes: here's one: ` and here's two: `` .
``````


:::

:::{warning}

To use the backquote markup with content containing backquotes which
are adjacent to the start or end markup, you need to pad it with
spaces.  So when quoting a single backquote, it needs to be written as
```@`` ` `` ```.  This also means you cannot use backquote markup to
specify a completely empty string.  It must always contain at least
one non-backquote character, e.g., `` @` ` ``.  If you really need
backquotes without whitespace padding, you can use a [hook](#hooks) to
intercept the backquote markup and strip it out.

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @`...` `` followed by
a newline becomes `` @`...`@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 4.0

Backquote markup was introduced in EmPy version 4.0.

:::


### Escape markup: `@\...`

**Escape markup** allows specifying individual non-printable
characters with a special readable syntax: `@\...`.  It is inspired
by and extends the string literal escape codes from languages such as
C/C++ and Python.

<a class="example" id="example-17"></a>

:::{admonition} Example 17: Escapes
_Source_: ⌨️
<div class="source">

``````
@# These are all a Latin uppercase A:
Binary: @\B{1000001}
Quaternary: @\q1001, @\Q{1001}
Octal: @\o101, @\O{101}
Hexadecimal (variable bytes): @\X{41}
Hexadecimal (one-byte): @\x41
Hexadecimal (two-byte): @\u0041
Hexadecimal (eight-byte): @\U00000041
By Unicode name: @\N{LATIN CAPITAL LETTER A}
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Binary: A
Quaternary: A, A
Octal: A, A
Hexadecimal (variable bytes): A
Hexadecimal (one-byte): A
Hexadecimal (two-byte): A
Hexadecimal (eight-byte): A
By Unicode name: A
``````

</div>

:::

The escape sequence type is indicated by the first character and then
consumes zero or more characters afterward, depending on the escape
sequence.  Some sequence sequences support a variable number of
characters, delimited by curly braces (`{...}`).

:::{seealso}

The list of all valid escape sequences is available in the `escapes`
help topic and is summarized [here](HELP.md#escape-sequences-summary).

:::

:::{versionadded} 1.5

Escape markup was introduced in EmPy version 1.5, and then reworked in
EmPy version 4.0.

:::


#### Named escape markup: `@\^{...}`

The escape markup for controls `@\^...` has an extended usage where
the character can be specified by a control code name.  The resulting
**named escape markup** takes the form of `@\^{...}` with the escape
code name between the curly braces.  The name of the escape code used
in the markup is case insensitive.

The mapping of escape names to characters is specified in the
configuration variable `controls`.  The keys of this
dictionary must be in uppercase and the values can be integers
(Unicode code point values), lists of integers, or strings.  They can
also take the form of a 2-tuple, where the first element is one of the
above values and the second element is a description string used for
displaying in help topics.

<a class="example" id="example-18"></a>

:::{admonition} Example 18: Named escapes
_Source_: ⌨️
<div class="source">

``````
Normal space: [ ]
Normal space by name: [@\^{SP}]
No-break space: [@\^{NBSP}]
Thin space: [@\^{THSP}]
En space: [@\^{ENSP}]
Em space: [@\^{EMSP}]
(Well, these would look right if it this were in a proportional font.)
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Normal space: [ ]
Normal space by name: [ ]
No-break space: [ ]
Thin space: [ ]
En space: [ ]
Em space: [ ]
(Well, these would look right if it this were in a proportional font.)
``````

</div>

:::

:::{note}

The dictionary mapping all named escape codes to characters is stored in the
`controls` configuration variable.  This can be
modified or completely replaced as desired.  The values will be
recoded to a native string and can be any of the following, in
order:

- a `str`, representing the string;
- a `bytes`, which will be decoded into a string according to the
  output encoding;
- an `int`, representing the Unicode code point value, which will be
  converted to a string via `chr`;
- a callable object, which will be called and then converted;
- any object with a `__str__` method, whose `str` value will be used;

or a `list` of the above values, which will be converted as above and
concatenated together.

Additionally, they can take the form of a 2-tuple, where the first
element is one of the above types and the second element is a
description string, used for rendering the value in help topics.

:::

:::{seealso}

The list of all valid control code names is available in the
`named` help topic and is summarized
[here](HELP.md#named-escapes-control-codes-summary).

:::

:::{versionadded} 4.0

Named escape markup was introduced in EmPy version 4.0.

:::


### Expression markup: `@(...)`

EmPy mainly processes markups by evaluating expressions and executing
statements.  Expressions are bits of Python code that return a value;
that value is then rendered into the output stream.  Simple examples
of Python expressions are `1 + 2`, `abs(-2)`, or `"test"*3`.

In EmPy, expressions are evaluated and expanded with the **expression
markup** `@(...)`.  By default, an expression that evaluates to
`None` does not print anything to the underlying output stream; this
is equivalent to it having returned `''`.

:::{tip}

If you want to change this behavior, specify your preferred value with
`--none-symbol` (_configuration variable:_ `noneSymbol`).

:::

<a class="example" id="example-19"></a>

:::{admonition} Example 19: Expressions
_Source_: ⌨️
<div class="source">

``````
The sum of 1 and 2 is @(1 + 2).
The square of 3 is @(3**2).
The absolute value of -12 is @(abs(-12)).
This prints "test" but does not print None: @(print("test", end='')).
This, however, does: @(repr(None)).
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The sum of 1 and 2 is 3.
The square of 3 is 9.
The absolute value of -12 is 12.
This prints "test" but does not print None: test.
This, however, does: None.
``````

</div>

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @(...) `` followed by
a newline becomes `` @(...)@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 1.0

Expression markup was introduced in EmPy version 1.0.

:::


### Additional expression markup

Several expression markup variants are available.

#### Simple expression markup: `@x`, `@x.a`, `@x[i]`, `@x(args...)`, _etc._

Often expressions are "simple" and unambiguous enough that needing to
use the full `@(...)` syntax is unnecessary.  In cases where a
single variable is being referenced unambiguously, the parentheses can
be left off to create **simple expression markup**:

<a class="example" id="example-20"></a>

:::{admonition} Example 20: Simple expressions, basic
_Source_: ⌨️
<div class="source">

``````
@# Set a variable to use.
@{x = 16309}@
The value of x is @x.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The value of x is 16309.
``````

</div>

:::

`@x` is precisely the same thing as `@(x)`.

This markup can be extended further.  Attribute references (`@x.a`),
indexing (`@x[i]`), and function calls (`@x(args...)`) can also be
simplified in this way.  They can also be chained together
arbitrarily, so `@object.attribute.subattribute`,
`@object.method(arguments...)`, `@object[index1][index2]`,
`@object[index].attribute`, `@object[index].method(arguments...)`,
etc. are all valid examples of simple expression markup.  Function
calls can even be chained in simple expression markup:
`@factory(identifier)(arguments...)`.  These simple expressions can
be extended arbitrarily.

<a class="example" id="example-21"></a>

:::{admonition} Example 21: Simple expressions, chaining
_Source_: ⌨️
<div class="source">

``````
@# Define some variables to use.
@{
import time

def mean(seq): # a function
    return sum(seq)/len(seq)

class Person: # a class

    def __init__(self, name, birth, scores):
        self.name = name
        self.birth = birth
        self.scores = scores

    def age(self):
        current = time.localtime(time.time()).tm_year
        return current - self.birth

person = Person("Fred", 1984, [80, 100, 70, 90]) # an instance of that class
}@
The name of person is @(person.name), or more simply @person.name.
The first letter is @(person.name[0]), or more simply @person.name[0].
He has @(len(person.scores)) scores, or more simply @len(person.scores).
His first score is @(person.scores[0]), or more simply @person.scores[0].
His average score is @(mean(person.scores)), or more simply @mean(person.scores).
His age is @(person.age()), or more simply @person.age().
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The name of person is Fred, or more simply Fred.
The first letter is F, or more simply F.
He has 4 scores, or more simply 4.
His first score is 80, or more simply 80.
His average score is 85.0, or more simply 85.0.
His age is 42, or more simply 42.
``````

</div>

:::

:::{note}

Final punctuation, including a period (`.`), is not interpreted as an
attribute reference and thus does not result in a parse error.  Thus
you can use end-of-sentence punctuation naturally after a simple
expression markup.

:::

If you wish to concatenate an expression with immediately following
text so that it will not be parsed incorrectly, either use whitespace
markup or just fall back to a full expression markup:

<a class="example" id="example-22"></a>

:::{admonition} Example 22: Simple expressions, concatenation
_Source_: ⌨️
<div class="source">

``````
@# Define a variable for use.
@{thing = 'cat'}@
@# Referencing `@things` to pluralize `@thing` will not work.  But:
The plural of @thing is @thing@ s.
Or:  The plural of @thing is @(thing)s.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The plural of cat is cats.
Or:  The plural of cat is cats.
``````

</div>

:::

:::{versionadded} 1.0

Simple expression markup was introduced in EmPy version 1.0.

:::


#### Functional expression markup: `@function{markup}{...}`

Arguments to function calls in EmPy expression markups use Python
expressions, not EmPy markup (_e.g._, `@f(x)` calls the function `f`
with the variable `x`).  To specify EmPy markup which is expanded and
then passed in to the function, there is **functional expression
markup** as an extension of simple expression markup.  Since each
argument to the function is expanded, the arguments are always
strings:

<a class="example" id="example-23"></a>

:::{admonition} Example 23: Functional expressions, one argument
_Source_: ⌨️
<div class="source">

``````
@{
def f(x):
    return '[' + x + ']'
}@
@# Note that the argument is expanded before being passed to the function:
This will be in brackets: @f{1 + 1 is @(1 + 1)}.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This will be in brackets: [1 + 1 is 2].
``````

</div>

:::

Functional expressions support the application of multiple arguments
by repeating the `{...}` suffix for as many arguments as is desired.
Each argument is expanded one at a time in order, from left to right:

<a class="example" id="example-24"></a>

:::{admonition} Example 24: Functional expressions, multiple arguments
_Source_: ⌨️
<div class="source">

``````
@{
def f(x, y, z):
    return x.lower() + ', ' + y.upper() + ', ' + z.capitalize()
}@
@# Multiple arguments are possible by repeating the pattern:
These expansions are separated by commas: @
@f{lowercase: @(1)}{uppercase: @(1 + 1)}{capitalized: @(1 + 1 + 1)}.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
These expansions are separated by commas: lowercase: 1, UPPERCASE: 2, Capitalized: 3.
``````

</div>

:::

The opening and closing curly braces can be repeated an equal number
of times to allow curly braces to be used within the markup:

<a class="example" id="example-25"></a>

:::{admonition} Example 25: Functional expressions, repeated braces
_Source_: ⌨️
<div class="source">

``````
@{
def f(*args):
    return ''.join('[' + x + ']' for x in args)
}@
@# Repeated matching curly braces allow internal curly braces to be used:
This contains internal curly braces: @f{{There are {curly braces} inside}}.
This contains more: @f{{{Even more {curly} {{braces}} here}}}.
@# To have internal curly braces at the start or end,
@# use escape markup or whitespace markup to separate them:
Internal curly braces with whitespace: @f{{@ {braces}@ }}.
Internal curly braces with escapes: @f{@\{braces@\}}.
Internal curly braces with hexdecimal escapes: @f{@\x7bbraces@\x7d}.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This contains internal curly braces: [There are {curly braces} inside].
This contains more: [Even more {curly} {{braces}} here].
Internal curly braces with whitespace: [{braces}].
Internal curly braces with escapes: [{braces}].
Internal curly braces with hexdecimal escapes: [{braces}].
``````

</div>

:::

:::{warning}

Functional expression markup is an extension of simple expression
markup so cannot be used in standard expression markup.  Further, it
cannot be seemlessly combined ("curried") with a normal function call.
Thus, `@f(1){a}{b}` is equivalent to `@(f(1)('a', 'b'))`, not
`@(f(1, 'a', 'b'))`.  A functional argument calls will terminate
simple expression parsing, so `@f{a}{b}(3)` is the same as `@(f('a',
'b'))(3)`, not `@f('a', 'b', 3)`; that is, trailing function calls
are not applied.

:::

:::{versionadded} 4.0

Functional expression markup was introduced in EmPy version 4.0.

:::


#### Extended expression markup: `@(...?...!...$...)`

Expression markup has an **extended expression markup** form which
allows more powerful manipulation of expressions.

##### Conditional expression markup: `@(...?...!...)`

The first form is **conditional expression markup** which allows for a
compact form of an `@[if]` statement with a ternary operator, similar
to C/C++'s `?` and `:` operators.  In EmPy, however, these are
represented with `?` and `!`, respectively.

:::{note}

C/C++'s use of `:` was changed to `!` for EmPy since `:` already has
special meaning in Python.  This syntax was originally added before
Python supported the `if/else` ternary expression, although EmPy's
syntax is more general and powerful.

:::

If a `?` is present in the expression, then the Python (not EmPy)
expression before the `?` is tested; if it is true, then the Python
expression following it is evaluated.  If a `!` is present afterward
and the originally expression was false, then the Python expression
following it is expanded (otherwise, nothing is).  It thus acts as an
if-then-else construct:

<a class="example" id="example-26"></a>

:::{admonition} Example 26: Extended expressions, conditional
_Source_: ⌨️
<div class="source">

``````
Four is an @(4 % 2 == 0 ? 'even' ! 'odd') number.
Five is @(5 % 2 == 0 ? 'also even' ! 'not').
Seven is an @(7 % 2 == 0 ? 'even' ! 'odd') number.
And this is blank: @(False ? 'this will expand to nothing')
@# Whitespace is not required:
Eleven is an @(11 % 2 == 0?'even'!'odd') number.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Four is an even number.
Five is not.
Seven is an odd number.
And this is blank: 
Eleven is an odd number.
``````

</div>

:::

##### Chained conditional expression markup: `@(...?...!... ...)`

These `?` and `!` sequences can be repeated indefinitely, forming an
if-else-if-else chain called **chained conditional expression
markup**, with a `!`  expression serving as the conditional test for
the next `?`:

<a class="example" id="example-27"></a>

:::{admonition} Example 27: Extended expressions, chained conditional
_Source_: ⌨️
<div class="source">

``````
@# Define a variable for use.
@{x = 3}@
x is @(x == 1 ? 'one' ! x == 2 ? 'two' ! x == 3 ? 'three' ! 'unknown').
``````

</div>

_Output_: 🖥️
<div class="output">

``````
x is three.
``````

</div>

:::

##### Except expression markup: `@(...$...)`

Finally, a `$` present toward the end of expression markup, whether or
not it includes an if-else chain, represents **except expression
markup**: If the main expression throws an exception, suppress it and
evaluate the Python (not EmPy) except expression instead.  This can be
combined with conditional or chained conditional expresion markup:

<a class="example" id="example-28"></a>

:::{admonition} Example 28: Extended expressions, except
_Source_: ⌨️
<div class="source">

``````
No exception:  2 + 2 = @(2 + 2 $ 'oops').
Division by zero is @(1/0 $ 'illegal').
Two divided by zero is @(2/0 % 2 == 0 ? 'even' ! 'odd' $ 'also illegal').
``````

</div>

_Output_: 🖥️
<div class="output">

``````
No exception:  2 + 2 = 4.
Division by zero is illegal.
Two divided by zero is also illegal.
``````

</div>

:::

:::{tip}

Except expression markup will not capture `SyntaxError`s, since it is
much more likely that this is a static error (_e.g._, a typographical
error) rather than a dynamic error.  This behavior can be modified by
changing `fallThroughErrors` in the
[configuration](#configuration).

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @(...) `` followed by
a newline becomes `` @(...)@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 1.3

Conditional expression markup was first introduced in EmPy version
1.3, updated to extended expressions (including exception handling) in
EmPy version 1.4, and was expanded to support if-else chained
conditional expressions in 4.0.

:::


### In-place expression markup: `@$...$...$`

Occasionally it's desirable to designate an expression that will be
evaluated alongside its evaluation which may change, but which will be
re-evaluated with subsequent updates, in other words, explicitly
identifying exactly what is being evaluated at the same time.  This is
similar to the notion of CVS or SVN keywords such as `$Date ...$`.
For this, there is **in-place expression markup** (`@$...$...$`).
They consist of two segments: first, the Python (not EmPy) expression
to evaluate, and the second, the result of that evaluation.  When
evaluating the markup, the second (result) section is ignored and
replaced with the evaluation of the first and a new in-place markup is
rendered.  For example:

<a class="example" id="example-29"></a>

:::{admonition} Example 29: In-place expressions
_Source_: ⌨️
<div class="source">

``````
This could be a code comment indicating the version of EmPy:
# @$empy.version$this text is replaced with the result$
Arbitrary Python expressions can be evaluated:
# @$__import__('time').asctime()$$
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This could be a code comment indicating the version of EmPy:
# @$empy.version$4.2.1$
Arbitrary Python expressions can be evaluated:
# @$__import__('time').asctime()$Sun Feb  8 16:45:25 2026$
``````

</div>

:::

:::{note}

The `$` character is a common choice for an alternate prefix.  If it
is chosen instead of the default `@`, the in-place expression markup
will be remapped to have the form `$@...@...@`; that is, the `@`
and `$` are swapped.  (This is done automatically for any prefix
collision with a markup indicator.)

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @$...$...$ `` followed by
a newline becomes `` @$...$...$@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 1.4

In-place markup was introduced in EmPy version 1.4.

:::


### Statement markup: `@{...}`

Again, EmPy mainly processes markups by evaluating expressions and
executing statements.  Statements include assignments, control
structures (`if`, `for`, function and class definitions, etc.)
Statements do not yield a value; they are used for side effects,
whether that's changing the state of the interpreter (setting or
changing variables, defining objects, calling functions, etc.) or
printing output.  Statements can also consist of expressions, so an
expression (such as `print("Hello, world!")`) can be used solely for
its side effects with the statement markup.  **Statement markup** sets
off a series of statements to be executed inside the `@{...}`
markup.  Since statements do not yield a value, they are executed but
the markup itself does not implicitly write anything.  Since the
executed statements are Python, multiline statements must be formatted
and indented according to Python's parsing rules:

<a class="example" id="example-30"></a>

:::{admonition} Example 30: Statements
_Source_: ⌨️
<div class="source">

``````
@# Note the use of whitespace markup below to consume trailing newlines.
@{x = 16309}@
x is now @x.
@{
if x > 0:
    category = 'positive'
else:
    category = 'non-positive'
print("x is {}.".format(category))
}@
@{
# Since statement markup does not write anything itself, this
# statement has no effect.
x + 123
}@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
x is now 16309.
x is positive.
``````

</div>

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @{...} `` followed by
a newline becomes `` @{...}@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 1.0

Statement markup was introduced in EmPy version 1.0.

:::


### Control markups: `@[...]`

EmPy supports a variety of control structures, analogous to the
builtin Python control structures (`if`, `while`, `for`, etc.), with
some additional markups for convenience.  This is done with **control
markup** indicated by `@[...]`.

Since EmPy cannot rely on source indentation to delimit control
structure syntax, all primary control markups must end with an
explicit `end` markup (_e.g._, `@[if ...]...@[end if]`).  The clauses
surrounded by control markup are EmPy (Python) markup and are expanded
according to the logic of each control markup; see below.

Unlike the Python control structures, the code that is expanded within
each subclause is EmPy code, not Python code.  Thus, control markups
can be nested arbitrarily (_e.g._, `@[while ...]@[for ...]@[if ...]...@[end
if]@[end for]@[end while]`).

::::{attention}

To use nested control markup that spans multiple lines and is more
readable, you can rely on whitespace markup to consume the newline
immediately following the control markup.  As an example:

<a class="example" id="example-31"></a>

:::{admonition} Example 31: Controls, idiom
_Source_: ⌨️
<div class="source">

``````
@# Note the user of whitespace markup to consume the trailing newlines.
Counting:
@[for i, x in enumerate(range(0, 5))]@
@x is @
@[  if x % 2 == 0]@
even@
@[  else]@
odd@
@[  end if]@
.
@[end for]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Counting:
0 is even.
1 is odd.
2 is even.
3 is odd.
4 is even.
``````

</div>

:::

This method of writing organizing control markup with `@[...]@` all
on a single line for clarity is idiomatic EmPy.  (This applies to all
markup with starting and ending delimiters.)  See [here](#idiom) for
more details.

::::

:::{hint}

Whitespace before the control keyword is ignored, so you can add
whitespace inside the markup to simulate Python indentation for
clarity, as the above example demonstrates.

:::

:::{tip}

Simple ("clean") control markup which does not contain arbitrary
Python expressions -- `@[try]`, `@[else]`, `@[except ...]`,
`@[finally]`, `@[continue]`, `@[break]` and `@[end ...]` -- can
include a Python-style comment for clarity:

<a class="example" id="example-32"></a>

:::{admonition} Example 32: Controls, clean
_Source_: ⌨️
<div class="source">

``````
@{
number = 42
test = 3%2 == 0
}@
@[if test]@
@number is even.
@# Say there were many more lines here ...
@[else # test]@
@number is not even.
@# And also here ...
@[end if # test]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
42 is not even.
``````

</div>

:::

:::

:::{versionadded} 3.0

Control markups were introduced in EmPy version 3.0 unless otherwise
noted below.

:::


#### If control markup: `@[if E]...@[end if]`

The simplest control markup is the **if control markup**.  It
precisely mimics the Python `if` branching control structure.  The
test expressions are Python expressions.  Like the native Python
control structure, it takes on the following forms:

- `@[if E]...@[end if]`
- `@[if E]...@[else]...@[end if]`
- `@[if E1]...@[elif E2]...@[end if]`
- `@[if E1]...@[elif E2]...@[else]...@[end if]`
- `@[if E1]...@[elif E2]...@[elif E3]...@[else]...@[end if]`
- `@[if E1]...@[elif E2]...@[elif E3]... ... @[else]...@[end if]`

where the _`E`s_ are Python expressions to be tested for truth.

Thus, as with the builtin Python `if` control structure, zero or more
`@[elif]` clauses can be used and the `@[else]` clause (only valid
at the end of the chain) is optional.  If there is no `@[else]`
clause and all the test expressions are false, nothing will be
expanded.

<a class="example" id="example-33"></a>

:::{admonition} Example 33: If controls
_Source_: ⌨️
<div class="source">

``````
@{
def even(x):
    return x % 2 == 0
}@
0 is @[if even(0)]even@[end if].
1 is @[if even(1)]even@[else]odd@[end if].
2 is @[if even(2)]even@[else]odd@[end if].
3 is @[if even(3)]even@[elif not even(3)]not even@[end if].
4 is @[if 0 == 1]wrong@[elif 1 == 2]wrong@[else]fine@[end if].
``````

</div>

_Output_: 🖥️
<div class="output">

``````
0 is even.
1 is odd.
2 is even.
3 is not even.
4 is fine.
``````

</div>

:::


#### Break and continue control markup: `@[break]`, `@[continue]`

The looping control markup structures below (`@[for]`, `@[while]`,
and `@[dowhile]`) all support **break** and **continue control
markup**.  These markups follow the native Python forms; `@[break]`
will exit out of the innermost looping control structure, and
`@[continue]` will restart the innermost looping control structure.

They take the following forms:

- `@[break]`
- `@[continue]`

What follows is a few examples using a `@[for]` loop:

<a class="example" id="example-34"></a>

:::{admonition} Example 34: Continue controls
_Source_: ⌨️
<div class="source">

``````
@# Print even numbers.
@[for n in range(10)]@
@[  if n % 2 != 0]@
@[    continue]@
@[  end if]@
@n is even.
@[end for]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
0 is even.
2 is even.
4 is even.
6 is even.
8 is even.
``````

</div>

:::

<a class="example" id="example-35"></a>

:::{admonition} Example 35: Break controls
_Source_: ⌨️
<div class="source">

``````
@# Print numbers up to (but not including) 5.
@[for n in range(10)]@
@[  if n >= 5]@
@[    break]@
@[  end if]@
@n is less than 5.
@[end for]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
0 is less than 5.
1 is less than 5.
2 is less than 5.
3 is less than 5.
4 is less than 5.
``````

</div>

:::


#### For control markup: `@[for N in E]...@[end for]`

A basic iteration markup is the **for control markup**.  It precisely
mimics the Python `for` looping control structure.  The iterator
expression is a Python expression.  Like the native Python control
structure, it takes on the following forms:

- `@[for N in E]...@[end for]`
- `@[for N in E]...@[else]...@[end for]`

where _`N`_ is a Python identifier and _`E`_ is a Python expression
which is iterable.

As with the native Python control structure, an `@[else]` clause is
supported; this is expanded if the loop exits without an intervening
break.

<a class="example" id="example-36"></a>

:::{admonition} Example 36: For controls
_Source_: ⌨️
<div class="source">

``````
@[for x in range(1, 6)]@
@x squared is @(x*x).
@[else]@
... and done.
@[end for]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
1 squared is 1.
2 squared is 4.
3 squared is 9.
4 squared is 16.
5 squared is 25.
... and done.
``````

</div>

:::


#### While control markup: `@[while E]...@[end while]`

The most general looping markup is the **while control markup**.  It
precisely mimics the Python `while` looping control structure.  The
test expression is a python expression.  Like the native Python
control structure, it takes on the following forms:

- `@[while E]...@[end while]`
- `@[while E]...@[else]...@[end while]`

where _`E`_ is a Python expression to be tested for truth.

As with the native Python control structure, an `@[else]` clause is
supported; this is invoked if the loop exits without an intervening
break.

<a class="example" id="example-37"></a>

:::{admonition} Example 37: While controls
_Source_: ⌨️
<div class="source">

``````
@{a = 1}@
@[while a <= 5]@
@a pound signs: @('#'*a).
@{a += 1}@
@[else]@
... and done.
@[end while]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
1 pound signs: #.
2 pound signs: ##.
3 pound signs: ###.
4 pound signs: ####.
5 pound signs: #####.
... and done.
``````

</div>

:::


#### Dowhile control markup: `@[dowhile E]...@[end dowhile]`

An alternate `while` control structure is provided by EmPy: **dowhile
control markup**.  This differs from the standard `while` markup only
in that the loop is always entered at least once; that is, the test
expression is not checked before the first iteration.  In this way, it
is similar to the `do ... while` control structure from C/C++.  It
takes the following forms:

- `@[dowhile E]...@[end dowhile]`
- `@[dowhile E]...@[else]...@[end dowhile]`

where _`E`_ is a Python expression to be tested for truth.

Like the native Python `while` control structure, an `@[else]` clause
is supported; this is invoked if the loop exits without an intervening
break.

<a class="example" id="example-38"></a>

:::{admonition} Example 38: Dowhile controls
_Source_: ⌨️
<div class="source">

``````
@# Stop when divisible by 5, but include 0 since it's the first iteration:
@{n = 0}@
@[dowhile n % 5 != 0]@
@n works@[if n % 5 == 0] (even though it's divisible by 5)@[end if].
@{n += 1}@
@[else]@
... and done.
@[end dowhile]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
0 works (even though it's divisible by 5).
1 works.
2 works.
3 works.
4 works.
... and done.
``````

</div>

:::

:::{versionadded} 4.0

Dowhile control markup was introduced in EmPy version 4.0.

:::


#### Try control markup: `@[try]...@[end try]`

**Try control markup** is the EmPy equivalent of a `try` statement.
As with the native Python statement, this markup can take on the
widest variety of forms.  They are:

- `@[try]...@[except]...@[end try]`
- `@[try]...@[except C]...@[end try]`
- `@[try]...@[except C as N]...@[end try]`
- `@[try]...@[except C, N]...@[end try]`
- `@[try]...@[except (C1, C2, ...) as N]...@[end try]`
- `@[try]...@[except C1]...@[except C2]...@[end try]`
- `@[try]...@[except C1]...@[except C2]... ... @[end try]`
- `@[try]...@[finally]...@[end try]`
- `@[try]...@[except ...]...@[finally]...@[end try]`
- `@[try]...@[except ...]...@[else]...@[end try]`
- `@[try]...@[except ...]...@[else]...@[finally]...@[end try]`

where the _`C`s_ are Python expressions to be treated as an exception
class or tuple of classes and _`N`_ is a Python identifier.

Its behavior mirrors in every way the native Python `try` statement.
The try clause will be expanded, and if an exception is thrown, the
first `@[except]` clause that matches the thrown exception (if there
are any) will be expanded.  If a `@[finally]` clause is present, that
will be expanded after any possible exception handling, regardless of
whether an exception was in fact thrown.  Finally, if there is at
least one `@[except]` clause, an `@[else]` may be present which will
be expanded in the event that no exception is thrown (but before any
`@[finally]` clause).

The argument to the `@[except]` markup indicates which type(s) of
exception should be handled and with what name, if any.  No argument
indicates that it will handle any exception.  A simple expression will
indicate an exception class, or a tuple of exception classes, that
will be handled.  The variable name of the thrown exception can be
captured and passed to the expansion with the `as` keyword, or a comma
(this latter notation is invalid in modern Python versions but is
still supported in EmPy regardless of the underlying Python version).

For example:

<a class="example" id="example-39"></a>

:::{admonition} Example 39: Try controls
_Source_: ⌨️
<div class="source">

``````
Garbage is @[try]@hugalugah@[except NameError]not defined@[end try].
Division by zero is @[try]@(1/0)@[except ZeroDivisionError]illegal@[end try].
An index error is @[try]@([][3])@[except IndexError as e]@e.__class__.__name__@[end try].
And finally: @[try]@(nonexistent)@[except]oops, @[finally]something happened@[end try].
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Garbage is not defined.
Division by zero is illegal.
An index error is IndexError.
And finally: oops, something happened.
``````

</div>

:::

:::{versionadded} 3.0

Try control markup was introduced in EmPy version 3.0, and was
expanded in 4.0 to include all modern valid uses of `@[else]` and
`@[finally]`.

:::


#### With control markup: `@[with E as N]...@[end with]`

EmPy supports a version of the `with` statement, which was introduced
in Python 2.5.  In EmPy, the **with control markup** is written as
`@[with]` and mirrors the behavior of the native `with` statement.
It takes the following forms:

- `@[with E as N]...@[end with]`
- `@[with N]...@[end with]`
- `@[with E]...@[end with]`

where _`E`_ is a Python expression which is iterable and _`N`_ is a
Python identifier.

All forms use context managers, just as with the native statement.
Context managers are objects which have `__enter__` and `__exit__`
methods, and the `@[with]` markup ensures that the former is called
before the markup's contents are expanded and that the latter is
always called afterward, whether or not an exception has been thrown.

The three forms of the `@[with]` markup mirror the uses of the `with`
keyword:  The user can specify an expression and a variable name with
the `as` keyword, or just a variable name, or just an expression (it
will be entered and exited, but the name of the resulting object will
not be available).  For example:

<a class="example" id="example-40"></a>

:::{admonition} Example 40: With controls
_Source_: ⌨️
<div class="source">

``````
@{
import os, sys

# Create a test file to use with the @[with ...] markup.
with open('/tmp/with.txt', 'w') as f:
    print("Hello, world!", file=f)
}@
@[with open('/tmp/with.txt') as f]@f.read()@[end with]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Hello, world!
``````

</div>

:::

:::{note}

Although the `with` keyword was only introduced in Python 2.5, the 
`@[with]` markup will work in any supported version of Python.

:::

:::{versionadded} 4.0

With control markup was introduced in EmPy version 4.0.

:::


#### Match control markup: `@[match E]@[case C]...@[end match]`

Python 3.10 introduces structural pattern matching with the
`match`/`case` control structure.  The analog to this in EmPy is the
**match control markup**.  It takes the following forms:

- `@[match E]@[case C1]...@[end match]`
- `@[match E]@[case C1]...@[case C2]...@[end match]`
- `@[match E]@[case C1]...@[case C2]...@[else]...@[end match]`

where _`E`_ is a Python expression to be matched and the _`C`s_ are
Python expressions to be tested as match cases.

The control markup behaves the same as the native Python control
structure: The first `@[case]` clause which matches is expanded
and the control finishes.  An optional `@[else]` clause can appear at
the end of the chain of cases and will be expanded if no previous
`@[case]` clause matches; `@[else]` is identical to `@[case
_]`.

Markup that is present between the `@[match]` markup and the first
`@[case]` is unconditionally expanded.  Typically this will be markup
that expands to nothing, but it can involve a preamble if desired.

<a class="example" id="example-41"></a>

:::{admonition} Example 41: Match controls
_Source_: ⌨️
<div class="source">

``````
@[for p in [(0, 0), (10, 0), (0, 10), (10, 10), (10, 20), (20, 10), 'oops']]@
@[  match p]@
@# Markup here is expanded unconditionally.
@p is @
@[    case (0, 0)]@
the origin.
@[    case (0, y)]@
y = @y.
@[    case (x, 0)]@
x = @x.
@[    case (x, y) if x == y]@
x = y = @x.
@[    case (x, y)]@
x = @x, y = @y.
@[    else]@
not a point.
@[  end match]@
@[end for]@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
(0, 0) is the origin.
(10, 0) is x = 10.
(0, 10) is y = 10.
(10, 10) is x = y = 10.
(10, 20) is x = 10, y = 20.
(20, 10) is x = 20, y = 10.
oops is not a point.
``````

</div>

:::

:::{warning}

Since the `@[match]` markup relies on the underlying Python
functionality, using this markup with a version of Python before 3.10
will result in a `CompatibilityError` being raised.

:::

:::{versionadded} 4.1

Match control markup was introduced in EmPy version 4.1.

:::


#### Defined control markup: `@[defined N]...@[end defined]`

Sometimes it's useful to know whether a name is defined in either the
locals or globals dictionaries.  EmPy provides a dedicated markup for
this purpose: **defined control markup**.  It takes the following
forms:

- `@[defined N]...@[end defined]`
- `@[defined N]...@[else]...@[end defined]`

where _`N`_ is a Python identifier.

When provided a name, it will expand the contained markup if that name
is defined in either the locals or globals.  `@[defined NAME]...@[end
defined]` is equivalent to `@[if 'NAME' in globals() or 'NAME' in
locals()]...@[end if]`.  An `@[else]` clause is also supported; if
present, this will be expanded if the name does _not_ appear in the
locals or globals.  If no `@[else]` clause is present and the name is
not defined, nothing will be expanded.

<a class="example" id="example-42"></a>

:::{admonition} Example 42: Defined controls
_Source_: ⌨️
<div class="source">

``````
@{cat = 'Boots'}@
Cat is @[defined cat]@cat@[else]not defined@[end defined].
Dog is @[defined dog]@dog@[else]not defined@[end defined].
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Cat is Boots.
Dog is not defined.
``````

</div>

:::

:::{versionadded} 4.0

Defined control markup was introduced in EmPy version 4.0.

:::

#### Def control markup: `@[def F(...)]...@[end def]`

EmPy supports defining functions which expand EmPy code, not Python
code as with the standard `def` Python statement.  This is called
**def control markup**.  It takes on the following form:

- `@[def F(...)]...@[end def]`

where _`F(...)`_ is a Python function signature.

Def control markup involves specifying the signature of the resulting
function (such as with the standard Python `def` statement) and
encloses the EmPy code that the function should expand.  It is then
defined in the interpreter's globals/locals and can be called like any
other Python function.

It is best demonstrated with a simple example:

<a class="example" id="example-43"></a>

:::{admonition} Example 43: Def controls
_Source_: ⌨️
<div class="source">

``````
@# Define an EmPy-native function.
@[def element(name, symbol, atomicNumber, group)]@
Element @name (symbol @symbol, atomic number @atomicNumber) is a @group@
@[end def]@
@# Now use it.
@element('hydrogen', 'H', 1, 'reactive nonmetal').
@element('helium', 'He', 2, 'noble gas').
@element('lithium', 'Li', 3, 'alkali metal').
@element('beryllium', 'Be', 4, 'alkaline earth metal').
@element('boron', 'B', 5, 'metalloid').
@element('carbon', 'C', 6, 'reactive nonmetal').
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Element hydrogen (symbol H, atomic number 1) is a reactive nonmetal.
Element helium (symbol He, atomic number 2) is a noble gas.
Element lithium (symbol Li, atomic number 3) is a alkali metal.
Element beryllium (symbol Be, atomic number 4) is a alkaline earth metal.
Element boron (symbol B, atomic number 5) is a metalloid.
Element carbon (symbol C, atomic number 6) is a reactive nonmetal.
``````

</div>

:::

:::{hint}

The markup `@[def FUNC(...)]DEFN@[end def]` is equivalent to the
following Python code:

```python
def FUNC(...):
    r"""DEFN"""
    return empy.expand(r"""DEFN""", locals())
```

It simply defines a Python function with the provided signature, a
docstring indicating its EmPy definition, and the function calls the
`expand` method on the pseudomodule/interpreter with the definition
and returns the results.

:::

Any valid Python function notation is allowed, including type hints;
note, though, that the return value of the defined function is an
expansion and thus should always be `str`:

<a class="example" id="example-44"></a>

:::{admonition} Example 44: Def controls: type hints
_Source_: ⌨️
<div class="source">

``````
@{
DIGITS = [
    'zero', 'one', 'two', 'three', 'four', 
    'five', 'six', 'seven', 'eight', 'niner',
]
}@
@[def words(number: int) -> str]@
@(' '.join(DIGITS[int(x)] for x in str(number)))@
@[end def]@
16309 in words is @words(16309).
``````

</div>

_Output_: 🖥️
<div class="output">

``````
16309 in words is one six three zero niner.
``````

</div>

:::

:::{tip}

Functions defined with def control markup are callable Python objects
like any other.  They can be called through any mechanism, whether
Python (`f(...)`), through EmPy markup (`@f(...)`), or even via
[functional expression markup](#functional-expression-markup)
(`@f{...}`).

:::


### Diacritic markup: `@^ CHAR DIACRITIC(S)`

EmPy provides a quick and convenient way to combine diacritics
(accents) to characters with **diacritic markup**.  Diacritic markup
consists of the prefix `@^`, followed by the base character, and then
either a single character representing the accent to apply or a
sequence of such characters enclosed in curly braces (`{...}`).

The first character is the base character to combine diacritics with,
and the remaining characters (possibly more than one if the curly
braces form is used) are diacritic codes corresponding to Unicode
combining characters that can be combined (or just appended) to the
base character.  These combining diacritics are simpler, more easily
entered characters that (at least in some cases) resemble the actual
desired combining character.  For instance, `'` (apostrophe)
represents the acute accent ◌́; `` ` `` (backquote) represents the grave accent ◌̀; `^` represents the circumflex
accent ◌̂, and so on:

<a class="example" id="example-45"></a>

:::{admonition} Example 45: Diacritics
_Source_: ⌨️
<div class="source">

``````
French: Voil@^a`, c'est ici que @^c,a s'arr@^e^te.
Spanish: Necesito ir al ba@^n~o ahora mismo.
Portuguese: Informa@^c,@^a~o @^e' poder.
Swedish: Hur m@^aonga kockar kr@^a:vs f@^o:r att koka vatten?
Vietnamese: Ph@^o{h?} b@^o` vi@^e^n ngon qu@^a'!
Esperanto: E@^h^o@^s^an@^g^e @^c^iu@^j^a@^u(de!
Shakespearean: All are punish@^e`d.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
French: Voilà, c'est ici que ça s'arrête.
Spanish: Necesito ir al baño ahora mismo.
Portuguese: Informação é poder.
Swedish: Hur många kockar krävs för att koka vatten?
Vietnamese: Phở bò viên ngon quá!
Esperanto: Eĥoŝanĝe ĉiuĵaŭde!
Shakespearean: All are punishèd.
``````

</div>

:::

:::{tip}

Curly braces can enclose zero or more characters representing
diacritics.  If they enclose zero, the diacritic markup has no effect
(`@^e{}` is no different from `e`).  If they enclose one, the results
are no different from not using curly braces (`@^e{'}` and `@^e'`
are have identical results).  Only when applying more than one
diacritic are curly braces required.

:::

By default, the base character and diacritics will be combined with
NFKC normalization -- this will, when possible, replace the base
character and its combiners with a single Unicode character
representing the combination, if one exists.  Normalization is not
required (and may sometimes fail when a suitable combined form does
not exist); in these cases, your system's Unicode renderer will cope
as best it can.  To change the normalization type, use
`-z/--normalization-form=F` (_configuration variable:_ `normalizationForm`).  To disable normalization,
set it to the empty string.

:::{note}

The dictionary mapping all diacritic codes to combining characters is stored in the
`diacritics` configuration variable.  This can be
modified or completely replaced as desired.  The values will be
recoded to a native string and can be any of the following, in
order:

- a `str`, representing the string;
- a `bytes`, which will be decoded into a string according to the
  output encoding;
- an `int`, representing the Unicode code point value, which will be
  converted to a string via `chr`;
- a callable object, which will be called and then converted;
- any object with a `__str__` method, whose `str` value will be used;

or a `list` of the above values, which will be converted as above and
concatenated together.

Additionally, they can take the form of a 2-tuple, where the first
element is one of the above types and the second element is a
description string, used for rendering the value in help topics.

:::

:::{seealso}

The list of all default diacritic codes is available in the
`diacritics` help topic and is summarized
[here](HELP.md#diacritic-combiners-summary).

:::

:::{versionadded} 4.0

Diacritic markup was introduced in EmPy version 4.0.

:::


### Icon markup: `@|...`

A customizable brief way to map "icon" keys -- short, user-specified
strings -- to arbitrary Unicode strings exists in the form of **icon
markup**.  Icon markup is set off with `@|...` and then followed by
an unambiguous, arbitrary-length sequence of characters (Unicode code
points) corresponding to one of its keys.

Keys can be arbitrary length and can consist of whatever characters
are desired (including letters, numbers, punctuation, or even Unicode
characters).  They are not delimited by whitespace; however, they must
be unambiguous, so if more than one key exists with the same prefixes
(say, `@|#+` and `@|#-`), a key cannot be assigned the common prefix
(`@|#`) as this would be found first and would hide the longer
prefixes.  Such a common prefix key should be set to the value `None`
(which indicates to the parser that the icon is potentially valid but
not yet complete).

This validation is done automatically when the icon markup is first
used: The dictionary of icons is traversed and any common prefixes not
defined in the dictionary are set to `None`.  In the event that this
auto-validation may be expensive and the user wishes to do it manually
to avoid this step, specify the
`--no-auto-validate-icons` command line option
(`autoValidateIcons` configuration variable) to disable
it.

<a class="example" id="example-46"></a>

:::{admonition} Example 46: Icons
_Source_: ⌨️
<div class="source">

``````
These are @|"(curly quotes.@|")
This is a royal flush: A@|%s K@|%s Q@|%s J@|%s T@|%s.
This is a check mark @|/ and this is an X mark @|\.
Smile! @|:) Laugh! @|:9 Cry! @|:5 Sleep! @|:Z
``````

</div>

_Output_: 🖥️
<div class="output">

``````
These are “curly quotes.”
This is a royal flush: A♠️ K♠️ Q♠️ J♠️ T♠️.
This is a check mark ✔️ and this is an X mark ❌️.
Smile! 😀 Laugh! 🤣 Cry! 🥲 Sleep! 😴
``````

</div>

:::

To customize icons, modify or replace the `icons`
configuration variable:

<a class="example" id="example-47"></a>

:::{admonition} Example 47: Icons, customization
_Source_: ⌨️
<div class="source">

``````
@# Replace the icons with just a few very serious ones.
@{
empy.config.icons = {
    'kitty': '\U0001f431',
    'cat': '\U0001f408',
}
}@
Counting: one two @|kitty @|cat five.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Counting: one two 🐱 🐈 five.
``````

</div>

:::

:::{note}

The dictionary mapping all icon keys to their substitutes is stored in the
`icons` configuration variable.  This can be
modified or completely replaced as desired.  The values will be
recoded to a native string and can be any of the following, in
order:

- a `str`, representing the string;
- a `bytes`, which will be decoded into a string according to the
  output encoding;
- an `int`, representing the Unicode code point value, which will be
  converted to a string via `chr`;
- a callable object, which will be called and then converted;
- any object with a `__str__` method, whose `str` value will be used;

or a `list` of the above values, which will be converted as above and
concatenated together.

Additionally, they can take the form of a 2-tuple, where the first
element is one of the above types and the second element is a
description string, used for rendering the value in help topics.

:::

:::{tip}

If you're finding problems with icons being ambiguous, you can add
delimiters at the end of the icon key to ensure that they are
unambiguous.  For example, the icons `@|!`, `@|!!` and `@|!?` would
normally be ambiguous.  However, wrapping them in, say, curly braces,
will remove the ambiguity: `@|{!}`, `@|{!!}`, `@|{!?}` are
unambiguous and can be used as icon keys.

:::

:::{seealso}

The list of all valid icon keys is available in the `icons` help topic
and is summarized [here](HELP.md#icons-summary).

:::

:::{warning}

The default set of icons were chosen by the author for his convenience
and to demonstrate what icon markup can do.  It is expected that users
using icon markup will modify (or more likely completely replace) the
icons dictionary to their liking.  Thus the default icons are subject
to change.

:::

:::{versionadded} 4.0

Icon markup was introduced in EmPy version 4.0.

:::


### Emoji markup: `@:...:`

A dedicated **emoji markup** is available to translate Unicode emoji
names, and Unicode names more generally, into Unicode glyphs.  Using
the markup is simple: Use the `@:...:` syntax and put the name of the
emoji character between the colons.  Since EmPy is often used in
wrapped text, any newlines in the emoji name will be replaced with
spaces.

By default it uses the builtin `unicodedata.lookup` function call
which allow the lookup of any Unicode code point by name, not just
emoji.  Whether names are case sensitive or not, or whether words are
separated by spaces or underscores or either, is module-dependent.
The builtin `unicodedata` module (the fallback if no emoji-specific
modules are installed) is case insensitive and requires spaces, not
underscores:

<a class="example" id="example-48"></a>

:::{admonition} Example 48: Emojis
_Source_: ⌨️
<div class="source">

``````
Latin capital letter A: @:LATIN CAPITAL LETTER A:
Latin small letter O with diaeresis: @:latin small letter o with diaeresis:
White heavy check mark: @:WHITE HEAVY CHECK MARK:
Volcano: @:VOLCANO:
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Latin capital letter A: A
Latin small letter O with diaeresis: ö
White heavy check mark: ✅
Volcano: 🌋
``````

</div>

:::

User-specified emojis can also be assigned to the
`emojis` configuration variable; these will be checked
before any emoji modules are queried.  Emojis in the `emojis`
dictionary are case sensitive:

<a class="example" id="example-49"></a>

:::{admonition} Example 49: Emojis, custom
_Source_: ⌨️
<div class="source">

``````
@# What's with this guy and cats?
@{
empy.config.emojis['kittycat'] = '\U0001f408'
}@
This is a kitty cat: @:kittycat:
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This is a kitty cat: 🐈
``````

</div>

:::

:::{note}

The dictionary mapping all emoji keys to their substitutes is stored in the
`emojis` configuration variable.  This can be
added to or completely replaced as desired.  The values will be
recoded to a native string and can be any of the following, in
order:

- a `str`, representing the string;
- a `bytes`, which will be decoded into a string according to the
  output encoding;
- an `int`, representing the Unicode code point value, which will be
  converted to a string via `chr`;
- a callable object, which will be called and then converted;
- any object with a `__str__` method, whose `str` value will be used;

or a `list` of the above values, which will be converted as above and
concatenated together.

Additionally, they can take the form of a 2-tuple, where the first
element is one of the above types and the second element is a
description string, used for rendering the value in help topics.

:::

#### Third-party emoji modules

The emoji markup can also use third-party emoji modules if they are
present.  These can be installed in the usual way with PyPI (_e.g._,
`python3 -m pip install emoji`) or any other preferred method.  The
following emoji modules are supported:


{#third-party-emoji-modules-table}
| Module | Function | Parameter | Capitalization | Word delimiters |
| --- | --- | --- | --- | --- |
| [`emoji`](https://pypi.org/project/emoji/) | `emojize` | `':%s:'` | lowercase | underscores
| [`emojis`](https://pypi.org/project/emojis/) | `encode` | `':%s:'` | lowercase | underscores
| [`emoji_data_python`](https://pypi.org/project/emoji_data_python/) | `replace_colons` | `':%s:'` | lowercase | underscores
| `unicodedata` (standard) | `lookup` | `'%s'` | both | spaces

On first usage, each module is checked to see if it is present and is
then registered in the order listed above.  When a lookup on a name is
performed, each module which is present is queried in order, and if it
finds the given name, that is used as output.  If no modules find the
name, by default an error is generated, but this behavior can be
changed with the `--ignore-emoji-not-found` command line
option.

The order in which modules are queried is also customizable with the
`--emoji-modules` command line option; specify the
sequence of emoji module names to test separated by commas.  Use the
`--no-emoji-modules` command line option to only enable
the builtin `unicodedata` module lookup, deactivating the use of any
custom modules which may be installed.  And use
`--disable-emoji-modules` to disable all emoji module
lookup; only the `emojis` configuration variable will be consulted.

In summary, after install the third-party emoji module you prefer --
or rely on the default standard module, `unicodedata` -- and using the
emoji markup will "just work."

If you're aware of other third-party emoji modules you'd like to see
supported, [contact the author](#contact).

:::{tip}

It's expected that the typical EmPy user will have at most one
third-party module installed, so no effort has been put in place to
avoid conflicts or redundancies regarding emoji names between them
other than specifying the desired lookup order.  Choose a third-party
module that works for you, or just rely on the builtin `unicodedata`
lookup table.

If you're relying on a third-party module to be present, you might
want to have your EmPy code explicitly import that module so that if
it's missing, the dependency will be called out prominently.

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @:...: `` followed by
a newline becomes `` @:...:@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 4.0

Emoji markup was introduced in EmPy version 4.0.

:::


### Significator markup: `@%[!]... NL`, `@%%[!]...%% NL`

**Significator markup** is a way to perform distinctive assignments
within an EmPy system which are easily parsed externally; for
instance, for specifying metadata for an EmPy source document; these
assignments are called **significators**.  In its simplest form, it
defines in a variable in the globals with the evaluation of a Python
value.  The significator `@%KEY VALUE` is equivalent to the Python
assignment statement `__KEY__ = VALUE`.

The name of the assigned variable is preceded and ended with a double
underscore (`__`).  (This behavior can be changed with
configurations.)  Note that the value assigned can be any Python
expression, not just a string literal:

<a class="example" id="example-50"></a>

:::{admonition} Example 50: Significators, basics
_Source_: ⌨️
<div class="source">

``````
@%title "A Tale of Two Cities"
@%author 'Charles Dickens'
@%year 1859
@%version '.'.join([str(x) for x in __import__('sys').version_info[0:2]])
The book is _@(__title__)_ (@__year__) by @__author__.
This version of Python is @__version__.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The book is _A Tale of Two Cities_ (1859) by Charles Dickens.
This version of Python is 3.10.
``````

</div>

:::

Whitespace is allowed between the `@%` markup introducer and the key,
and any (non-newline) whitespace is allowed between the key and the
value.  The ending newline is always consumed.

A variant of significator markup can span multiple lines.  Instead of
using `@%` and a newline to delimit the significator, use `@%%` and
`%%` followed by a newline:

<a class="example" id="example-51"></a>

:::{admonition} Example 51: Significators, multiline
_Source_: ⌨️
<div class="source">

``````
@%%longName "This is a potentially very long \
name which can span multiple lines." %%
@%%longerName """This is a triple quoted string
which itself contains newlines.  Note the newlines
are preserved.""" %%
@%%longExpression [[1, 2, 3],
[4, 5, 6],
[7, 8, 9]] %%
Long name: @__longName__
Longer name: @__longerName__
Long expression: @__longExpression__
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Long name: This is a potentially very long name which can span multiple lines.
Longer name: This is a triple quoted string
which itself contains newlines.  Note the newlines
are preserved.
Long expression: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
``````

</div>

:::

::::{note}

When using multiline significators, the value must still be a valid
Python expression.  So the significator

``````
@%%bad 1 + 2 + 3 + 
4 + 5 + 6 %%
``````

is a syntax error due to the intervening newline.  To correct this,
use a backslash character (`\`) to escape the newline or enclose the
value expression in parentheses:

``````
@%%good1 1 + 2 + 3 + \
4 + 5 + 6 %%
@%%good2 (1 + 2 + 3 +
4 + 5 + 6) %%
``````

::::

Two more subvariants of significator markup exists, one for each of
these two variants.  Frequently significator values will just be
string literals and for uniformity users may wish to not deal with
full Python expressions.  For these purposes, significator values can
be **stringized**, or treated merely as strings with no Python
evaluation.  Simply insert a `!` after the `@%` or `@%%` markup
introducer and before the name of the key:

<a class="example" id="example-52"></a>

:::{admonition} Example 52: Significators, stringized
_Source_: ⌨️
<div class="source">

``````
@# These values are all implicit strings.
@%!single This is on a single line.
@%%!multi This is on
multiple lines. %%
Single line: @__single__
Multiple lines: @__multi__
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Single line: This is on a single line.
Multiple lines: This is on
multiple lines.
``````

</div>

:::

Finally, the values for both single and multiline significator markups
are optional.  If the markup is not stringized, the value will be
`None`; if stringized, it will be the empty string (`''`):

<a class="example" id="example-53"></a>

:::{admonition} Example 53: Significators, optional values
_Source_: ⌨️
<div class="source">

``````
@%none
@%!empty
This is a None: @repr(__none__).
This is an empty string: @repr(__empty__).
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This is a None: None.
This is an empty string: ''.
``````

</div>

:::

:::{hint}

Significators can appear anywhere in an EmPy document, but typically
are used at the beginning.

:::

:::{tip}

A compiled regular expression object is returned by the
`significatorRe` configuration method and can be used to
systematically find all the significators in a given text.

The values of a non-stringinized significators can be any Python
expression, so can include side effects from prior EmPy expansions.
It's best practice, however, to only have significator values depend
on the value of previous significators, so that trimmed down
processors can evaluate them without having to expand the entire
document.

:::

:::{versionadded} 1.2

Significator markup was introduced in EmPy version 1.2.  Stringified
and multiline variants were introduced in version 4.0.

:::


### Context markups

Contexts are objects which track the current progress of an EmPy
interpreter through its source document(s) for the purposes of error
reporting.  This is handled automatically by the EmPy system, but they
can be modified through the API or with context markup.

:::{versionadded} 3.0.2

Context markups were introduced in EmPy version 3.0.2.

:::

#### Context name markup: `@?... NL`

The **context name markup** can be used to change the current context
name with `@?... NL`; it uses as the new name what follows on the
same line and consumes everything up to and including the newline.
Whitespace surrounding the context name is ignored.

<a class="example" id="example-54"></a>

:::{admonition} Example 54: Context names
_Source_: ⌨️
<div class="source">

``````
@?Test
This context is now: @empy.getContext().
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This context is now: Test:2:22.
``````

</div>

:::


#### Context line markup: `@!... NL`

The **context line markup** can be used to change the current context
line with `@!... NL`; it uses as the new line what follows on the
same line and consumes everything up to and including the newline.
Whitespace surrounding the context name is ignored.  If the remaining
text is not parseable as an integer, it is a parse error.

<a class="example" id="example-55"></a>

:::{admonition} Example 55: Context lines
_Source_: ⌨️
<div class="source">

``````
@!1000
This context is now: @empy.getContext().
Note that the line is 1001 since it's the next line after the markup.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This context is now: <example 55 "Context lines">:1001:22.
Note that the line is 1001 since it's the next line after the markup.
``````

</div>

:::


### Extension markup: `@((...))`, `@[[...]]`, `@{{...}}`, `@<...>`, ...

Markup can be provided with customizable user-defined meanings with
**extension markup**.  Use of these markups calls a method on an
extension instance installed with the running interpreter, and
serializes its return result.  Once installed, an extension cannot be
uninstalled (but you can dynamically change its behavior).  Each of
these methods has the following signature:

> `method(contents: str, depth: int, locals: Optional[dict]) -> str`

It takes the following arguments:

{#extension-arguments-table}
| Argument | Type | Description |
| --- | --- | --- |
| `contents` | `str` | The contents inside the markup |
| `depth` | `int` | The number of opening and closing markup characters |
| `locals` | `Optional[dict]` | The locals dictionary or `None` |

The return value is a string to serialize to the output.  Of course,
these methods can perform any other desired side effects.
Encountering extension markup with no installed extension, or with an
extension that has no correspondingly defined method is an error.

The following extension markups are available out of the box:

{#extension-markups-table}
| Markup | Start | Extension method name | Minimum depth |
| --- | --- | --- | --- |
| `@((...))` | `((` | `parentheses` | 2 |
| `@[[...]]` | `[[` | `square_brackets` | 2 |
| `@{{...}}` | `{{` | `curly_braces` | 2 |
| `@<...>` | `<` | `angle_brackets` | 1 |

Extension markup is parsed so that the number of starting and ending
characters can be of any depth but the number of starting and ending
characters must match.  For angle brackets extension markup, any depth
can be used.  For the others, a depth of 2 or more is needed (since
only one character, _e.g._, `@(...)`, is a different markup).  For
additional user-specified markups, it is up to the user.

<a class="example" id="example-56"></a>

:::{admonition} Example 56: Extensions
_Source_: ⌨️
<div class="source">

``````
@{
import em

class Extension(em.Extension):

    def parentheses(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('parentheses', contents, depth)
    def square_brackets(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('square_brackets', contents, depth)
    def curly_braces(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('curly_braces', contents, depth)
    def angle_brackets(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('angle_brackets', contents, depth)

empy.installExtension(Extension())
}@
Parentheses: @((This is a test.))
Parentheses: @(((This is a test.)))
Parentheses: @((((This is a test.))))
Square brackets: @[[This is a test.]]
Square brackets: @[[[This is a test.]]]
Square brackets: @[[[[This is a test.]]]]
Curly braces: @{{This is a test.}}
Curly braces: @{{{This is a test.}}}
Curly braces: @{{{{This is a test.}}}}
Angle brackets: @<This is a test.>
Angle brackets: @<<This is a test.>>
Angle brackets: @<<<This is a test.>>>
Angle brackets: @<<<<This is a test.>>>>
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Parentheses: [parentheses] "This is a test." (depth 2)
Parentheses: [parentheses] "This is a test." (depth 3)
Parentheses: [parentheses] "This is a test." (depth 4)
Square brackets: [square_brackets] "This is a test." (depth 2)
Square brackets: [square_brackets] "This is a test." (depth 3)
Square brackets: [square_brackets] "This is a test." (depth 4)
Curly braces: [curly_braces] "This is a test." (depth 2)
Curly braces: [curly_braces] "This is a test." (depth 3)
Curly braces: [curly_braces] "This is a test." (depth 4)
Angle brackets: [angle_brackets] "This is a test." (depth 1)
Angle brackets: [angle_brackets] "This is a test." (depth 2)
Angle brackets: [angle_brackets] "This is a test." (depth 3)
Angle brackets: [angle_brackets] "This is a test." (depth 4)
``````

</div>

:::

Extensions can be used to define entirely new markup in this way.  The
first optional argument to the `Extension` constructor is a dict or
list or 2-tuples mapping first characters to method names.  If it is a
list, it will be added to the default mapping:

<a class="example" id="example-57"></a>

:::{admonition} Example 57: Extensions, additions
_Source_: ⌨️
<div class="source">

``````
@{
import em

class Extension(em.Extension):

    def __init__(self):
        super().__init__([('/', 'slashes')])

    def parentheses(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('parentheses', contents, depth)
    def square_brackets(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('square_brackets', contents, depth)
    def curly_braces(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('curly_braces', contents, depth)
    def angle_brackets(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('angle_brackets', contents, depth)

    def slashes(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('slashes', contents, depth)

empy.installExtension(Extension())
}@
Parentheses: @((This is a test.))
Square brackets: @[[This is a test.]]
Curly braces: @{{This is a test.}}
Angle brackets: @<This is a test.>
Slashes: @/This is a test./
Slashes: @//This is a test.//
Slashes: @///This is a test.///
Slashes: @////This is a test.////
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Parentheses: [parentheses] "This is a test." (depth 2)
Square brackets: [square_brackets] "This is a test." (depth 2)
Curly braces: [curly_braces] "This is a test." (depth 2)
Angle brackets: [angle_brackets] "This is a test." (depth 1)
Slashes: [slashes] "This is a test." (depth 1)
Slashes: [slashes] "This is a test." (depth 2)
Slashes: [slashes] "This is a test." (depth 3)
Slashes: [slashes] "This is a test." (depth 4)
``````

</div>

:::

:::{warning}

If the opening and closing characters are the same, the markup must
contain at least one other character in order to be recognized.  For
instance, if an extension is added for `@/.../`, the character
sequence `@//` is not an extension markup with a depth of one
containing the empty string; it is the unfinished start of an
extension markup of at least depth two.  `@/ /`, with a space in
between, on the other hand, would be valid.

:::

The first constructor argument can also be a dict, in which case the
mapping is completely replaced:

<a class="example" id="example-58"></a>

:::{admonition} Example 58: Extensions, replacement
_Source_: ⌨️
<div class="source">

``````
@{
import em

class Extension(em.Extension):

    def __init__(self):
        super().__init__({
            '((': 'parens',
            '[[': 'brackets',
            '{{': 'braces',
            '<': 'angles',
            '/': 'slashes',
        })

    def parens(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('parens', contents, depth)
    def brackets(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('brackets', contents, depth)
    def braces(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('braces', contents, depth)
    def angles(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('angles', contents, depth)
    def slashes(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('slashes', contents, depth)

empy.installExtension(Extension())
}@
Parentheses: @((This is a test.))
Square brackets: @[[This is a test.]]
Curly braces: @[[This is a test.]]
Angle brackets: @<This is a test.>
Slashes: @/This is a test./
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Parentheses: [parens] "This is a test." (depth 2)
Square brackets: [brackets] "This is a test." (depth 2)
Curly braces: [brackets] "This is a test." (depth 2)
Angle brackets: [angles] "This is a test." (depth 1)
Slashes: [slashes] "This is a test." (depth 1)
``````

</div>

:::

Note that the markup opening can either be one character or two.  This
is so that some extension markup can exist alongside of existing
markup (_e.g._, `@((...))` is extension markup whereas `@(...)` is
expression markup).

Finally, you can manually install token class factories corresponding
to your extension:

<a class="example" id="example-59"></a>

:::{admonition} Example 59: Extensions, manual
_Source_: ⌨️
<div class="source">

``````
@{
import em

class Extension(em.Extension):

    def __init__(self):
        super().__init__({})

    def the_colons(self, contents, depth, locals):
        return '[{}] "{}" (depth {})'.format('the_colons', contents, depth)

empy.installExtension(Extension())
factory = empy.config.getFactory()
factory.addToken(empy.config.createExtensionToken(';', 'the_colons', ':'))
}@
The colons: @;This is a test.:
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The colons: [the_colons] "This is a test." (depth 1)
``````

</div>

:::

:::{attention}

Note that when markup which has starting and ending delimiters appears
alone on a line, the trailing newline will be rendered in the output.
To avoid these extra newlines, use a trailing `@` to turn it
into whitespace markup which consumes that trailing newline, so _e.g._
`` @<...> `` followed by
a newline becomes `` @<...>@`` followed by a newline.  This is idiomatic for
suppressing unwanted newlines.  See [here](#idiom) for more details.

:::

:::{versionadded} 4.1

Extension markup was introduced in EmPy version 4.1.

:::


## Features

Various additional features are available in a running EmPy system.
The following subsections list the various features available in the
EmPy system and in which version they were introduced.

{#features-table}
| Feature | Description | Ver. |
| --- | --- | --- |
| [Pseudomodule](#pseudomodule-interpreter) | The pseudomodule/interpreter available in an EmPy system | 1.0 |
| [Commands](#commands) | Preprocessing and postprocessing commands | 1._x_ |
| [Plugins](#plugins) | Plugins for the interpreter | 4.1 |
| [Cores](#cores) | Alternatives for the underlying language interpreter | 4.1 |
| [Extensions](#extensions) | Specify user-customizable markup | 4.1 |
| [Callbacks](#callbacks) | Specify user-customizable markup for `@<...>` [deprecated] | 3.3 |
| [Finalizers](#finalizers) | Functions to be called on interpreter shutdown | 2.1 |
| [Diversions](#diversions) | Defer and playing back output | 1.0 |
| [Filters](#filters) | Filter output | 1.3 |
| [Modules](#modules) | Import native EmPy modules | 4.2 |
| [Hooks](#hooks) | Inspect and modify interpreter behavior | 2.0---4.0 |

### Pseudomodule/interpreter

The pseudomodule/interpreter can be accessed by a running EmPy system by
referencing its name (which defaults to
`empy`) in the globals:

<a class="example" id="example-2"></a>

:::{admonition} Example 2: Pseudomodule sample
_Source_: ⌨️
<div class="source">

``````
This version of EmPy is @empy.version.
The prefix in this interpreter is @empy.getPrefix() @
and the pseudomodule name is @empy.config.pseudomoduleName.
Do an explicit write: @empy.write("Hello, world!").
The context is currently @empy.getContext().
Adding a new global in a weird way: @
@empy.updateGlobals({'q': 789})@
Now q is @q!
You can do explicit expansions: @empy.expand("1 + 1 = @(1 + 1)").
q is @(empy.defined('q') ? 'defined' ! 'undefined').
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This version of EmPy is 4.2.1.
The prefix in this interpreter is @ and the pseudomodule name is empy.
Do an explicit write: Hello, world!.
The context is currently <example 2 "Pseudomodule sample">:5:26.
Adding a new global in a weird way: Now q is 789!
You can do explicit expansions: 1 + 1 = 2.
q is defined.
``````

</div>

:::

:::{important}

The pseudomodule and interpreter are one and the same object; the
terms _pseudomodule_ and _interpreter_ are used interchangeably.  The
interpreter exposes itself as the pseudomodule
`empy` in a running EmPy system; this
pseudomodule is never imported explicitly.

:::

:::{versionadded} 1.0

The pseudomodule was introduced in EmPy version 1.0.

:::


#### Interpreter attributes and methods

##### Interpreter attributes

The following attributes are set on the pseudomodule after it is
initialized.

`version: str`

: The version of EmPy.

`compat: list[str]`

: A list of strings indicating the "compatibility features" that were
  automatically enabled to support or call out earlier versions of
  Python.  Possible strings are:

  {#compatibility-features-table}
  | Feature | Description | Affected versions |
  | --- | --- | --- |
  | `BaseException` | `BaseException` class does not exist | before 2.5 |
  | `FileNotFoundError` | `FileNotFoundError` class does not exist | before 3.3 |
  | `callable` | `callable` builtin does not exist | from 3.0 up to 3.2 |
  | `chr/decode` | Substituted an implementation of `chr` for narrow Unicode builds using `decode` | before 3.0 |
  | `chr/uliteral` | Substituted an implementation of `chr` for narrow Unicode builds using `uliteral` | before 2.6 |
  | `narrow` | Python was built with narrow Unicode (strings natively stored as UCS-2) | -- |
  | `!modules` | EmPy module support disabled; `importlib` not available (or IronPython) | before 3.4 |
  | `!codecs.open` | `codecs.open` is deprecated (use `open` instead) | 3.14 and up |

  :::{note}

  Whether Python builds are narrow is a function of how the Python
  interpreter was built, and not directly correlated with the Python
  version.

  :::

`executable: str`

: The path to the EmPy interpreter that is being used by the system
  (analogous to `sys.executable`).

`argv: list[str]`

: The arguments (analogous to `sys.argv`) used to start the
  interpreter.  The first element is the EmPy document filename and
  the remaining elements are the arguments, if any.  If no EmPy
  document was specified, `<->` is used.

`config: Configuration`

: The [configuration](#configuration) instance that the interpreter is
  using.

`core: Core`

: The [core](#cores) used by this interpreter.

`extension: Optional[Extension]`

: The [extension](#extensions), if any, used by this interpreter.

`enabled: bool`

: A Boolean indicating whether or not output is currently enabled.
  This can be disabled and re-enabled with [switch
  markup](#switch-markup).

`ok: bool`

: A Boolean indicating whether or not the interpreter is still active.

`error: Optional[Error]`

: If an error occurs, the instance of it will be assigned to this
  attribute.  When using `invoke`, this will determine whether or not
  a failure exit code is returned.  No error is indicated by `None`.

##### Interpreter constructor

{#constructor}
`__init__(**kwargs)`

: The constructor.  It takes the following keyword arguments (listed
  in alphabetical order), all of which have reasonable defaults.

  {#constructor-arguments-table}
  | Argument | Type | Meaning | Default |
  | --- | --- | --- | --- |
  | `argv` | `Sequence[str]` | The system arguments to use | `['<->']` |
  | `callback` | `Callable` | A custom callback to register [deprecated] | `None` |
  | `config` | `Confguration` | The configuration instance to use | default |
  | `core` | `Core` | The interpreter core to use | `Core()` |
  | `dispatcher` | `bool \| Callable` | Dispatch errors or raise to caller? | `True` |
  | `executable` | `str` | The path to the EmPy executable | `".../em.py"` |
  | `extension` | `Extension` | The extension to install on this interpreter | `None` |
  | `filespec` | `tuple` | A 3-tuple of the input filename, output mode, and buffering | `None` |
  | `filters` | `Sequence[Filter]` | The list of filters to install | `[]` |
  | `finalizers` | `Sequence[Callable]` | The list of finalizers to install | `[]` |
  | `globals` | `dict` | The globals dictionary to use | `{}` |
  | `handler` | `Callable` | The error handler to use | default |
  | `hooks` | `Sequence[Hook]` | The list of hooks to install | `[]` |
  | `ident` | `str` | The identifier of the interpreter (used for debugging) | `None` |
  | `immediately` | `bool` | Declare the interpreter ready immediately after initialization? | `True` |
  | `input` | `file` | The input file to use for interactivity | `sys.stdin` |
  | `origin` | `bool` | Is this the top-level interpreter? | `False` |
  | `output` | `file` | The output file to use | `sys.stdout` |
  | `root` | `str \| tuple \| Context` | The root interpreter context filename | `'<root>'` |

  The ordering of the arguments does not matter.  Missing arguments
  have reasonable defaults and unrecognized arguments are ignored.

  :::{important}

  The order of the `Interpreter` constructor arguments has changed
  over time and is subject to change in the future, so you must use
  keyword arguments to prevent any ambiguity, _e.g._:

  ```python
  import em

  myConfig = em.Configuration(...)
  myGlobals = {...}
  myOutput = open(...)
  interp = em.Interpreter(
      config=myConfig,
      globals=myGlobals,
      output=myOutput,
      ...)
  ```

  :::

  {#constructor-arguments}
  The allowed arguments are:

  `argv: Optional[Sequence[str]]`

  : The list of EmPy arguments.  The first element is always the EmPy
    executable, with the remaining elements the actual arguments.  If
    not specified, a reasonable default is used with no arguments.

  `callback: Optional[Callable]`

  : The custom [callback](#callbacks) to register.  Defaults to no
  callback.

    :::{warning}

    Custom callbacks are deprecated in favor of
    [extensions](#extensions) but registering them is supported for
    backward compatibility providing that extensions are not also
    being used.

    :::

  `config: Optional[Configuration]`

  : The [configuration](#configurations) to use for this interpreter.
    If not specified, a default configuration will be created and
    used.

    :::{note}

    The current configuration of an interpreter can be modified while
    the interpreter is running and the changes will be effective as
    they occur.  Configurations can also be shared between multiple
    interpreters if desired.

    :::

  `core: Core`

  : The [core](#cores) to use with this interpreter.  See [Cores](#cores)
    for more information.

  `dispatcher: bool | Callable`

  : The [dispatcher](#error-dispatchers) to use when an error is
    encountered.  Dispatchers determine whether the error handler will
    be called (`True`), whether the error will be reraised to the
    caller (`False`), or something else (a custom callable). See
    [Error dispatchers](#error-dispatchers) for more information.

  `executable: str`

  : A string representing the path to the EmPy executable.

  `extension: Optional[Extension]`

  : The [extension](#extensions) to install, if any.

  `filespec: Optional[tuple[str, str, int | str]]`

  : An optional 3-tuple of the filename, the file open mode, and the
    buffering mode of the EmPy script to be loaded.  When using the
    command line arguments, this will be handled automatically.

  `filters: Sequence[Filter]`

  : A list of [filters](#filters) to install at startup.  Defaults to
  none.

  `finalizers: Sequence[Callable]`

  : A list of [finalizers](#finalizers) to install at startup.
  Defaults to none.

  `globals: Optional[dict]`

  : The globals dictionary to use for this interpreter.  If not
    specified, an empty dictionary will be created and used.

  `handler: Optional[Callable]`

  : The [error handler](#error-handlers) to set.  If not specified,
    use the default handler.

  `hooks: Sequence[Hook]`

  : A list of [hooks](#hooks) to install at startup.  Defaults to none.

  `ident: Optional[str]`

  : The name of the interpreter, printed when calling `repr` on the
    interpreter/pseudomodule object.  Used only for debugging;
    defaults to `None`.

  `immediately: bool`

  : A Boolean which indicates whether or not the [`ready`](#ready)
    method will be called before the constructor exits.  This is only
    relevant for hooks which implement the `atReady` method.  Defaults
    to true.

  `input: Optional[file]`

  : The input file to use for interactive mode and pausing at end.
    Defaults to `sys.stdin`.

  `origin: bool`

  : Is this the top-level interpreter?  This is used to determine
    whether to report an error if the proxy is not properly cleaned up
    on its shutdown, suggesting that the proxy reference count got out
    of sync due to a subinterpreter not being properly shutdown.
    Defaults to `False` for a new interpreter, but calling the global
    `invoke` function (as the main em.py executable does) sets it
    to `True` for the main interpreter.

  `output: Optional[file]`

  : The output file to use.  Defaults to `sys.stdout`.

  `root: str | (tuple[str] | tuple[str, int] | tuple[str, int, int]) | Context`

  : The root [context](#context-formatting) to use, which appears at
    the bottom of every Python error traceback (`-r/--raw-errors`).
    This can be a string, representing the filename; a tuple with
    between 1 and 3 parameters, with the full 3-tuple consisting of
    the name, the line number, and the column number; or an instance
    of the `Context` class, which will be cloned.

  :::{warning}

  The five `...Func` arguments (`evalFunc`, `execFunc`, `definerFunc`,
  `matcherFunc` and `serializerFunc`) collectively define the behavior
  of the underlying interpreter (which defaults to Python).
  Alternates can be specified if desired; however, specifying these
  arguments in the constructor in this way is now deprecated.  (These
  arguments to the constructor are still supported for backward
  compatibility, however.)  Instead, use [cores](#cores).  Using cores
  is as simple as passing the functions (which, if not specified, will
  default to the standard Python interpreter) to a `Core` constructor
  and using that:

  ```python
  import em

  core = em.Core(
      evaluate=evalFunc,
      execute=execFunc,
      serialize=serializerFunc,
      define=definerFunc,
      match=matcherFunc,
  )
  interp = em.Interpreter(core=core, ...)
  ```

  :::

  :::{seealso}

  The list of `Interpreter` constructor arguments is available in the
  `constructor` help topic and is summarized
  [here](HELP.md#interpreter-constructor-arguments-summary).

  :::

##### Interpreter methods

These methods involve the interpreter directly.

:::{note}

Most interpreter methods return `None` so they can be called from
EmPy expression markup.

:::

{#context-management}
`__enter__()`/`__exit__(*exc)`

: The interpreter presents a context manager interface and so can be
  used with the `with` Python control structure; this will
  automatically shut down the interpreter after the control structure
  ends, _e.g._:
  
  ```python
  import em

  with em.Interpreter(...) as interp:
      ... manipulate interp here ...
  # interp is now shutdown
  ```

{#reset}
`reset([clearStacks: bool])`

: Reset the interpreter to a pristine state.  If `clearStacks` is true,
  reset the stacks as well.

{#ready}
`ready()`

: Declare the interpreter ready for processing.  This calls the
  `atReady` hook.  By default this is called before the
  [constructor](#constructor) exits, but the user can do this
  explicitly by passing `False` to the `immediately` constructor
  argument and calling it when they wish to declare the interpreter
  ready.

{#shutdown}
`shutdown()`

: Shutdown the interpreter.  No further expansion must be done.  This
  method is idempotent.

    :::{important}
  
  When you create an interpreter, you must call its `shutdown` method
  when you are done.  This is required to remove the proxy on
  `sys.stdout` that EmPy requires for proper operation and restore your
  Python environment to the state it was before creating the
  interpreter.  This can be accomplished by creating the interpreter in
  a `with` statement -- interpreters are also context managers -- or by
  creating it and shutting it down in a `try`/`finally` statement.
  
  This is not needed when calling the `expand` global function; it
  creates and shuts down an ephemeral interpreter automatically.
  
  :::
    
##### Interpreter file-like methods

These methods mimic a file so the interpreter can be treated as a
file-like object (_e.g._, `print("Lorem ipsum", file=empy)`.

`write(data: str)`

: Write the string data to the output stream.

`writelines(lines: list[str])`

: Write the sequence of strings to the output stream.

`flush()`

: Flush the output stream.

`close()`

: Close the output stream.  Note this will never close the fundamental
  output stream; it will only flush it, and it will only close other
  streams when they are not at the bottom of the stream stack.

`serialize(thing: object)`

: Write a string version of the object to the output stream.  This
  will reference `--none-symbol` (_configuration variable:_ `noneSymbol`) if the object is
  `None`.

##### Interpreter stream methods

`top() -> Stream`

: Get the top-level stream.

`push()`

: Push this interpreter onto the proxy's stream stack.

`pop()`

: Pop this interpreter off the proxy's stream stack.

`clear()`

: Clear the interpreter's stacks.

##### Interpreter high-level methods

`go(inputFilename, inputMode, [preprocessing, [postprocessing]])`

: Process the main document with the given mode.  Optionally process
  the specified commands before processing the main document and
  after, respectively.

`interact()`

: Go into interactive mode.

`file(file, [locals, [dispatcher]])`

: Process the given file, expanding its contents.  This will defer to
  one of the below methods based on the buffering setting.

`fileLines(file, [locals, [dispatcher]])`

: Process the given file line by line.

`fileChunks(file, [bufferSize, [locals, [dispatcher]]])`

: Process the given file in chunks with the optional buffer size; if
  not specified, the configuration default will be used.

`fileFull(file, [locals, [dispatcher]])`

: Process the given file as a single chunk that's read in all at once.

`string(string, [locals, [dispatcher]])`

: Process the given string, expanding its contents.

`import_(filename, module, [locals, [dispatcher]])`

: Import an EmPy module contained in `filename` to the module object
  `module`.

`process(command: Command)`

: Process a command.

`processAll(commands: Sequence[Command])`

: Process a sequence of commands.

##### Interpreter context methods

These methods manipulate the interpreter's context stack.

`identify() -> tuple[char, int, int, int]`

: Get a 4-tuple of the current context, consisting of the filename,
  the line number, the column number, and the number of characters
  (Unicode code points) processed.

`getContext() -> Context`

: Get the current context object.

`newContext([name: str, [line: int, [column: int, [chars: int]]]]) -> Context`

: Create a new context and return it.

`pushContext(context: str | tuple | Context)`

: Push the given context on top of the context stack.

`popContext()`

: Pop the top context off the context stack; do not return it.

`setContext(context: str | tuple | Context)`

: Replace the context on the top of the context stack with the given
  context.

`setContextName(name: str)`

: Set the top context's name to the given value.

`setContextLine(line: int)`

: Set the top context's line to the given value.

`setContextColumn(column: int)`

: Set the top context's column to the given value.

`setContextData([name: str, [line: int, [column: int, [chars: int]]]])`

: Set the top context's name, line, and/or column to the given value(s).

`restoreContext(oldContext: str | tuple | Context)`

: Restore the top context on the stack to the given context.

##### Interpreter finalizer methods

These methods manipulate the interpreter's finalizers.

`clearFinalizers()`

: Clear all finalizers from this interpreter.

`appendFinalizer(finalizer: Callable)`/`atExit(finalizer: Callable)`

: Append the given finalizer to the finalizers list for this
  interpreter.  `atExit` is an alias for backward compatibility
  [deprecated].

`prependFinalizer(finalizer: Callable)`

: Prepend the given finalizer to the finalizers list for this
  interpreter.

`setFinalizers(finalizers: list[Callable])`

: Replace the current list of finalizers wih the given sequence.

##### Interpreter globals methods

These methods manipulate the interpreter's globals.

`getGlobals() -> dict`

: Get the current globals dictionary.

`setGlobals(globals: dict)`

: Set the current globals dictionary,.

`updateGlobals(moreGlobals: dict)`

: Update the current globals dictionary, adding this dictionary's
  entries to it.

`clearGlobals()`

: Clear the current globals dictionary completely.

`saveGlobals([deep: bool])`

: Save a copy of the globals off to on the history stack.  If deep is
  true, do a deep copy (defaults to false).

`restoreGlobals([destructive: bool])`

: Restore the globals dictionary on the top of the globals history
  stack.  If destructive is true (default), pop it off when done.

`flattenGlobals([skipKeys: Sequence[str]])`

: Flatten the interpreter namespace into the globals.  If `skipKeys`
  is specified, skip over those keys; otherwise, use the defaults from
  the configuration.

##### Interpreter expansion methods

These methods are involved with markup expansion.

`include(fileOrFilename, [locals, [name]])`

: Include the given EmPy (not Python) document (or filename, which is
  opened) and process it with the given optional locals dictionary and
  context name.

`expand(data, [locals, [name], [dispatcher]]) -> str`

: Create a new context and stream to evaluate the EmPy data, with the
  given optional locals and context name and return the result.  If
  the expansion raises an exception, by default it will be raised up
  to the caller; set `dispatcher` to true to have the interpreter
  handle it with its formal error handler mechanism.  Set `dispatcher`
  to another callable to do custom dispatching.  See [Error
  dispatchers](#error-dispatchers) for more information.

`defined(name, [locals]) -> bool`

: Return a Boolean indicating whether the given name is present in the
  interpreter globals (or the optional locals, if provided).
  
`lookup(name, [locals]) -> object`

: Lookup the value of a name in the globals (and optionally the
  locals, if provided) and return the value.

`evaluate(expression, [locals, [write]]) -> object`

: Evaluate the given Python expression in the interpreter, with the
  given optional locals dictionary.  If write is true, write it to the
  output stream, otherwise return it (defaults to false).

`execute(statements, [locals])`

: Execute the given Python statements in the interpreter, with the
  given optional locals dictionary.

`single(source, [locals]) -> Optional[object]`

: Execute the given Python expression or statement, with the given
  optional locals dictionary.  This compiles the code with the
  `single` Python compilation mode which supports either.  Return the
  result or `None`.  This method is not used internally by the EmPy
  system and is not used by interpreter [cores](#cores) but is
  available for embedding.

`atomic(name, value, [locals])`

: Do an atomic assignment of the given name and value in the
  interpreter globals.  If the optional locals dictionary is provided,
  set it in the locals instead.

`assign(name, value, [locals])`

: Do a potentially complex assignment of the given name "lvalue" and
  "rvalue."  Unlike `atomic`, `assign` can support tuple assignment.

`significate(key, [value, [locals]])`

: Declare a significator with the given key and optional value (if not
  specified, defaults to `None`).  If the optional locals dictionary
  is provided, set it in the locals instead.

`quote(string) -> str`

: Given an EmPy string, return it quoted.

`escape(string) -> str`

: Given an EmPy string, escape non-ASCII characters in it and return.

`getPrefix() -> str`

: Get this interpreter's prefix.

`setPrefix(char)`

: Set this interpreter's prefix.

##### Interpreter diversion methods

These methods manipulate the interpreter's diversions.

`stopDiverting()`

: Stop any current diversion.

`createDiversion(name)`

: Create a new diversion with the given name but do not start diverting
  to it.

`retrieveDiversion(name, [default]) -> Diversion`

: Get the diversion with the given name.  If `default` is provided,
  return that if the diversion does not exist; otherwise, raise an
  error.

`startDiversion(name)`

: Start diverting to a diversion with the given name, creating if it
  necessary.

`playDiversion(name, [drop])`

: Play the diversion with the given name, optionally dropping it
  (default is true).

`replayDiversion(name, [drop])`

: Play the diversion with the given name, optionally dropping it
  (default is false).

`dropDiversion(name)`

: Drop the diversion with the given name without playing it.

`playAllDiversions()`

: Play all diversions in sorted order by name, dropping them.

`replayAllDiversions()`

: Replay all diversions in sorted order by name, leaving them in
  place.

`dropAllDiversions()`

: Drop all diversions without playing them.

`getCurrentDiversionName() -> Optional[str]`

: Get the name of the current diversion or `None` if there is no
  current diversion.

`getAllDiversionNames() -> list[str]`

: Get a list of the names of all diversions in sorted order.

`isExistingDiversionName(name) -> bool`

: Is the given name the name of an existing diversion?

##### Interpreter filter methods

These methods manipulate the interpreter's filters.

`resetFilter()`

: Reset the filtering system so there are no filters.

`getFilter() -> Filter`

: Get the top-most filter.

`getLastFilter() -> Filter`

: Get the bottom-most filter.

`setFilter(*filters)`

: Set the top-most filter(s) to the given filter chain, replacing any
  current chain.  More than one filter can be specified as separate
  arguments.

`prependFilter(filter)`

: Prepend the given filter to the current filter chain.

`appendFilter(filter)`

: Append the given filter to the current filter chain.

`setFilterChain(filters)`

: Set the filter chain to the given list of filters, replacing any
  current chain.

##### Interpreter core methods

These methods involve cores, an optional EmPy feature for overriding
the core behavior of an EmPy interpreter.  See [cores](#cores) for
details.

`hasCore() -> bool`

: Does this interpreter currently have a core installed?  (A default
  core is installed by the time initialization ends, regardless of
  whether or not a custom core has been specified.)

`getCore() -> Core`

: Get the core currently inserted into this interpreter.

`insertCore(core: Optional[Core])`

: Insert a core on this interpreter.  If `core` is `None`, a default
  core will be created and used.  This will detach any previous core.

`ejectCore()`

: Eject this interpreter's core.  Since cores may have a reference
  back to the interpreter, this will cut any cyclical link that may be
  present.  This method is automatically called by the interpreter's
  `shutdown` method.  Note that once this method is called, no further
  expansion can be performed by this interpreter until a new core is
  installed.  This method is idempotent.

`resetCore()`

: Reset the core for this interpreter to the default (Python).

##### Interpreter extension methods

`hasExtension() -> bool`

: Does this interpreter already have an extension installed?

`getExtension() -> Optional[Extension]`

: Get the extension installed in this interpreter, or `None` if there
  is no such extension.

`installExtension(Extension)`

: Install an extension in this interpreter.  This can only be done
  once per interpreter.

`callExtension(name: str, contents: str, depth: int)`

: Calls the extension as if extension markup had been encountered and
  serializes the result.

##### Interpreter hook methods

These methods manipulate the interpreter's hooks.

`invokeHook(_name: str, **kwargs)`

: Invoke the hooks associated with the given name and keyword
  arguments dictionary.  This is the primary method called when hook
  events are invoked.

`areHooksEnabled() -> bool`

: Are hooks currently enabled?

`enableHooks()`

: Enable hooks.

`disableHooks()`

: Disable hooks.  Any existing hooks will not be called until
  `enableHooks` is called.

`getHooks() -> list[Hook]`

: Get the current list of hooks.

`prependHook(hook: Hook)`

: Prepend the given hook to the list of hooks.

`appendHook(hook: Hook)`

: Append the given hook to the list of hooks.

`removeHook(hook: Hook)`

: Remove the given hook from the list of hooks.

`clearHooks()`

: Clear the list of hooks.

##### Interpreter callback methods

These methods manipulate the interpreter's custom callback.  A
callback is a callable object which takes one argument:  the content
to process.

`hasCallback() -> bool`

: Does this interpreter have a custom callback registered?

`getCallback() -> Optional[Callable]`

: Return the interpreter's registered custom callback or `None` if
  none is registered.

`registerCallback(callback)`

: Register the given callback with the interpreter, replacing any
  existing callback.

`deregisterCallback()`

: Remove the current interpreter's registered callback, if any.

`invokeCallback(contents)`

: Manually invoke the interpreter's custom callback as if the custom
  markup `@<...>` were expanded.

:::{warning}

Custom callbacks are deprecated in favor of [extensions](#extensions)
but registering them and querying them is supported for backward
compatibility, providing that extensions are not also being used.

:::

##### Interpreter error handler methods

These methods manipulate the interpreter's error handler.  A handler
is a callable object which takes three arguments: the type of the
error, the error instance itself, and a traceback object.

`defaultHandler(type, error, traceback)`

: The default EmPy error handler.  This can be called manually by
  custom error handlers if desired.

`getHandler() -> Callable`

: Get the current error handler, or `None` for the default.

`setHandler(handler, [exitOnError])`

: Set the error handler.  If `exitOnError` is not `None` (defaults to
  false), also set the interpreter's configuration's `exitOnError`
  configuration variable.  This default is `False` (so that custom
  error handlers do not automatically exit, which is usually the
  intent).

`resetHandler([exitOnError])`

: Reset the error handler to the default.  `exitOnError` is as above.

`invokeHandler(*exc)`

: Manually invoke the error handler.  The arguments should be the
  3-tuple of the return value of `sys.exc_info` as a single argument
  or as three positional arguments, _e.g._:

  ```python
  interp.invokeHandler(sys.exc_info()) # or: *sys.exc_info()
  ```

`getExitCode() -> int`

: Get the exit code that will be returned by the process given the
  current state of the `error` attribute.

##### Interpreter emoji methods

`initializeEmojiModules([moduleNames: Sequence[str]])`

: Initialize the allowed emoji modules to use by name.  If the names
  list is not specified, use the defaults.

`getEmojiModule(moduleName: str) -> Module`

: Get the initialized module abstraction corresponding to the given
  module name.

`getEmojiModuleNames() -> list[str]`

: Return the list of available emoji modules by name in their proper
  order.

`substituteEmoji(text: str) -> str`

: Use the emoji facilities to lookup the given emoji name and return
  the result as if the emoji markup `@:...:` were expanded.

:::{seealso}

The list of pseudomodule/interpreter attributes in methods is
available in the `pseudo` help topic and is summarized
[here](HELP.md#pseudomodule-attributes-and-methods-summary).

:::


#### Commands

**Commands** are self-contained objects which encapsulate some
interpreter processing.  They are primarily used by the command line
options system, but can be used by users.

Commands are created from the `Command` class and its subclasses, and
are processed by an interpreter with its `process` method.  Commands
are created with a single argument which represents the action of that
particular command.  For instance, this EmPy code will use commands to
include an EmPy document:

<a class="example" id="example-60"></a>

:::{admonition} Example 60: Commands
_Source_: ⌨️
<div class="source">

``````
@{
import em

# Create a temporary file for this example.
with open('/tmp/include.em', 'w') as file:
    file.write("Hello, world!  1 + 1 = @(1 + 1).\n")
}@
Including a file with a command:
@empy.process(em.DocumentCommand('/tmp/include.em'))@
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Including a file with a command:
Hello, world!  1 + 1 = 2.
``````

</div>

:::

A sequence of commands can be processed with the `processAll` method.

The following `Command` subclasses are available, alongside their
corresponding command line options.  Preprocessing commands are
processed before the main EmPy document is expanded; postprocessing
commands after.

{#command-classes-table}
| Subclass | Behavior | Preprocessing option | Postprocessing option |
| --- | --- | --- | --- |
| `ImportCommand` | Import a Python module | `-I/--import=MODULES` |
| `DefineCommand` | Define a Python variable | `-D/--define=DEFN` |
| `StringCommand` | Define a Python string variable | `-S/--string=STR` |
| `DocumentCommand` | Expand an EmPy document by filename | `-P/--preprocess=FILENAME` | `-Q/--postprocess=FILENAME` |
| `ExecuteCommand` | Execute a Python statement | `-E/--execute=STATEMENT` | `-K/--postexecute=STATEMENT` |
| `FileCommand` | Execute a Python file by filename | `-F/--file=FILENAME` | `-G/--postfile=FILENAME` |
| `ExpandCommand` | Expand some EmPy markup | `-X/--expand=MARKUP` | `-Y/--postexpand=MARKUP` |

Users can define their own subclasses of `Command`.

:::{seealso}

For more details on the various commands, check the corresponding
[](#command-line-options).

:::

:::{versionchanged} 4.0

Commands were codified and introduced in EmPy version 4.0, though the
equivalent command line options that they correspond to were available
going back to 1._x_.

:::


#### Plugins

**Plugins** are objects which can be installed in an interpreter and
contain a reference back to it via an `interp` attribute.  They are
registered with the interpreter by calling an interpreter method such
as `insertCore`, `installExtension`, etc. and are either callable
objects (callbacks, finalizers) or are objects defined with attributes
approrpiate to their purpose (cores, extensions).  User-defined
plugins can derive from `em.Plugin` or the relevant subclass (_e.g._,
`em.Core` or `em.Extension`).

The types of plugins available follow below.

{#plugins-table}
| Type | Purpose | Subclass | Install, uninstall method | Ver. |
| --- | --- | --- | --- | --- |
| [](#cores) | Modify core interpreter behavior | `em.Core` | `insertCore`, `ejectCore` | 4.1 |
| [](#extensions) | Add additional customized markup | `em.Extension` | `installExtension`, `uninstallExtension` | 4.1 |
| [](#callbacks) | Add custom markup callback (deprecated) | `emlib.Callback` | `registerCallback` | 3.3 |
| [](#finalizers) | Functions to call before shutdown | `emlib.Finalizer` | `append`/`prependFinalizer`, `clearFinalizers` | 2.1 |
| [](#hooks) | Objects to adjust interepter behavior by event | `emlib.Hook` | `addHook`/`appendHook`/`prependHook`, `removeHook` | 2.0 |
| [Handlers](#error-handlers) | Error handlers | `emlib.Handler` | `setHandler`, `resetHandler` | 4.0 |

:::{versionadded} 4.1

Plugins were codified and introduced in EmPy version 4.1.

:::


##### Cores

By default, of course, EmPy expands expressions and statements through
the underlying Python interpreter.  But even this behavior is
configurable with interpreter **cores**.  Cores can be used
to completely change the underlying language that EmPy expands to
(provided, naturally, that it can be implemented with Python).

Cores (represented with the base class `Core` in the module `em`)
support any language/system that can be encapsulated with a set of
globals, optional locals, and with these actions (methods on the core
object): evaluating an expression, executing a statement, serializing
an object, defining an EmPy function (`@[def]`), and defining the
behavior of the `@[match]` markup.

As with the Python languge, expressions which are evaluated must
return a value (or `None`); statements which are executed do not
return a value.  Beyond this definition, the meaning and purpose of
these operations in a custom core are completely configurable.

Cores can be created by either providing a set of callables to the
`Core` constructor or by deriving a subclass from `Core` and providing
overriding methods.  The default implementation (`Core()`) provides
the implementation for Python.  Not specifying a core defaults to this
implementation.

{#core-constructor}
The default `Core` class has the following constructor:

`__init__(**kwargs)`

: Create a core with optional keyword arguments.  The arguments can be
  callables representing the different possible operations on a core,
  or an interpreter to attach to immediately after initialization:

  {#core-constructor-arguments-table}
  | Argument | Description | Default |
  | --- | --- | --- |
  | `evaluate` | Evaluate an expression; return the result | `eval` |
  | `execute` | Execute a statement; no return result | `exec` |
  | `serialize` | Serialize an object; return the result | `str` |
  | `define` | Functionality of `@[def ...]` markup; no return result | (Python-specific) |
  | `match` | Functionality of `@[match ...]` markup; no return result | (Python-specific) |

  An optional `interp` argument is also allowed which will
  automatically attach this core to the given interpreter after
  intiialization.

{#core-methods}
The expected signatures of these methods are as follows:

`evaluate(code: str, globals: dict, [locals: dict]) -> object`

: Evaluate the string expression given the current globals (and
  optional locals) and return the result.

`execute(code: str, globals: dict, [locals: dict])`

:  Execute the string statements given the current globals (and
  optional locals).  The return value is ignored.

`serialize(thing: object) -> str`

: Take a custom object and render it as a string for output.

`define(signature: str, definition: str, [globals: dict, [locals: dict]])`

: Implement the behavior of the `def` control markup `@[def
  SIGNATURE]DEFINITION@[end def]`, given the globals dictionary
  (defaults to the interpreter's globals), and an optional locals
  dictionary.  The meaning of the signature and the definition are
  arbitrary.  The return value is ignored.

`match(expression: str, cases: list[Core.Case], [locals: dict])`

: Implement the behavior of the `match` control markup `@[match
  EXPRESSION]...@[end match]`.  The first argument is the string
  expression to test.  `cases` is a list of `Core.Case` instances each
  representing a `case` statements.  Cases have the following
  attributes:
  
  {#match-arguments-table}
  | Attribute | Type | Purpose |
  | --- | --- | --- |
  | `expression` | `str` | A string representing the case test expression |
  | `tokens` | `list[Token]` | The list of tokens to run if there is a match |

  The `expression` instance is the string representing the case test,
  and the second element is a list of `Core.Case` instances
  representing the case expression to test and the markup to expand
  when the case matches.  Also passed are is an optional locals
  dictionary.  The return result is ignored.

Cores are installed in an interpreter by either providing one as the
`core` [constructor argument](#constructor-arguments) or by calling
the `insertCore` interpreter method.  An interpreter's core can be
reset to the default (Python implementation core) with the `resetCore`
method, or can be manually removed by calling the `ejectCore` method
(though this will leave the interpeter non-functional until a new core
is installed).  An interpreter's core can also be modified dynamically
while an interpreter is running.

Cores must be attached to the interpreter in order to make sure that
cores have a reference to the interpreter (needed for the default
`define` method).  This is done automatically when calling
`insertCore`.

As mentioned earlier, cores can be created in one of two ways.  First,
a subclass of `Core` in the `em` module can be instantiated and
provided to the interpreter:

```python
import em

class MyCore(em.Core):

    def evaluate(self, code, globals, locals=None): ...
    def execute(self, code, globals, locals=None): ...
    def serialize(self, thing): ...
    def define(self, signature, definition, globals, locals=None): ...
    def match(self, expression, casePairs, globals, locals=None): ...

core = MyCore(...)
interp = em.Interpreter(core=core)
```

Alternatively, the core methods can be implemented as standalone
callables and passed to the `Core` constructor:

```python
import em

def myEvaluate(code, globals, locals=None): ...
def myExecute(code, globals, locals=None): ...
def mySerialize(thing): ...
def myDefine(signature, definition, globals, locals=None): ...
def myMatch(expression, casePairs, globals, locals=None): ...

core = em.Core(
    evaluate=myEvaluate,
    execute=myExecute,
    serialize=mySerialize,
    define=myDefine,
    match=myMatch,
)
interp = em.Interpreter(core=core)
```

For methods which are not defined in a subclass or specified as a
callable in the constructor, the default (Python implementation core)
will be used.  The source code for the default definitions can be used
as a guide.

:::{tip}

The `define` method and `match` method correspond to the `@[def ...]`
and `@[match ...]` markup, respectively.  If these markups will not
be used by the user, these methods can be left unimplemented as they
will never be called.  It's good form, however, to have them raise
`NotImplementedError` in that case just to make the intent clear.

:::

:::{versionadded} 4.1

Interpreter cores were introduced in EmPy version 4.1.

:::


##### Extensions

Starting with EmPy 4.1, users can now specify their own customizable
markup with **extensions**.  They define methods which are called when
the respective custom markup is expanded and are installed via the
`installExtension` method on the interpreter.

Extensions are documented in the [Extension markup](#extension-markup)
section.

:::{versionadded} 4.1

Extensions were introduced in EmPy version 4.1, replacing custom
markup which was introduced in EmPy version 3.3.

:::


##### Callbacks

:::{warning}

Custom callbacks are deprecated in favor of [extensions](#extensions)
but registering them and querying them is supported for backward
compatibility providing that extensions are not also being used.

:::

Before the introduction of [extensions](#extensions), only one markup
was available for customization: the **custom markup**, `@<...>`.
This meaning of this markup was provided with the use of a Python
callable object referred to as a **custom callback**, or just
"callback," which can be set or queried using pseudomodule functions.
At most one custom callback can be registered, and once registered, it
cannot be deregistered or replaced.

When the custom markup `@<...>` is encountered, the contents inside
the markup are passed to the current custom callback.  Its return
value, if not `None`, is then written to the output stream.  The
custom callback may also perform operations with side effects
resulting in output, as well.  If there are multiple opening angle
brackets, an equal number of closing angle brackets will be required
to match.  This allows the embedding of `<` and `>` in the contents
passed to the callback.

The custom callback is a callable object which, when invoked, is
passed a single argument: a string representing the contents of what
was found inside the custom markup `@<...>`.  Only one custom
callback can be registered at a time.

To register a callback, call `empy.registerCallback`.  To see if there
is a callback registered, use `empy.hasCallback`.  To retrieve the
callback registered with the interpreter, use `empy.getCallback`; if
no callback is registered, `None` will be returned.  Finally, to
invoke the callback explicitly just as if the custom markup had been
encountered, call `empy.invokeCallback`.  For instance, `@<This
text>` would be equivalent to the call `@empy.invokeCallback("This
text")`.

Invoking a callback (either explicitly with `empy.invokeCallback` or
by processing a `@<...>` custom markup) when no callback has been
registered is an error.

<a class="example" id="example-61"></a>

:::{admonition} Example 61: Custom markup
_Source_: ⌨️
<div class="source">

``````
@{
def callback(contents):
    return contents.upper()
empy.registerCallback(callback)
}@
This will be in uppercase: @<This is a test>.
This will also contain angle brackets: @<<This is <also a> test>>.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This will be in uppercase: THIS IS A TEST.
This will also contain angle brackets: THIS IS <ALSO A> TEST.
``````

</div>

:::

:::{deprecated} 4.1

Custom markup was introduced in EmPy version 3.3 and transformed into
extension markup in EmPy version 4.1.

:::


##### Finalizers

Every interpreter must shutdown (by the `shutdown` method being called
either implicitly or explicitly).  As part of this process, the
interpreter runs any **finalizers** that may have been registered with
the interpreter.

Finalizers are callable objects which take zero arguments.  The
interpreter contains a list of finalizers which are called
sequentially in order.  If a finalizer raises an exception, any
remaining finalizers will not be called that exception will be
propagated up.

To add a finalizer, call either the `appendFinalizer` or
`preprendFinalizer` interpreter methods.  To clear all finalizers,
call the `clearFinalizers` method.

:::{note}

`atExit` is an alias for `appendFinalizer` for backward compatibility
and is now deprecated.

:::

<a class="example" id="example-62"></a>

:::{admonition} Example 62: Finalizers
_Source_: ⌨️
<div class="source">

``````
@# These are printed in reverse order.
@empy.appendFinalizer(lambda: empy.write("This is the last line.\n"))@
@empy.appendFinalizer(lambda empy=empy: empy.write("This is the penultimate line.\n"))@
@{
import em

class Finalizer:

    def __init__(self, interp):
        self.interp = interp

    def __call__(self):
        self.interp.write("This is the third to last line.\n")

finalizer = Finalizer(empy)
empy.appendFinalizer(finalizer)
}@
This is the first line.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This is the first line.
This is the third to last line.
This is the penultimate line.
This is the last line.
``````

</div>

:::

:::{tip}

The finalizers are callables which take no arguments when they are
called, so if they need a reference to the interpreter to do their
work (as the example above), they need to arrange it with a closure,
an implicit argument, or by implementing it as an instance which
contains a reference to the interpreter.  All three of these
approaches are illustrated in the example above.

:::

:::{warning}

When using the `expand` method or standalone function, the `StringIO`
object which is used to capture the output of the ephemeral
interpreter is processed before finalizers are handled.  Thus
attempting to use finalizers to generate output will not work as
expected.  Instead, create a dedicated interpreter (Python code):

```python
import em

source = ... # the EmPy source to expand

with em.StringIO() as file:
    with em.Interpreter(output=file, ...) as interp:
        interp.string(source)
    output = file.getvalue()
```

:::

:::{versionadded} 2.1

Finalizers were introduced in EmPy version 2.1.

:::


##### Handlers

Error handlers are also implemented as plugins.  See [Error
handlers](#error-handlers).



### Diversions

EmPy supports an extended form of **diversions**, which are a
mechanism for deferring and playing back output on demand, similar to
the functionality included in [m4](https://www.gnu.org/software/m4/).
Multiple "streams" of output can be diverted (deferred) and played
back (undiverted) in this manner.  A diversion is identified with a
name, which is any immutable object such an integer or string.
Diversions can be played back multiple times ("replayed") if desired.
When recalled, diverted code is *not* resent through the EmPy
interpreter (although a [filter](#filters) could be set up to do
this).

By default, no diversions take place.  When no diversion is in effect,
processing output goes directly to the specified output file.  This
state can be explicitly requested at any time by calling the
`empy.stopDiverting` function.  It is always legal to call this
function, even when there is currently no active diversion.

When diverted, however, output goes to a deferred location which can
then be recalled later.  Output is diverted with the
`empy.startDiversion` function, which takes an argument that is the
name of the diversion.  If there is no diversion by that name, a new
diversion is created and output will be sent to that diversion; if the
diversion already exists, output will be appended to that preexisting
diversion.

Output send to diversions can be recalled in two ways.  The first is
through the `empy.playDiversion` function, which takes the name of the
diversion as an argument.  This plays back the named diversion, sends
it to the output, and then erases that diversion.  A variant of this
behavior is the `empy.replayDiversion`, which plays back the named
diversion but does not eliminate it afterwards; `empy.replayDiversion`
can be repeatedly called with the same diversion name, and will replay
that diversion repeatedly.  `empy.createDiversion` will create a
diversion without actually diverting to it, for cases where you want
to make sure a diversion exists but do not yet want to send anything
to it.

The diversion object itself can be retrieved with
`empy.retrieveDiversion`.  Diversions act as writable file-objects,
supporting the usual `write`, `writelines`, `flush`, and `close`
methods.  The data that has been diverted to them can be manually
retrieved in one of two ways; either through the `asString` method,
which returns the entire contents of the diversion as a single string,
or through the `asFile` method, which returns the contents of the
diversion as a readable (not writable) file-like object.

Diversions can also be explicitly deleted without playing them back
with the `empy.dropDiversion` function, which takes the desired
diversion name as an argument.

Additionally there are three functions which will apply the above
operations to all existing diversions: `empy.playAllDiversions`,
`empy.replayAllDiversions`, and `empy.dropAllDiversions`.  The
diversions are handled in lexicographical order by their name.  Also,
all three will do the equivalent of a `empy.stopDiverting` call before
they do their thing.

The name of the current diversion can be requested with the
`empy.getCurrentDiversionName` function; also, the names of all
existing diversions (in sorted order) can be retrieved with
`empy.getAllDiversionNames`.  `empy.isExistingDiversionName` will
return whether or not a diversion with the given name exists.

When all processing is finished, the equivalent of a call to
`empy.playAllDiversions` is done.  This can be disabled with the
`--no-auto-play-diversions` (_configuration variable:_ `autoPlayDiversions = False`) option.

<a class="example" id="example-3"></a>

:::{admonition} Example 3: Diversions sample
_Source_: ⌨️
<div class="source">

``````
This text is output normally.
@empy.startDiversion('A')@
(This text was diverted!)
@empy.stopDiverting()@
This text is back to being output normally.
Now playing the diversion:
@empy.playDiversion('A')@
And now back to normal output.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This text is output normally.
This text is back to being output normally.
Now playing the diversion:
(This text was diverted!)
And now back to normal output.
``````

</div>

:::

:::{versionadded} 1.0

Diversions were introduced in EmPy version 1.0.

:::


### Filters

EmPy also supports dynamic **filters**.  Filters are put in place
immediately before the final output file, and so are only invoked
after all other processing has taken place (including interpreting and
diverting).  Filters take input, remap it, and then send it to the
output.  They can be chained together where a series of filters point
to each other in series and then finally to the output file.

The current top-level filter can be retrieved with `empy.getFilter`
(or `empy.getFirstFilter`).  The last filter in the chain (the one
just before the underlying file) can be retrieved with
`empy.getLastFilter`.  The filter can be set with `empy.setFilter`
(which allows multiple arguments to constitute a chain).  To append a
filter at the end of the chain (inserting it just before the
underlying output file), use `empy.appendFilter`.  To prepend it to
the top of the chain, use `empy.prependFilter`.  A filter chain can be
set directly with `empy.setFilterChain`.  And a filter chain can be
reset with `empy.resetFilter`, removing all filters.

Filters are, at their core, simply file-like objects (minimally
supporting `write`, `flush`, and `close` methods that behave in the
usual way) which, after performing whatever processing they need to
do, send their work to the next file-like object or filter in line,
called that filter's "sink."  That is to say, filters can be "chained"
together; the action of each filter takes place in sequence, with the
output of one filter being the input of the next.  The final sink of
the filter chain will be the output file.  Additionally, filters
support a `_flush` method (note the leading underscore) which will
always flush the filter's underlying sink; this method should be not
overridden.

Filters also support two additional methods, not part of the
traditional file interface: `attach`, which takes as an argument a
file-like object (perhaps another filter) and sets that as the
filter's "sink" -- that is, the next filter/file-like object in line.
`detach` (which takes no arguments) is another method which flushes
the filter and removes its sink, leaving it isolated.  Finally,
`next`, if present, is an attribute which references the filter's sink
-- or `None`, if the filter does not yet have a sink attached.

To create your own filter, you can create an object which supports the
above described interface, or simply derive from the `Filter` class
(or one of its subclasses) in the `emlib` module and override the
relevant methods.

<a class="example" id="example-4"></a>

:::{admonition} Example 4: Filters sample
_Source_: ⌨️
<div class="source">

``````
@{
# For access to the filter classes.
import emlib
}@
This text is normal.
@empy.appendFilter(emlib.FunctionFilter(lambda x: x.upper()))@
This text is in all uppercase!
@empy.appendFilter(emlib.FunctionFilter(lambda x: '[' + x + ']'))@
Now it's also surrounded by brackets!
(Note the brackets are around output as it is sent, 
not at the beginning and end of each line.)
@empy.resetFilter()@
Now it's back to normal.
``````

</div>

_Output_: 🖥️
<div class="output">

``````
This text is normal.
THIS TEXT IS IN ALL UPPERCASE!
[NOW IT'S ALSO SURROUNDED BY BRACKETS!
(NOTE THE BRACKETS ARE AROUND OUTPUT AS IT IS SENT, 
NOT AT THE BEGINNING AND END OF EACH LINE.)
]Now it's back to normal.
``````

</div>

:::

:::{versionadded} 1.3

Filters were introduced in EmPy version 1.3.

:::


### Modules

EmPy also supports importing EmPy files as modules, the same way that
Python can import Python modules.  EmPy modules are files with an .em
extension (configurable with `--module-extension` (_configuration variable:_ `moduleExtension`))
located somewhere in the path.  EmPy modules are imported with the
`import` statement just as Python modules are imported.  EmPy modules
are implemented via the [`importlib`
architecture](https://docs.python.org/3/library/importlib.html).

EmPy modules are activated by installing a meta path finder instance
in `sys.meta_path`.  By default, it is installed before all the other
meta path finders; this can be modified with
`--module-finder-index` (_configuration variable:_ `moduleFinderIndex`) -- it represents the index
to insert the module finder in the meta path, with 0 being at the
beginning; a negative value indicates that it should be appended.
This module path finder is installed when the first interpreter is
created.  It is designed so that if an `import` statement is executed
outside of a running interpreter, the EmPy module path finder will be
skipped.  When all interpreters are shut down, the meta path finder is
_not_ removed from the `sys.meta_path`; it will become inactive since
there is no proxy installed and no interpreter is running.

EmPy modules are real modules and have the same attributes and
behavior as Python modules.  When an EmPy module is imported, the
module is created, and then the pseudodmodule is placed in that
module's globals (or its attributes are if `-f/--flatten` (_environment variable:_ `EMPY_FLATTEN`, _configuration variable:_ `doFlatten = True`) is
set).  After the module is so prepared, the source file is expanded in
a new interpreter context with the globals pointing to that new
module's contents.

Module support can disabled with `-g/--disable-modules` (_configuration variable:_ `supportModules = False`).

An EmPy document can determine whether it is being imported as a
module or expanded naturally by checking for the globals `__name__`,
`__file__`, and/or `__spec__`.

As an example, given this file names.em somewhere in your path:

```empy
@[def upper(x)]@x.upper()@[end def]@
@[def capitalize(x)]@x.capitalize()@[end def]@
@[def lower(x)]@x.lower()@[end def]@
```

Then (with test.em found in /tmp):

<a class="example" id="example-63"></a>

:::{admonition} Example 63: Modules
_Source_: ⌨️
<div class="source">

``````
@{
import names
}@
Module: @names
Uppercase: @names.upper("uppercase").
Lowercase: @names.lower("LOWERCASE").
Capitalize: @names.capitalize("capitalize").
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Module: <module 'names' from '/tmp/names.em'>
Uppercase: UPPERCASE.
Lowercase: lowercase.
Capitalize: Capitalize.
``````

</div>

:::

:::{important}

To change the filename extension used to detect EmPy modules (which
defaults to `.em`), use
`--module-extension` (_configuration variable:_ `moduleExtension`).

The meta path finder used to support modules is installed when the
first interpreter is created; therefore, changing the configuration
variable `moduleExtension` after the interpreter is up
and running will have no effect.  To use configurations to change the
module extension, use the `-c/--config-file=FILENAME` (_environment variable:_ `EMPY_CONFIG`) or
`--config=STATEMENTS` command line options, or create a
`Configuration` object manually and then pass it into the first
`Interpreter` instance created.  For instance, to use the extension
.empy:

<pre class="shell">
<b><i>% em.py --module-extension=.empy ...</i></b>

<i>...</i></pre>


or

<pre class="shell">
<b><i>% em.py --config='moduleExtension = ".empy"' ...</i></b>

<i>...</i></pre>


or (Python code):

```python
import em

config = em.Configuration(modulExtension='.empy')
interp = em.Interpreter(config=config, ...)
... use interp here ...
```

:::

:::{tip}

By default, any output generated by an EmPy module _will_ be sent to
the output.  Often this is undesirable, as modules are usually
intended to define classes, functions and variables, rather than
generate output.  Using natural spacing between these definitions --
particularly if they are long and take up many lines themselves --
would normally result in blank lines in the output.  For instance,
when importing this module:

```empy
@[def f(...)]@
... definition of f here ...
@[end def]@

@[def g(...)]@
... definition of g here ...
@[end def]@
```

a blank line will be rendered since there is one between the two
function definitions.  That is unfortunate since the blank line helps
to separate the two definitions.  This can be addressed in a few
different ways:

1. Wrap the EmPy module in [switch markup](#switch-markup) (`@-
   ... NL`, `@+ ... NL`).  Something like this:

   ```empy
   @-
   @[def f(...)]@
   ... definition of f here ...
   @[end def]@

   @[def g(...)]@
   ... definition of g here ...
   @[end def]@
   @+
   ```

2. Systematically use whitespace markup `@ NL` or comment markup
   `@#` to make sure no blank lines are generated:

   ```empy
   @[def f(...)]@
   ... definition of f here ...
   @[end def]@
   @
   @[def g(...)]@
   ... definition of g here ...
   @[end def]@
   ```

3. Disable module output with `-j/--disable-import-output` (_configuration variable:_ `enableImportOutput = False`).

:::

:::{warning}

EmPy modules rely on the `importlib` architecture in order to
function, so they are only supported in Python version 3.4 and up.
Also, IronPython's `importlib` implementation is flawed, so modules do
not function in IronPython.

To see if modules are not supported in your interpreter, verify that
the string `!modules` is an element of the `compat`
pseudomodule/interpreter attribute.

:::

:::{versionadded} 4.2

Modules were introduced in EmPy version 4.2.

:::


### Hooks

The EmPy system allows for the registration of **hooks** with a
running EmPy interpreter.  Hooks are objects, registered with an
interpreter, whose methods represent specific hook events.  Any number
of hook objects can be registered with an interpreter, and when a hook
is invoked, the associated method on each one of those hook objects
will be called by the interpreter in sequence.  The method name
indicates the type of hook, and it is called with a keyword list of
arguments corresponding the event arguments.

To use a hook, derive a class from `emlib.Hook` and override the
desired methods (with the same signatures as they appear in the base
class).  Create an instance of that subclass, and then register it
with a running interpreter with the `empy.addHook` function.  A hook
instance can be removed with the `empy.removeHook` function.

More than one hook instance can be registered with an interpreter; in
such a case, the appropriate methods are invoked on each instance in
the order in which they were appended.  To adjust this behavior, an
optional `prepend` argument to the `empy.addHook` function can be used
dictate that the new hook should placed at the *beginning* of the
sequence of hooks, rather than at the end (which is the default).
Also there are explicit `empy.appendHook` and `empy.prependHook`
functions.

All hooks can be enabled and disabled entirely for a given
interpreter; this is done with the `empy.enableHooks` and
`empy.disableHooks` functions.  By default hooks are enabled, but
obviously if no hooks have been registered no hooks will be called.
Whether hooks are enabled or disabled can be determined by calling
`empy.areHooksEnabled`.  To get the list of registered hooks, call
`empy.getHooks`.  All the hooks can be removed with `empy.clearHooks`.
Finally, to invoke a hook manually, use `empy.invokeHook`.

For a list of supported hooks, see [](#hook-methods) or the `Hook`
class definition in the `emlib` module.  (There is also an
`AbstractHook` class in this module which does not have blank stubs
for existing hook methods in case a user wishes to create them
dynamically.)

For example:

<a class="example" id="example-5"></a>

:::{admonition} Example 5: Hooks sample
_Source_: ⌨️
<div class="source">

``````
@# Modify the backquote markup to prepend and append backquotes
@# (say, for a document rendering system, cough cough).
@{
import emlib

class BackquoteHook(emlib.Hook):

    def __init__(self, interp):
        self.interp = interp
    
    def preBackquote(self, literal):
        self.interp.write('`' + literal + '`')
        return True # return true to skip the standard behavior

empy.addHook(BackquoteHook(empy))
}@
Now backquote markup will render with backquotes: @
@`this is now in backquotes`!
``````

</div>

_Output_: 🖥️
<div class="output">

``````
Now backquote markup will render with backquotes: `this is now in backquotes`!
``````

</div>

:::

:::{versionchanged} 4.0

Hooks were originally introduced in EmPy version 2.0, much improved in
version 3.2, and revamped again in version 4.0.

:::


#### Hook methods

##### Hook `at...` methods

These hooks are called when a self-contained event occurs.

`atInstallProxy(proxy, new)`

: A `sys.stdout` proxy was installed.  The Boolean value `new`
  indicates whether or not the proxy was preexisting.

`atUninstallProxy(proxy, done)`

: A `sys.stdout` proxy was uninstalled.  The Boolean value `done`
  indicates whether the reference count went to zero (and so the proxy
  has been completely removed).

`atStartup()`

: The interpreter has started up.

`atReady()`

: The interpreter has declared itself ready for processing.

`atFinalize()`

: The interpreter is finalizing.

`atShutdown()`

: The interpreter is shutting down.

`atParse(scanner, locals)`

: The interpreter is initiating a parse action with the given scanner
  and locals dictionary (which may be `None`).

`atToken(token)`

: The interpreter is expanding a token.

`atHandle(info, fatal, contexts)`

: The interpreter has encountered an error.  The `info` parameter is a
  3-tuple error (error type, error, traceback) returned from
  `sys.exc_info`, `fatal` is a Boolean indicating whether the
  interpreter should exit afterwards, and `contexts` is the context
  stack.

`atInteract()`

: The interpreter is going interactive.

##### Hook context methods

`pushContext(context)`

: This context is being pushed.

`popContext(context)`

: This context has been popped.

`setContext(context)`

: This context has been set or modified.

`restoreContext(context)`

: This context has been restored.

##### Hook `pre...`/`post...` methods

The `pre...` hooks are invoked before a token expands.  The hook can
return a true value to indicate that it has intercepted the expansion
and the token should cancel native expansion.  Not explicitly
returning anything, as in standard Python, is equivalent to returning
`None`, which is a false value, which continues expansion:

<a class="example" id="example-64"></a>

:::{admonition} Example 64: Hook `pre...` methods
_Source_: ⌨️
<div class="source">

``````
@{
import emlib

class Hook(emlib.Hook):

    def __init__(self, interp):
        self.interp = interp

    def preString(self, string):
        self.interp.write('[' + string + ']')
        return True

empy.addHook(Hook(empy))
}@
@# Now test it:
@"Hello, world!"
``````

</div>

_Output_: 🖥️
<div class="output">

``````
["Hello, world!"]
``````

</div>

:::

:::{tip}

It's typical to want to have an instance of the
interpreter/pseudomodule available to the hook, but it is neither done
automatically nor is it required.

:::

The `post...` hooks are invoked after a non-intercepted token finishes
expanding.  Not all `pre...` hooks have a corresponding `post...`
hook.  The `post...` hooks take at most one argument (the result of
the token expansion, if applicable) and their return value is ignored.

`preLineComment(comment)`, `postLineComment()`

: The line comment `@#... NL` with the given text.

`preInlineComment(comment)`, `postInlineComment()`

: The inline comment `@*...*` with the given text.

`preWhitespace(whitespace)`

: The whitespace token `@ WS` with the given whitespace.

`prePrefix()`

: The prefix token `@@`.

`preString(string)`, `postString()`

: The string token `@'...'`, etc. with the given string.

`preBackquote(literal)`, `postBackquote(result)`

: The backquote token `` @`...` `` with the given literal.

`preSignificator(key, value, stringized)`, `postSignificator()`

: The significator token `@%... NL`, etc. with the given key, value
  and a Boolean indicating whether the significator is stringized.

`preContextName(name)`, `postContentName()`

: The context name token `@?...` with the given name.

`preContextLine(line)`, `postContextLine()`

: The context line token `@!...` with the given line.

`preExpression(pairs, except, locals)`, `postExpression(result)`

: The expression token `@(...)` with the given if-then run pairs, the
  except run, and the locals dictionary (which may be `None`).

`preSimple(code, subtokens, locals)`, `postSimple(result)`

: The simple expression token `@word` (etc.) with the given code,
  subtokens and locals.

`preInPlace(code, locals)`, `postInPlace(result)`

: The in-place expression token `@$...$...$` with the given code
  (first section) and locals (which may be `None`).

`preStatement(code, locals)`, `postStatement()`

: The statement token `@{...}` with the given code and locals (which
  may be `None`).

`preControl(type, rest, locals)`, `postControl()`

: The control token `@[...]` of the given type, with the rest run and
  locals (which may be None).

`preEscape(code)`, `postEscape()`

: The control token `@\...` with the resulting code.

`preDiacritic(code)`, `postDiacritic()`

: The diacritic token `@^...` with the resulting code.

`preIcon(code)`, `postIcon()`

: The icon token `@|...` with the resulting code.

`preEmoji(name)`, `postEmoji()`

: The emoji token `@:...:` with the given name.

`preExtension(name, contents, depth)`, `postExtension(result)`

: An extension with the given name, contents and depth was invoked.

`preCustom(contents)`, `postCustom()`

: The custom token `@<...>` with the given contents.


##### Hook `before...`/`after...` methods

The `before...` and `after...` hooks are invoked before and after (go
figure) mid-level expansion activities are performed.  Any `locals`
argument indicates the locals dictionary, which may be `None`.

If the expansion returns something relevant, it is passed as a
`result` argument to the corresponding `after...` method.

`beforeProcess(command, n)`, `afterProcess()`

: The given command (with index number) is being processed.

`beforeInclude(file, locals, name)`, `afterInclude()`

: The given file is being processed with the given name.

`beforeExpand(string, locals, name)`, `afterExpand(result)`

: `empy.expand` is being called with the given string and name.

`beforeTokens(tokens, locals)`, `afterTokens(result)`

: The given list of tokens is being processed.

`beforeFileLines(file, locals)`, `afterFileLines()`

: The given file is being read by lines.

`beforeFileChunks(file, locals)`, `afterFileChunks()`

: The given file is being read by buffered chunks.

`beforeFileFull(file, locals)`, `afterFileFull()`

: The given file is being read fully.

`beforeString(string, locals)`, `afterString()`

: The given string is being processed.

`beforeQuote(string)`, `afterQuote(result)`

: The given string is being quoted.

`beforeEscape(string)`, `afterEscape(result)`

: The given string is being escaped.

`beforeSignificate(key, value, locals)`, `afterSignificate()`

: The given key/value pair is being processed.

`beforeCallback(contents)`, `afterCallback()`

: The custom callback is being processed with the given contents.

`beforeAtomic(name, value, locals)`, `afterAtomic()`

: The given atomic variable setting with the name and value is being
  processed.

`beforeMulti(names, values, locals)`, `afterMulti()`

: The given complex variable setting with the names and values is
  being processed.

`beforeImport(name, locals)`, `afterImport()`

: A module with the given name is being imported.

`beforeFunctional(code, lists, locals)`, `afterFunctional(result)`

: A functional markup is with the given code and argument lists (of
  EmPy code) is being processed.

`beforeEvaluate(expression, locals, write)`, `afterEvaluate(result)`

: An evaluation markup is being processed with the given code and a
  Boolean indicating whether or not the results are being written
  directly to the output stream or returned.

`beforeExecute(statements, locals)`, `afterExecute()`

: A statement markup is being processed.

`beforeSingle(source, locals)`, `afterSingle(result)`

: A "single" source (either an expression or a statement) is being
  compiled and processed.

`beforeFinalizer(final)`, `afterFinalizer()`

: The given finalizer is being processed.  If the `beforeFinalizer`
  hook returns true for a particular finalizer, then that finalizer
  will not be called.

:::{seealso}

The list of hook methods is available in the `hooks` help topic and is
summarized [here](HELP.md#hook-methods-summary).

:::


## Customization

The behavior of an EmPy system can be customized in various ways.

### Command line options

EmPy uses a standard GNU-style command line options processor with
both short and long options (_e.g._, `-p` or `--prefix`).  Short
options can be combined into one word, and options can have values
either in the next word or in the same word separated by an `=`.  An
option consisting of only `--` indicates that no further option
processing should be performed.

EmPy supports the following options:

`-V/--version`

: Print version information exit.  Repeat the option for more details
  (see below).

`-W/--info`

: Print additional information, including the operating system, Python
  implementation and Python version number.
  
`-Z/--details`

: Print all additional details about the running environment,
  including interpreter, system, platform, and operating system
  release details.

`-h/--help`

: Print basic help and exit.  Repeat the option for more extensive
  help.  Specifying `-h` once is equivalent to `-H default`; twice to
  `-H more`, and three or more times to `-H all` (see below).

`-H/--topics=TOPICS`

: Print extended help by topic(s).  Topics are a comma-separated list
  of the following choices:

  {#help-topics-table}
  | Topic | Description |
  | --- | --- |
  | `usage` | Basic command line usage |
  | `options` | Command line options |
  | `simple` | Simple (one-letter) command line options (why not) |
  | `markup` | Markup syntax |
  | `escapes` | Escape sequences |
  | `environ` | Environment variables |
  | `pseudo` | Pseudomodule attributes and functions |
  | `variables` | Configuration variable attributes |
  | `methods` | Configuration methods |
  | `hook` | Hook methods |
  | `named` | Named escapes (control codes) |
  | `diacritics` | Diacritic combiners |
  | `icons` | Icons |
  | `emojis` | User-specified emojis (optional) |
  | `hints` | Usage hints |
  | `topics` | This list of topics |
  | `default` | `usage,options,markup,hints` and `topics` |
  | `more` | `usage,options,markup,escapes,environ,hints` and `topics` |
  | `all` | `usage,options,markup,escapes,environ,pseudo,config,controls,diacritics,icons,hints` |

  As a special case, `-H` with no topic argument is treated as `-H
  all` rather than error.

`-v/--verbose`

: The EmPy system will print debugging information to `sys.stderr` as
  it is doing its processing.

{#prefix}
`-p/--prefix=CHAR` (_environment variable:_ `EMPY_PREFIX`, _configuration variable:_ `prefix`)

: Specify the desired EmPy prefix.  It must consist of a single
  Unicode code point (or character).  To specify no prefix (see
  below), provide an empty string, the string `None` or `none`, or set
  the `prefix` configuration variable to `None`.
  Defaults to `@`.

`--no-prefix`

: Specify that EmPy use no prefix.  In this mode, will only process
  text and perform no markup expansion.  This is equivalent to
  specifying `-p ''`, or setting the `prefix`
  configuration variable to `None`.

`-q/--no-output`

: Use a null file for the output file.  This will result in no output
  at all.

`-m/--pseudomodule=NAME` (_environment variable:_ `EMPY_PSEUDO`, _configuration variable:_ `pseudomoduleName`)

: Specify the name of the EmPy pseudomodule/interpreter.  Defaults to
  `empy`.

`-f/--flatten` (_environment variable:_ `EMPY_FLATTEN`, _configuration variable:_ `doFlatten = True`)

: Before processing, move the contents of the
  `empy` pseudomodule into the globals, just
  as if `empy.flattenGlobals()` were executed immediately after
  starting the interpreter.  This is the equivalent of executing `from
  empy import *` (though since the pseudomodule is not a real module
  that statement is invalid).  _e.g._, `empy.include` can be referred
  to simply as `include` when this flag is specified on the command
  line.

`-k/--keep-going` (_configuration variable:_ `exitOnError = False`)

: Don't exit immediately when an error occurs.  Execute the error
  handler but continue processing EmPy tokens.

`-e/--ignore-errors` (_configuration variable:_ `ignoreErrors = True`)

: Ignore errors completely.  No error dispatcher or handler is
  executed and token processing continues indefinitely.  Implies
  `-k/--keep-going`.

`-r/--raw-errors` (_environment variable:_ `EMPY_RAW_ERRORS`, _configuration variable:_ `rawErrors = True`)

: After logging an EmPy error, show the full Python traceback that
  caused it.  Useful for debugging.

`-s/--brief-errors` (_configuration variable:_ `verboseErrors = False`)

: When printing an EmPy error, show only its arguments and not its
  keyword arguments.  This is in contrast to the default (verbose)
  where keyword arguments are shown.

`--verbose-errors` (_configuration variable:_ `verboseErrors = True`)

: When printing an EmPy error, show both its arguments and its
  (sorted) keyword arguments.  This is the default.
  
`-i/--interactive` (_environment variable:_ `EMPY_INTERACTIVE`, _configuration variable:_ `goInteractive = True`)

: Enter interactive mode after processing is complete by continuing to
  process EmPy markup from the input file, which is by default
  `sys.stdin`); this can be changed with the `input` interpreter
  attribute.  This is helpful for inspecting the state of the
  interpreter after processing.

`-d/--delete-on-error` (_environment variable:_ `EMPY_DELETE_ON_ERROR`, _configuration variable:_ `deleteOnError`)

: If an error occurs, delete the output file; requires the use of the
  one of the output options such as `-o/--output=FILENAME`.  This is
  useful when running EmPy under a build systemn such as GNU Make.  If
  this option is not selected and an error occurs, the output file
  will stop when the error is encountered.

`-n/--no-proxy` (_environment variable:_ `EMPY_NO_PROXY`, _configuration variable:_ `useProxy`)

: Do not install a proxy in `sys.stdout`.  This will make EmPy thread
  safe but writing to `sys.stdout` will not be captured or processed
  in any way.

`--config=STATEMENTS`

: Perform the given configuration variable assignments.  This option
  can be specified multiple times.

`-c/--config-file=FILENAME` (_environment variable:_ `EMPY_CONFIG`)

: Read and process the given configuration file(s), separated by the
  platform-specific path delimiter (`;` on Windows, `:` on other
  operating systems).  This option can be specified multiple times.
  
`--config-variable=NAME` (_configuration variable:_ `configVariableName`)

: Specify the variable name corresponding to the current configuration
  when configuration files are processed.  Defaults to
  `_`.
  
`-C/--ignore-missing-config` (_configuration variable:_ `missingConfigIsError = False`)

: Ignore missing files while reading and processing configurations.
  By default, a missing file is an error.
  
`-o/--output=FILENAME`

: Specify the file to write output to.  If this argument is not used,
  final output is written to the underlying `sys.stdout`.
  
`-a/--append=FILENAME`

: Specify the file to append output to.  If this argument is not used,
  final output is appended to the underlying `sys.stdout`.

`-O/--output-binary=FILENAME`

: Specify the file to write output to and open it as binary.

`-A/--append-binary=FILENAME`

: Specify the file to append output to and open it as binary.

`--output-mode=MODE`

: Specify the output mode to use.

`--input-mode=MODE`

: Specify the input mode to use.  Defaults to `'r'`.

`-b/--buffering` (_environment variable:_ `EMPY_BUFFERING`, _configuration variable:_ `buffering`)

: Specify the buffering to use.  Use an integer to specify the maximum
  number of bytes to read per block or one of the following string
  values:

  {#buffering-names-table}
  | Name | Value | Description |
  | --- | --- | --- |
  | `full` | -1 | Use full buffering |
  | `none` | 0 | Use no buffering |
  | `line` | 1 | Use line buffering |
  | `default` | 16384 | Default buffering |

  If the choice of buffering is incompatible with other settings, a
  `ConfigurationError` is raised.  This option has no effect on
  interactive mode, as `sys.stdin` is already open.  Defaults to
  16384.

`--default-buffering`

: Use default buffering.

`-N/--no-buffering`

: Use no buffering.

`-L/--line-buffering`

: Use line buffering.

`-B/--full-buffering`

: Use full buffering.

{#import}
`-I/--import=MODULES`

: Import the given Python (not EmPy) module(s) into the interpreter
  globals before main document processing begins.  To specify more
  than one module to import, separate the module names with commas or
  use multiple `-I/--import=MODULES` options.
  
  Variations on the Python `import` statement can be expressed with
  the following arguments **shortcuts**:

  {#import-shortcuts-table}
  | Argument shortcut patterns | Python equivalent |
  | --- | --- |
  | `X` | `import X` |
  | `X as Y` | `import X as Y` |
  | `X=Y` | `import X as Y` |
  | `X:Y` | `from X import Y` |
  | `X:Y as Z` | `from X import Y as Z` |
  | `X:Y=Z` | `from X import Y as Z` |
  | `X,Y` | `import X, Y` |

  For convenience, any `+` character in the argument is replaced with
  a space.

`-D/--define=DEFN`

: Define the given variable into the interpreter globals before main
  document processing begins.  This is executed as a Python assignment
  statement (`variable = ...`); if it does not contain a `=`
  character, then the variable is defined in the globals with the
  value `None`.

`-S/--string=STR`

: Define the given string variable into the interpreter globals before
  main document processing begins.  The value is always treated as a
  string and never evaluated; if it does not contain a `=` character,
  then the variable is defined as the empty string (`''`).

`-P/--preprocess=FILENAME`

: Process the given EmPy (not Python) file before main document
  processing begins.

`-Q/--postprocess=FILENAME`

: Process the given EmPy (not Python) file after main document
  processing begins.

`-E/--execute=STATEMENT`

: Execute the given arbitrary Python (not EmPy) statement before main
  document processing begins.

`-K/--postexecute=STATEMENT`

: Execute the given arbitrary Python (not EmPy) statement after main
  document processing begins.

`-F/--file=FILENAME`

: Execute the given Python (not EmPy) file before main document
  processing begins.

`-G/--postfile=FILENAME`

: Execute the given Python (not EmPy) file after main document
  processing begins.

`-X/--expand=MARKUP`

: Expand the given arbitrary EmPy (not Python) markup before main
  document processing begins.

`-Y/--postexpand=MARKUP`

: Expand the given arbitrary EmPy (not Python) markup after main
  document processing begins.

`--preinitializer=FILENAME`

: Execute the given Python file locally just before creating the main
  interpreter.  Used for tests.

`--postinitializer=FILENAME`

: Execute the given Python file locally just after shutting edown the
  main interpreter.  Used for tests.

`-w/--pause-at-end` (_configuration variable:_ `pauseAtEnd`)

: Prompt for a line of input after all processing is complete.  Useful
  for systems where the window running EmPy would automatically
  disappear after EmPy exits (_e.g._, Windows).  By default, the input
  file used is `sys.stdin`, so this will not work when redirecting
  stdin to an EmPy process.  This can be changed with the `input`
  interpreter attribute.

`-l/--relative-path` (_configuration variable:_ `relativePath`)

: Prepend the location of the EmPy script to Python's `sys.path`.
  This is useful when the EmPy scripts themselves import Python .py
  modules in that same directory.

`--replace-newlines` (_configuration variable:_ `replaceNewlines = True`)

: Replace newlines in (Python) expressions before evaluation.

`--no-replace-newlines` (_configuration variable:_ `replaceNewlines = False`)

: Don't replace newlines in (Python) expressions before evaluations.
  This is the default.

`--ignore-bangpaths` (_configuration variable:_ `ignoreBangpaths = True`)

: Treat bangpaths as comments.  By default, bangpaths (starting lines
  that begin with the characters `#!`) are treated as comments and
  ignored.

`--no-ignore-bangpaths` (_configuration variable:_ `ignoreBangpaths = False`)

: Do not treat bangpaths as comments.  This is the opposite of the
  default.

`--none-symbol` (_configuration variable:_ `noneSymbol`)

: The string to write when expanding the value `None`.  Defaults to
  `None`, which will result in no output.

`--no-none-symbol`

: Do not write any preset string when expanding `None`; equivalent to
  setting `noneSymbol` to `None`.

`--expand-user` (_configuration variable:_ `expandUserConstructions = True`)

: Expand user constructions (`~user`) in configuration file pathnames.
  This is the default.

`--no-expand-user` (_configuration variable:_ `expandUserConstructions = False`)

: Do not expand user constructions (`~user`) in configuration file
  pathnames.  By default they are expanded.

`--auto-validate-icons` (_configuration variable:_ `autoValidateIcons = True`)

: Auto-validate icons when an icon markup is first used.  This is the
  default.  See below.

`--no-auto-validate-icons` (_configuration variable:_ `autoValidateIcons = False`)

: Do not auto-validate icons when an icon markup is first used.  See
  below.

`--starting-line` (_configuration variable:_ `startingLine`)

: Specify an integer representing the default starting line for
  contexts.  Default is 1.

`--starting-column` (_configuration variable:_ `startingColumn`)

: Specify an integer representing the default starting column for
  contexts.  Default is 1.

`--emoji-modules` (_configuration variable:_ `emojiModuleNames`)

: A comma-separated list of emoji modules to try to use for the emoji
  markup (`@:...:`).  See below.  Defaults to
  `emoji,emojis,emoji_data_python,unicodedata`.

`--no-emoji-modules`

: Only use `unicodedata` as an emoji module; disable all the
  third-party emoji modules.

`--disable-emoji-modules`

: Disable all emoji module usage; just rely on the `emojis` attribute
  of the configuration.  See below.

`--ignore-emoji-not-found` (_configuration variable:_ `emojiNotFoundIsError = False`)

: When using emoji markup (`@:...:`), do not raise an error when an
  emoji is not found; just pass the `:...:` text through.

`-u/--binary/--unicode` (_environment variable:_ `EMPY_BINARY`, _configuration variable:_ `useBinary`)

: Operate in binary mode; open files in binary mode and use the
  `codecs` module for Unicode support.  This is necessary in older
  versions of Python 2._x_.

`-x/--encoding=E`

: Specify both input and output Unicode encodings.  Requires
  specifying both an input and an output file.

`--input-encoding=E` (_environment variable:_ `EMPY_INPUT_ENCODING`, _configuration variable:_ `inputEncoding`)

: Specify the input Unicode encoding.  Requires specifying an input
  file rather than `sys.stdout`.

  :::{note}
  
  Specifying a non-default encoding when using interactive mode
  (`sys.stdin`) raises a `ConfigurationError`.
  
  :::

`--output-encoding=E` (_environment variable:_ `EMPY_OUTPUT_ENCODING`, _configuration variable:_ `outputEncoding`)

: Specify the output Unicode encoding.  Requires specifying an output
  file rather than `sys.stdout`.
  
  :::{note}
  
  Specifying a non-default encoding when using `sys.stdout` raises a
  `ConfigurationError`.
  
  :::

`-y/--errors=E`

: Specify both [input and output Unicode error
  handlers](https://docs.python.org/3/library/functions.html#open).

`--input-errors=E` (_environment variable:_ `EMPY_INPUT_ERRORS`, _configuration variable:_ `inputErrors`)

: Specify the [input Unicode error
  handler](https://docs.python.org/3/library/functions.html#open).

  :::{note}
  
  Specifying a non-default error handler when using interactive mode
  (`sys.stdin`) raises a `ConfigurationError`.
  
  :::

`--output-errors=E` (_environment variable:_ `EMPY_OUTPUT_ERRORS`, _configuration variable:_ `outputErrors`)

: Specify the [output Unicode error
  handler](https://docs.python.org/3/library/functions.html#open).

  :::{note}
  
  Specifying a non-default error handler when using `sys.stdout`
  raises a `ConfigurationError`.
  
  :::

`-z/--normalization-form=F` (_configuration variable:_ `normalizationForm`)

: Specify the Unicode normalization to perform when using the
  diacritics markup (`@^...`).  Specify an empty string (`''`) to
  skip normalization.  Defaults to `NFKC`
  for modern versions of Python and `''` for very old versions of
  Python 2._x_.

`--auto-play-diversions` (_configuration variable:_ `autoPlayDiversions = True`)

: Before exiting, automatically play back any remaining diversions.
  This is the default.

`--no-auto-play-diversions` (_configuration variable:_ `autoPlayDiversions = False`)

: Before exiting, do not automatically play back any remaining
  diversions.  By default such diversions are played back.

`--check-variables` (_configuration variable:_ `checkVariables = True`)

: When modifying configuration variables, by default the existence and
  proper type of these variables is checked; anomalies will raise a
  `ConfigurationError`.  This is the default.

`--no-check-variables` (_configuration variable:_ `checkVariables = False`)

: When modifying configuration variables, normally the existence and
  types of these variables is checked and if it doesn't exist or it is
  attempting to be assigned to an incompatible type, it will raise a
  `ConfigurationError`.  To override this behavior, use this flag.

`--path-separator` (_configuration variable:_ `pathSeparator`)

: The path separator delimiter for specifying configuration paths.
  Defaults to `;` on Windows and `:` on all other platforms.

`--enable-modules` (_configuration variable:_ `supportModules = True`)

: Enable EmPy module support (Python 3.4 and up).  The default.

`-g/--disable-modules` (_configuration variable:_ `supportModules = False`)

: Disable EmPy module support.

`--module-extension` (_configuration variable:_ `moduleExtension`)

: The filename extension for EmPy modules.  Defaults to `.em`.

`--module-finder-index` (_configuration variable:_ `moduleFinderIndex`)

: When installing the module finder, which allows EmPy module support,
  use this index as the position in the meta path to insert it.  Zero
  means that it will be inserted into the front of the list, one means
  the second, etc.  Negative values means that it will appended.
  Defaults to zero.

`--enable-import-output` (_configuration variable:_ `enableImportOutput = True`)

: Output from imported EmPy modules is allowed through.  The default.

`-j/--disable-import-output` (_configuration variable:_ `enableImportOutput = False`)

: Output from imported EmPy modules is suppressed.

`--context-format` (_configuration variable:_ `contextFormat`)

: Specify the format for printing contexts.  See below.

`--success-code=N` (_configuration variable:_ `successCode`)

: Specify the exit code for the Python interpreter on success.
  Defaults to 0.

`--failure-code=N` (_configuration variable:_ `failureCode`)

: Specify the exit code for the Python interpreter when a processing
  error occurs.  Defaults to 1.

`--unknown-code=N` (_configuration variable:_ `unknownCode`)

: Specify the exit code for the Python interpreter when an invalid
  configuration (such as unknown command line options) is encountered.
  Defaults to 2.

:::{seealso}

The list of command line options is available in the `options` help
topic and is summarized [here](HELP.md#command-line-options-summary).

:::


### Environment variables

The following environment variables are supported:

`EMPY_OPTIONS`

: Specify additional command line options to be used.  These are in
  effect added to the start of the command line and parsed before
  any explicit command line options and processing begins.
  
  For example, this will run the EmPy interpreter as if the `-r` and
  `-d` command line options were specified:
  
<pre class="shell">
<b><i>% export EMPY_OPTIONS='-r -d'; em.py ...</i></b>
</pre>

  
`EMPY_CONFIG` (_command line option:_ `-c/--config-file=FILENAME`)

: Specify the configuration file(s) to process before main document
  processing begins.

`EMPY_PREFIX` (_command line option:_ `-p/--prefix=CHAR`, _configuration variable:_ `prefix`)

: Specify the prefix to use when processing.

`EMPY_PSEUDO` (_command line option:_ `-m/--pseudomodule=NAME`, _configuration variable:_ `pseudomoduleName`)

: Specify the name of the pseudomodule/interpreter to use when
  processing.

`EMPY_FLATTEN` (_command line option:_ `-f/--flatten`, _configuration variable:_ `doFlatten = True`)

: If defined, flatten the globals before processing.

`EMPY_RAW_ERRORS` (_command line option:_ `-r/--raw-errors`, _configuration variable:_ `rawErrors = True`)

: If defined, after an error occurs, show the full Python tracebacks
  of the exception.

`EMPY_INTERACTIVE` (_command line option:_ `-i/--interactive`, _configuration variable:_ `goInteractive = True`)

: If defined, enter interactive mode by processing markup from
  `sys.stdin` after main document processing is complete.

`EMPY_DELETE_ON_ERROR` (_command line option:_ `-d/--delete-on-error`, _configuration variable:_ `deleteOnError`)

: If defined, when an error occurs, delete the corresponding output
  file.

`EMPY_NO_PROXY` (_command line option:_ `-n/--no-proxy`, _configuration variable:_ `useProxy`)

: If defined, do not install a `sys.stdout` proxy.

`EMPY_BUFFERING` (_command line option:_ `-b/--buffering`, _configuration variable:_ `buffering`)

: Specify the desired file buffering.

`EMPY_BINARY` (_command line option:_ `-u/--binary/--unicode`, _configuration variable:_ `useBinary`)

: If defined, use binary mode.

`EMPY_ENCODING`

: Specify the desired input and output Unicode encodings.

`EMPY_INPUT_ENCODING` (_command line option:_ `--input-encoding=E`, _configuration variable:_ `inputEncoding`)

: Specify the desired input Unicode encoding only.

`EMPY_OUTPUT_ENCODING` (_command line option:_ `--output-encoding=E`, _configuration variable:_ `outputEncoding`)

: Specify the desired output Unicode encoding only.

`EMPY_ERRORS`

: Specify the desired input and output Unicode error handler.

`EMPY_INPUT_ERRORS` (_command line option:_ `--input-errors=E`, _configuration variable:_ `inputErrors`)

: Specify the desired input Unicode error handler.

`EMPY_OUTPUT_ERRORS` (_command line option:_ `--output-errors=E`, _configuration variable:_ `outputErrors`)

: Specify the desired output Unicode error handler.

:::{seealso}

The list of environment variables is available in the `environ` help
topic and is summarized [here](HELP.md#environment-variables-summary).

:::

:::{versionadded} 2.2

Environment variables were first introduced in EmPy version 2.2, and
revamped in version 4.0.

:::


### Configuration

**Configurations** are objects which determine the behavior of an EmPy
interpreter.  They can be created with an instance of the
`Configuration` class and have a set of attributes (**configuration
variables**) which can be modified.  Most configuration variables
correspond to a command line option.  The configuration instance also
contains supporting methods which are used by the interpreter which
can be overridden.

When configuration variables are modified, they are by default checked
to make sure have a known name and that they have the correct type; if
not, a `ConfigurationError` will be raised.  This behavior can be
disabled with `--no-check-variables` (_configuration variable:_ `checkVariables = False`).

When a configuration is assigned to an interpreter, it exists as a
`config` attribute of the `empy` pseudomodule and can be modified by a
running EmPy system.  Configurations can be shared between multiple
interpreters if desired.

<a class="example" id="example-65"></a>

:::{admonition} Example 65: Configuration instances
_Source_: ⌨️
<div class="source">

``````
@{
empy.config.prefix = '$'
}$
${
print("The EmPy prefix is now $, not @!")
}$
``````

</div>

_Output_: 🖥️
<div class="output">

``````
The EmPy prefix is now $, not @!
``````

</div>

:::

:::{tip}

This example shows a quirk of changing configurations in the middle of
processing an EmPy document; the prefix changes from a `@` to a `$`
by the end of the first statement markup, so a `$` and a newline is
required to suppress the trailing newline; a `@` would have been sent
to the output unchanged since it is no longer the prefix.  Use
[command line options](#command-line-options), [environment
variables](#environment-variables) or [configuration
files](#configuration-files) to avoid issues like this, as they are
processed before any EmPy document.

For changing the prefix, use `-p/--prefix=CHAR` (_environment variable:_ `EMPY_PREFIX`, _configuration variable:_ `prefix`).

:::


#### Configuration files

**Configuration files** are snippets of Python (not EmPy) code which
can be executed under an EmPy system to modify the current
configuration.  By convention they have the extension .conf. though
this is not a requirement.  Configuration files are processed before
any expansion begins and are specified with the `-c/--config-file=FILENAME` (_environment variable:_ `EMPY_CONFIG`) command line option; a list of configuration files can be
specified with a `:` delimiter (`;` on Windows); the delimiter can be
specified with `--path-separator` (_configuration variable:_ `pathSeparator`).  A nonexistent
configuration file specified in this way is an error unless
`-C/--ignore-missing-config` (_configuration variable:_ `missingConfigIsError = False`) is specified.

When a configuration file is processed, its contents are executed in a
Python (not EmPy) interpreter and then any resulting variable
assignments are assigned to the configuration instance.  So:

``````python
prefix = '$'
``````

is a simple configuration file which will change the EmPy prefix to
`$`.

Any resulting variable beginning with an underscore will be ignored.
Thus these variables can be used as auxiliary variables in the
configuration file.  For example, this configuration file will define
custom emojis for the numbered keycaps:

``````python
emojis = {}
for _x in range(10):
    emojis[str(_x)] = '{}\ufe0f\u20e3'.format(_x)
``````

Finally, when a configuration file is processed, the current
configuration instance is presented as a variable named `_` (this can
be changed with `--config-variable=NAME` (_configuration variable:_ `configVariableName`)).  The
following example does the same as the previous example but uses the
dedicated variable:

``````python
_.emojis.update(((str(_x), '{}\ufe0f\u20e3'.format(_x)) for _x in range(10)))
``````

:::{tip}

To make a set of configuration files automatic loaded by EmPy, use the
`EMPY_CONFIG` environment variable in your startup
shell:

% export EMPY_CONFIG=~/path/to/default.conf

To make a more general set of _options_ available, set `EMPY_OPTIONS`.

:::


#### Configuration variables

The following configuration variables (attributes of a `Configuration`
object) exist with the given types and their corresponding command
line options and environment variables.  Default values are shown
after a `=` sign.  When a corresponding command line option exists,
See [](#command-line-options) for more detailed information.

`name: str = 'default'`

: The name of this configuration.  It is for organizational purposes
  and is not used directly by the EmPy system.

`notes = None`

: Arbitrary data about this configuration.  It can be anything from an
  integer to a string to a dictionary to a class instance, or its
  default, `None`.  It is for organizational purposes and is not used
  directly by the EmPy system.
  
`prefix: str = '@'` (_command line option:_ `-p/--prefix=CHAR`, _environment variable:_ `EMPY_PREFIX`)

: The prefix the interpreter is using to delimit EmPy markup.  Must be
  a single Unicode code point (character).

`pseudomoduleName: str = 'empy'` (_command line option:_ `-m/--pseudomodule=NAME`, _environment variable:_ `EMPY_PSEUDO`)

: The name of the pseudomodule for this interpreter.

`verbose: bool = False`

: If true, print debugging information before processing each EmPy
  token.
  
`rawErrors: bool = False` (_command line option:_ `-r/--raw-errors`, _environment variable:_ `EMPY_RAW_ERRORS`)

: If true, print a Python traceback for every exception that is thrown.

`exitOnError: bool = True` (_command line option:_ `-k/--keep-going`)

: If true, exit the EmPy interpreter after an error occurs.  If false,
  processing will continue despite the error.

`ignoreErrors: bool = False` (_command line option:_ `-e/--ignore-errors`)

: If true, all errors are ignored by the EmPy interpreter.  Setting
  this to true also implies `exitOnError` is false.

`contextFormat: str = '%(name)s:%(line)d:%(column)d'` (_command line option:_ `--context-format`)

: The string format to use to render contexts.  EmPy will
  automatically determine whether or not it should use the `%`
  operator or the `str.format` method with this format.  See
  [](#context-formatting) for more details.

`goInteractive: bool = False` (_command line option:_ `-i/--interactive`, _environment variable:_ `EMPY_INTERACTIVE`)

: When done processing the main EmPy document (if any), go into
  interactive mode by running a REPL loop with `sys.stdin`.  If such
  document is specified (_i.e._, EmPy is invoked with no arguments),
  go into interactive mode as well.
  
`deleteOnError: bool = False` (_command line option:_ `-d/--delete-on-error`, _environment variable:_ `EMPY_DELETE_ON_ERROR`)

: If an output file is chosen (_e.g._, with `-o/--output=FILENAME` or one
  of the other such options) and an error occurs, delete the output
  file.  If this is set to true with output set to `sys.stdout`, a
  ConfigurationError will be raised.
  
`doFlatten: bool = False` (_command line option:_ `-f/--flatten`, _environment variable:_ `EMPY_FLATTEN`)

: Flatten the contents of the `empy`
  pseudomodule into the globals rather than having them all under the
  pseudomodule name.
  
`useProxy: bool = True` (_command line option:_ `-n/--no-proxy`, _environment variable:_ `EMPY_NO_PROXY`)

: If true, install a proxy object for `sys.stdout`.  This should be
  true if any output is being done via `print` or `sys.stdout.write`.
  
`relativePath: bool = False` (_command line option:_ `-l/--relative-path`)

: If true, the directory of the EmPy script's path will be prepended
  to Python's `sys.path`.

`buffering: int = 16384` (_command line option:_ `-b/--buffering`, _environment variable:_ `EMPY_BUFFERING`)

: Specify the buffering for the input and output files.

`replaceNewlines: bool = False` (_command line option:_ `--no-replace-newlines`)

: If true, newlines in emoji names, Unicode character name escape
  markup, and code evaluation will be changed to spaces.  This can
  help when writing EmPy with a word-wrapping editor.
  
`ignoreBangpaths: bool = True` (_command line option:_ `--no-ignore-bangpaths`)

: If true, a bangpath (the first line of a file which starts with
  `#!`) will be treated as an EmPy comment, allowing the creation of
  EmPy executable scripts.  If false, it will not be treated specially
  and will be rendered to the output.
  
`noneSymbol: str = None` (_command line option:_ `--none-symbol`)

: When an EmPy expansion evaluates to None (_e.g._, `@(None)`), this
  is the string that will be rendered to the output stream.  If set to
  `None` (the default), no output will be rendered.

`missingConfigIsError: bool = True` (_command line option:_ `-C/--ignore-missing-config`)

: If a configuration file is specified with `-c/--config-file=FILENAME` but
  does not exist, if this variable is true an error will be raised.
  
`pauseAtEnd: bool = False` (_command line option:_ `-w/--pause-at-end`)

: When done processing EmPy files, read a line from `sys.stdin` before
  exiting the interpreter.  This can be useful when testing under
  consoles on Windows.

`startingLine: int = 1` (_command line option:_ `--starting-line`)

: The line to start with in contexts.

`startingColumn: int = 1` (_command line option:_ `--starting-column`)

: The column to start with in contexts.

`significatorDelimiters: tuple = ('__', '__')`

: A 2-tuple of strings representing the prefix and suffix to add to
  significator names in order to determine what name to give them in
  the globals.

`emptySignificator: object = None`

: The default value to use for non-stringized significators.

`autoValidateIcons: bool = True` (_command line option:_ `--no-auto-validate-icons`)

: When icons are used with a custom dictionary, a preprocessing phase
  needs to be done to make sure that all icon starting substrings are
  marked in the `icons` dictionary.  If this variable is false, this
  extra processing step will not be done; this is provided if the user
  wants to specify their own properly-validated icons dictionary and
  wishes to avoid a redundant step.
  
`emojiModuleNames: list[str] = ['emoji', 'emojis', 'emoji_data_python', 'unicodedata']` (_command line option:_ `--emoji-modules`)

: The list of names of supported emoji modules that the EmPy system
  will attempt t use at startup.

`emojiNotFoundIsError: bool = True` (_command line option:_ `--ignore-emoji-not-found`)

: If true, a non-existing emoji is an error.

`useBinary: bool = False` (_command line option:_ `-u/--binary/--unicode`, _environment variable:_ `EMPY_BINARY`)

: If true, open files in binary mode.

`inputEncoding: str = 'utf-8'` (_command line option:_ `--input-encoding=E`, _environment variable:_ `EMPY_INPUT_ENCODING`)

: The file input encoding to use.  This needs to be set before files
  are opened to take effect.

`outputEncoding: str = 'utf-8'` (_command line option:_ `--output-encoding=E`, _environment variable:_ `EMPY_OUTPUT_ENCODING`)

: The file output encoding to use.  This needs to be set before files
  are opened to take effect.
  
`inputErrors: str = 'strict'` (_command line option:_ `--input-errors=E`, _environment variable:_ `EMPY_INPUT_ERRORS`)

: the file input error handler to use.  This needs to be set before files
  are opened to take effect.

`outputErrors: str = 'strict'` (_command line option:_ `--output-errors=E`, _environment variable:_ `EMPY_OUTPUT_ERRORS`)

: The file output error handler to use.  This needs to be set before files
  are opened to take effect.

`normalizationForm: str = 'NFKC'` (_command line option:_ `-z/--normalization-form=F`)

: The normalization form to use when applying diacritic combiners.
  Set to `None` or `''` in order to skip normalization.
  
`autoPlayDiversions: bool = True` (_command line option:_ `--no-auto-play-diversions`)

: If diversions are extant when an interpreter is ready to exist, if
  this variable is true then those diversions will be undiverted to
  the output stream in lexicographical order by name.

`expandUserConstructions: bool = True` (_command line option:_ `--no-expand-user`)

: If true, when processing configuration files, call
  `os.path.expanduser` on each filename to expand `~` and `~user`
  constructions.
  
`configVariableName: str = '_'` (_command line option:_ `--config-variable=NAME`)

: When processing configuration files, the existing configuration
  object can be referenced as a variable.  This indicates its name.

`successCode: int = 0` (_command line option:_ `--success-code=N`)

: The exit code to return when a processing is successful.

`failureCode: int = 1` (_command line option:_ `--failure-code=N`)

: The exit code to return when an error occurs during processing.

`unknownCode: int = 2` (_command line option:_ `--unknown-code=N`)

: The exit code to return when a configuration error is found (and
  processing never starts).
  
`checkVariables: bool = True` (_command line option:_ `--no-check-variables`)

: If true, configuration variables will be checked to make sure they
  are known variables and have the proper type on assignment.

`pathSeparator: str = ';'` (Windows) or `':'` (others) ` ` (_command line option:_ `--path-separator`)

: The path separator to use when specifying multiple filenames with
  `-c/--config-file=FILENAME`.  Defaults to `;` on Windows and `:` on other
  platforms.

`supportModules: bool = True` (_command line option:_ `-g/--disable-modules`)

: Enable EmPy module support?  Requires Python 3.4 or greater.

`moduleExtension: str = '.em'` (_command line option:_ `--module-extension`)

: The filename extension to use for modules.

`moduleFinderIndex: int = 0` (_command line option:_ `--module-finder-index`)

: The integer index where to insert the module finder (required for
  module support) into the meta path.  0 would insert the finder into
  the beginning of the meta path (so it is checked before any other
  finder, such as the native Python module finder), 1 would place it
  in the second position, etc.  A negative index indicates that the
  finder should be appended to the meta path, so that it is checked
  last.

`enableImportOutput: bool = True` (_command line option:_ `-j/--disable-import-output`)

: By default output is handled normally when a module is imported; any
  markup expanded that renders output in a module will be sent to the
  output stream.  This is sometimes undesirable, so this can be
  disabled by setting this configuration variable to false.

`duplicativeFirsts: list[str] = ['(', '[', '{']`

: The list of first markup characters that may be duplicated to
  indicate variants.  For instance, `@(...)` is expression markup,
  but `@((...))` is parenthesis extension markup.

`openFunc = None`

: The file open function to use.  If `None`, defaults to either
  `codecs.open` or `open`, depending on Python version.

`controls: dict = {...}`

: The controls dictionary used by the [named escape
  markup](#named-escape-markup).

`diacritics: dict = {...}`

: The diacritic combiners dictionary used by the [diacritic
  markup](#diacritic-markup).

`icons: dict = {...}`

: The icons dictionary used by the [icon markup](#icon-markup).

`emojis: dict = {...}`

: The custom emojis dictionary which is referenced first by the [emoji
  markup](#emoji-markup).  Defaults to an empty dictionary.

:::{seealso}

The list of configuration variables is available in the `variables`
help topic and is summarized
[here](HELP.md#configuration-variables-summary).

:::

:::{versionadded} 4.0

Configuration objects were introduced in EmPy version 4.0; previously
an underused options dictionary was introduced in version 2.2.2.

:::


#### Configuration methods

The following methods are supported by configuration instances:

`__init__(**kwargs)`

: The constructor.  Takes a set of keyword arguments that are then set
  as attributes in the configuration instance.  So
  
  ```python
  config = em.Configuration(prefix='$')
  ```
  
  is a shorter form of
  
  ```python
  config = em.Configuration()
  config.prefix = '$'
  ```

`isInitialized() -> bool`

: Has this instance been initialized?  Before initialization, no
  typechecking is done even if `checkVariables` is set.

`check(inputFilename: str, outputFilename: str)`

: Check the file settings against these filenames and raise a
  `ConfigurationError` is there appears to be an inconsistency.

`has(name: str) -> bool`

: Is this name an existing configuration variable?

`get(name: str, [default: object]) -> bool`

: Get the value of this configuration variable or return this default
  if it does not exist.

`set(name: str, value: object)`

: Set the configuration variable to the given value.

`update(**kwargs)`

: Set a series of configuration variables via a set of keyword
  arguments.

`clone(deep: bool = False) -> Configuration`

: Clone this configuration and return it.  If `deep` is true, make it
  a deep copy.

`run(statements: str)`

: Execute a series of configuration commands.

`load(filename: str, [required: bool])`

: Load and execute a configuration file.  If `required` is true, raise
  an exception; if false, ignore; if `None`, use the default for this
  configuration.

`path(path: str, [required: bool])`

: Load and execute one or more configuration files separated by the
  path separator.  `required` argument is the same as for `load`
  above.

`hasEnvironment(name: str) -> bool`

: Is the given environment variable defined, regardless of its value?

`environment(name: str, [default: str, blank: str])`

: Get the value of the environment variable.  If it is not defined,
  return `default`; if it is defined but is empty, return `blank`.

`hasDefaultPrefix() -> bool`

: Is the `prefix` configuration variable set to the
  default?

`has{Full|No|Line|Fixed}Buffering() -> bool`

: Is buffering set to full, none, line, or some fixed value,
  respectively?

`createFactory([tokens: Sequence[Token]]) -> Factory`

: Create a token factory from the list of token classes and return it.
  If `tokens` is not specified, use the default list.

`adjustFactory()`

: Adjust an existing factory to take into account a non-default prefix.

`getFactory([tokens: Sequence[Token], [force: bool]])`

: Get a factory, creating one if one has not yet been created, with
  the given `tokens` list (if not specified, a default list will be
  used).  If `force` is true, then create a new one even if one
  already exists.

`resetFactory()`

: Clear the current factory, if any.

`createExtensionToken(first: str, name: str, [last: str])`

: Create a new extension token class with the first character `first`
  and with method name `name`.  If `last` is specified, use that as
  the last character; otherwise, guess for a closed form with a
  default of `first`.

`hasBinary() -> bool`

: Is binary (formerly called Unicode) support enabled?

`enableBinary([major: int, minor: int])`

: Enable binary support, conditionally if `major` and `minor` (the
  major and minor versions of Python) are specified and binary support
  is needed for this version of Python.

`disableBinary()`

: Turn off binary/Unicode support.

`isDefaultEncodingErrors([encoding: str, errors: str, asInput: bool]) -> bool`

: Are both the file encoding and file error handler the default?
  Check for input if `asInput` is true, otherwise check for output.

`getDefaultEncoding([default: str]) -> str`

: Get the current default encoding, overriding with `default` if
  desired.

`open(filename: str, mode: Optional[str] = None, buffering: int = -1, [encoding: Optional[str], errors: Optional[str], expand: bool]) -> file`

: The main wrapper around the `open`/`codecs.open` call, allowing for
  seamless file opening in both binary and non-binary mode across all
  supported Python versions.

`significatorReString() -> str`

: Return a regular expression string that will match significators in
  EmPy code with this configuration's prefix.
  
  :::{hint}
  
  It can be used in Python like this:
  
  ```python
  data = open('script.em', 'r').read()
  for result in empy.config.significatorRe().findall(data):
      string2, key2, value2, string1, key1, value1 = result
      if key1:
          print("Single line significator: {} = {}{}".format(
              key1, value1, ' (stringized)' if string1 else ''))
      else: # key2
          print("Multi-line significator: {} = {}{}".format(
              key2, value2, ' (stringized)' if string2 else ''))
  ```

  :::

`significatorRe([flags: int]) -> re.Pattern`

: Return a compiled regular expression pattern object for this
  configuration's prefix.  Override the `re` `flags` if desired.

`significatorFor(key: str) -> str`

: Return the significator variable name for this significator key.

`setContextFormat(rawFormat: str)`

: Set the context format for this configuration.  See [context
  formatting](#context-formatting).

`renderContext(context) -> str`

: Render the given context using the existing context format string.

`calculateIconsSignature() -> tuple`

: Calculate the icons signature to try to detect any accidental
  changes.

`signIcons()`

: Calculate the icons signature and update the configuration with it.

`transmogrifyIcons([icons: dict])`

: Process the icons dictionary and make sure any keys' prefixes are
  backfilled with `None` values.  This is necessary for the
  functioning of the [icon markup](#icon-markup).  This method will be
  called automatically unless `autoValidateIcons` is
  false.

`validateIcons([icons: dict])`

: Check whether the icons have possibly changed and transmogrify them
  if necessary.

`initializeEmojiModules([moduleNames: Sequence])`

: Scan for existing emoji modules and set up the appropriate internal
  data structures.  Use the list of module names in the configuration
  if `moduleNames` is not specified.

`substituteEmoji(text: str) -> str`

: Perform emoji substitution with the detected emoji modules.

`isSuccessCode(code: int) -> bool`

: Is this exit code a success code?

`isExitError(error: Optional[Error]) -> bool`

: Is this exception instance an exit error rather than a real error?

`errorToExitCode(error: Optional[Error]) -> int`

: Return an appropriate exit code for this error.

`isNotAnError(error: Optional[Error]) -> bool`

: Does this exception instance not represent an actual error?

`formatError(error: Optional[Error][, prefix: str, suffix: str]) -> str`

: Return a string representing the details of the given exception
  instance, with an optional prefix and suffix.

:::{seealso}

The list of configuration methods is available in the `methods` help
topic and is summarized [here](HELP.md#configuration-methods-summary).

:::


### Error handling

#### Error dispatchers

When an error occurs in an EmPy system, first an **error dispatcher**
is invoked.  The purpose of the dispatcher is to determine at a
high-level what should be done about the error.  A dispatcher is a
zero-argument callable which primarily determines whether the error
should be handled by the running interpreter, whether it should be
raise to the parent caller rather than handled by the interpreter, or
some other custom behavior.

When specified in the [`Interpreter` constructor](#constructor) or one
of the high-level interpreter methods (_e.g._, `file` or `string`), it
can take on a few special values:

{#error-dispatchers-table}
| Value | Meaning | Corresponding method |
| --- | --- | --- |
| `None` | Use interpreter default | -- |
| `True` | Interpreter should handle error | `dispatch` |
| `False` | Interpreter should reraise error | `reraise` |

:::{note}

For standalone interpreters and its high-level methods, the default
dispatcher is `True` (`dispatch`); that is, the interpeter will handle
the error itself.  When calling the `expand` interpreter method or the
global `expand` function, the dispatcher is `False` (`reraise`); in
other words, calls to `expand` will result in any occurring errors
being raised to the caller rather than handled by the interpteter.

:::

:::{versionadded} 4.0.1

Error dispatchers were introduced in EmPy version 4.0.1.

:::


#### Error handlers

Once an error is dispatched to the interpteter, it is handled by an
**error handler**.  An error handler is a callable object that will
respond to the error and take any necessary action.  If no
user-specified error handler is set, the default error handler is
used, which prints a formatted EmPy error message to `sys.stderr`.

An error handler is a callable object with the following signature:

> `handler(type, error, traceback) -> bool`

It takes the error type, the error instance, and the traceback object
corresponding to an exception (a tuple of which is the return value of
`sys.exc_info()`) and returns an optional Boolean.  If the return
value is true, the default handler will _also_ be invoked after the
error handler is called.  (Not explicitly returning anything will
implicitly return `None`, which is a false value.)

The current error that the interpreter has encountered is set in the
interpreter's `error` attribute (with `None` indicating no error).
The error handler can manually set this attribute to `None` to clear
the error if desired.

After the error handler(s) have been called, the interpreter will then
decide how to resolve the error.  If the `error` attribute of the
interpreter is still non-`None` and the configuration variable
`exitOnError` is true (option: `-k/--keep-going`), the
interpreter will exit.  If the `error` attribute is `None`, it will
continue running.

If the `ignoreErrors` configuration variable (option:
`-e/--ignore-errors`) is true, then no error dispatchers or error
handlers will be called.

:::{versionadded} 4.0

Error handlers were introduced in EmPy version 4.0.

:::


#### Error classses

The following error classes are used by EmPy:

{#error-classes-table}
| Class | Base class | Meaning |
| --- | --- | --- |
| `Error` | `Exception` | Base error class |
| `ConsistencyError` | `Error` | An error involving inconsistent settings has occurred |
| `ProxyError` | `ConsistencyError` | A consistency error involving misuse of the stdout proxy |
| `DiversionError` | `Error` | An error involving diversions has occurred |
| `FilterError` | `Error` | An error involving filters has occurred |
| `CoreError` | `Error` | An error involving cores has occurred |
| `ExtensionError` | `Error` | An error involving extensions has occurred |
| `StackUnderflowError` | `Error` | A stack has underflowed (internal error) |
| `UnknownEmojiError` | `Error` | An unknown emoji was requested |
| `StringError` | `Error` | An old-style string error (used internally) |
| `InvocationError` | `Error` | An error invoking the interpreter has occurred |
| `ConfigurationError` | `Error` | An error involving a bad configuration has occurred |
| `CompatibilityError` | `ConfigurationError` | An error involving backward compatibility has occurred |
| `ConfigurationFileNotFoundError` | `ConfigurationError` | A requested configuration file was missing |
| `ParseError` | `Error` | Invalid EmPy syntax was encountered |
| `TransientParseError` | `ParseError` | Invalid EmPy syntax was encountered (but may be resolved by reading further data) |

## Reference

The following reference material is available:

### Getting version and debugging information

To print the version of EmPy you have installed, run:

% em.py -V # or: --version
Welcome to EmPy version 4.2.1.

To print additional information including the Python implementation
and version, operating system, and machine type, run:

% em.py -W # or: --info
Welcome to EmPy version 4.2.1, in CPython/3.10.12, on Linux (POSIX), with x86_64, under GCC/11.4.0.

For diagnostic details (say, to report a potential problem to the
developer), run:

% em.py -Z # or: --details
Welcome to EmPy version 4.2.1, in CPython/3.10.12, on Linux (POSIX), with x86_64, under GCC/11.4.0.
Details:
- basic/context: --
- basic/framework/name: GCC
- basic/framework/version: 11.4.0
- basic/implementation: CPython
- basic/machine: x86_64
...

### Examples and testing

For quick examples of EmPy code, check out the examples throughout
this document.  For a more expansive tour of examples illustrating
EmPy features, check out tests/sample/sample.em.  For a real-world
example, check out README.md.em, which is the EmPy source file from
which this documentation is generated.

EmPy has an extensive testing system.  (If you have EmPy installed via
an operating system package that does not include the test system and
you wish to use it, [download the tarball](#getting-the-software).)

EmPy's testing system consists of the shell script test.sh and two
directories: tests and suites.  The tests directory contains the
unit/system tests, and the suites directory contains files with lists
of tests to run.  The test.sh shell script will run with any modern
Bourne-like shell.

Tests can be run changing to the directory where test.sh and both the
tests and suites directories are located, and then executing
`./test.sh` followed by the tests desired to be run following on the
command line.  For example, this runs a quick test:

% ./test.sh tests/sample/sample.em
tests/sample/sample.em (python3) [PASS]

PASSES: 1/1
All tests passed (python3) in 0.217 s = 217 ms/test.

Specifying a directory will run all the tests contained in that
directory and all its subdirectories:

% ./test.sh tests/common/trivial
tests/common/trivial/empty.em (python3) [PASS]
tests/common/trivial/long.em (python3) [PASS]
tests/common/trivial/medium.em (python3) [PASS]
tests/common/trivial/short.em (python3) [PASS]
tests/common/trivial/short_no_newline.em (python3) [PASS]

PASSES: 5/5
All tests passed (python3) in 0.400 s = 80 ms/test.

Suites can be run by using the `@` character before the filename.  A
suite is a list of tests, one per line, to run.  Blank lines and lines
starting with `#` are ignored:

% cat suites/default
# Run tests for Python versions from 3.4 up.
tests/common
tests/python2.7
tests/python3.4
tests/python3
tests/sample

% ./test.sh @suites/default
tests/common/callbacks/deregister.em (python3) [PASS]
tests/common/callbacks/get_none.em (python3) [PASS]
tests/common/callbacks/get_one.em (python3) [PASS]
...
PASSES: 566/566
All tests passed (python3).

To test a version of Python other than the default (that is, other
than a Python 3._x_ interpreter named `python3`), specify it with the
`-p` option to the test script and use that version's test suite.  To
test CPython 2.7, for instance:

% ./test.sh -p python2.7 @suites/python2.7
tests/common/callbacks/deregister.em (python2.7) [PASS]
tests/common/callbacks/get_none.em (python2.7) [PASS]
tests/common/callbacks/get_one.em (python2.7) [PASS]
...

Suites for all supported interpreters and versions are provided.  For
example, if you have PyPy 3.10 installed:

% ./test.sh -p pypy3.10 @suites/pypy3.10
tests/common/callbacks/deregister.em (pypy3.10) [PASS]
tests/common/callbacks/get_none.em (pypy3.10) [PASS]
tests/common/callbacks/get_one.em (pypy3.10) [PASS]
...

To only report errors ("quiet mode"), use the `-q` option:

% ./test.sh -q @suites/default
PASSES: 566/566
All tests passed (python3).

For more information about the testing tool, run:

% ./test.sh -h # or: --help
Usage: ./test.sh [<option>...] [--] (<file> | <directory> | @<suite>)...
Example: ./test.sh -p python3 @suites/python3

Run one or more EmPy tests, comparing the results to exemplars, and
return an exit code indicating whether all tests succeeded or whether
there were some failures.  If no tests are specified, this help is
displayed.  Test filenames, directory names, and suite names cannot
contain spaces.  All tests are run from the current working directory.

...

:::{note}

All tests can be run by specifying tests as the test directory
(`./test.sh ... tests`) and will automatically skip in the
inappropriate tests; however, specifying the version-specific suite
(_e.g._, `@suites/python3`) will be more efficient.

:::

:::{versionchanged} 4.0

A simple benchmark test system was introduced in EmPy version 2.1, and
was expanded to full unit and system test suites for all supported
versions of Python in EmPy version 4.0.

:::


### Embedding EmPy

EmPy can be easily embedded into your Python programs.  Simply ensure
that the em.py file is available in the `PYTHONPATH` and import `em`
as a module:

```python
import em

print(em)
```

To embed an interpreter, create an instance of the `Interpreter`
class.  The interpreter constructor requires keyword arguments, all
with reasonable defaults; [see here for the list](#constructor).  One
important argument to an interpreter is a
[configuration](#configuration), which, if needed, should be
constructed first and then passed into the interpreter.  If no
configuration is specified, a default instance will be created and
used:

```python
import em

config = em.Configuration(...)
interp = em.Interpreter(config=config, ...)
try:
    ... do some things with interp ...
finally:
    interp.shutdown()
```

Then call interpreter methods on it such as `write`, `evaluate`,
`execute`, `expand`, `string`, `file`, `expand`, and so on.  The full
list of interpreter methods is [here](#interpreter-methods).
Exceptions that occur during processing will be handled by the
interpreter's error handler.

:::{important}

When you create an interpreter, you must call its `shutdown` method
when you are done.  This is required to remove the proxy on
`sys.stdout` that EmPy requires for proper operation and restore your
Python environment to the state it was before creating the
interpreter.  This can be accomplished by creating the interpreter in
a `with` statement -- interpreters are also context managers -- or by
creating it and shutting it down in a `try`/`finally` statement.

This is not needed when calling the `expand` global function; it
creates and shuts down an ephemeral interpreter automatically.

:::

Calling the interpreter's `shutdown` can be handled with either with a
`try`/`finally` statement or a `with` statement:

```python
import em

interp = em.Interpreter(...)
try:
    ... do some things with the interpreter ...
finally:
    interp.shutdown()
    
# or ...

with em.Interpreter(...) as interp:
    ... do other things with the interpreter ...
```

:::{warning}

If you receive a `ProxyError` mentioning when quitting your program,
you are likely not calling the `shutdown` method on the interpreter.
Make sure to call `shutdown` so the interpreter can clean up after
itself.

:::

:::{note}

The `empy` pseudmodule is itself an instance
of the `Intrerpreter` class, and its `config` attribute is an instance
of the `Configuration` class, so within an EmPy interpreter you can
access the `Interpreter` and `Configuration` classes without needing
to import the `em` module:

```empy
@{
Configuration = empy.config.__class__
Interpreter = empy.__class__

config = Configuration(...)
interp = Interpreter(config=config, ...)
...
}@
```

:::

There is also a global `expand` function which will expand a single
string and return the results, creating and destroying an ephemeral
interpreter to do so.  You can use this function to do a one-off
expansion of, say, a large file:

```python
import em

data = open('tests/sample/sample.em').read()
print(em.expand(data))
```

If an exception occurs during `expand` processing, the exception will
be raised to the caller.


### Standard modules

A fully-functional EmPy system contains the following standard
modules.


#### `empy` pseudomodule

The pseudomodule is not an actual module, but rather the instance of
the running EmPy interpreter exposed to the EmPy system.  It is
automatically placed into the interpreter's globals and cannot be
imported explicitly.  See
[Pseudomodule/interpreter](#pseudomodule-interpreter) for details.


#### `em` module

The primary EmPy module.  It contains the `Configuration` and
`Interpreter` classes as well as all supporting logic.  An EmPy system
can be functional with only this module present if needed.
  
It also includes the following global functions:

{#details}
`details(level, [prelim, postlim, file])`
  
: Write details about the running system to the given file, which
  defaults to `sys.stdout`.  The `level` parameter is an attribute of
  the `em.Version` class (effectively an enum).  `prelim` and
  `postlim` indicate preliminary and postliminary text to output
  before and after the details (and have reasonable defaults).

  :::{note}

  This function requires the `emlib` standard module to be installed
  to function most effectively.

  :::

{#expand}
`expand(data, **kwargs) -> str`

: Create a ephemeral interpreter with the given kwargs, expand data,
  shut the interpreter down, and then return the expansion.  The
  function takes the same keyword arguments as the [`Interpreter`
  constructor](#constructor), with the following additions:

  {#expand-arguments-table}
  | Argument | Meaning | Default |
  | --- | --- | --- |
  | `dispatcher` | Dispatch errors or raise to caller? | `False` |
  | `locals` | The locals dictionary | `{}` |
  | `name` | The context filename | `"<expand>"` |

  If the markup that is being expanded causes an exception to be
  raised, by default the exception will be let through to the caller.

  :::{important}

  As with the [`Interpreter` constructor](#constructor), the order of
  the `expand` arguments has changed over time and is subject to
  change in the future, so you must use keyword arguments to prevent
  any ambiguity, _e.g._:
  
  ```python
  myConfig = em.Configuration(...)
  myGlobals = {...}
  myOutput = open(...)
  result = em.expand(source, config=myConfig, globals=myGlobals, ...)
  ```

  Attempts have been made to make the `expand` function as backward
  compatible (to 3._x_) as feasible, but some usages are ambiguous or
  do not have direct mappings to configurations.  A
  `CompatibilityError` will be raised in these cases; if you encounter
  this, redesign your use of `expand` to be compatible with [the
  modern usage](#expand).  In particular, in 3._x_, additional keyword
  arguments were used to indicate the locals dictionary; in 4._x_,
  keyword arguments are used for all arguments so the locals
  dictionary must be specified as a distinct `locals` keyword
  argument:

  ```python
  myGlobals = {...}
  myLocals = {...}
  result = em.expand(source, globals=myGlobals, locals=myLocals)
  ```

  :::

  :::{warning}

  Not all of the `Interpreter` constructor arguments are compatible
  with the `expand` function.  The `filters`, `handler`, `input` and
  `output` arguments are immediately overridden by the inherent nature
  of the ephemeral interpreter and so would not behave as expected.
  Thus, if they are specified, a `ConfigurationError` will be raised.
  For more detailed configuration of an interpreter, it's better to
  create one yourself rather than rely on `expand`.

  :::

{#invoke}
`invoke(args, **kwargs)`
  
: Invoke the EmPy system with the given command line arguments
  (`sys.argv[1:]`, not `sys.argv`) and optional string settings.  This
  is the entry point used by the main EmPy function.  The remaining
  keyword arguments correspond to the [`Interpreter`
  constructor](#constructor) arguments.

  :::{warning}

  Since the `invoke` function configures and manages the lifetime of
  an `Interpreter`, not all of the constructor arguments are
  compatible with it.  Specifically, the `filespec` and `immediately`
  arguments need to be managed by the function and so specifying a
  starting value is nonsensical.  Thus, if they are specified, a
  `ConfigurationError` will be raised.

  :::


#### `emlib` module

The EmPy supporting library.  It contains various support classes,
including the base classes `Filter` and `Hook` to assist in creating
this supporting functionality.


#### `emhelp` module

The EmPy help system.  It can be accessed from the main executable
with the `-h/--help` and `-H/--topics=TOPICS` command
line options.  If the emlib module is not available to the executable,
the help system will return an error.


#### `emdoc` module

The EmPy documentation system, used to create this document.

:::{note}

Unlike the other EmPy modules, `emdoc` requires a modern Python 3._x_
interpreter.

:::


### Using EmPy with build tools

If you're using EmPy to process documents within a build system such
as GNU Make or Ninja, you'll want to use the `-o/--output=FILENAME` (or
`-a/--append=FILENAME`) and `-d/--delete-on-error` options together.  This
will guarantee that a file will be output (or appended) to a file
without shell redirection, and that the file will be deleted if an
error occurs.  This will prevent errors from leaving a partial file
around which subsequent invocations of the build system will mistake
as being up to date.  The invocation of EmPy should look like this
(the `--` is not required if the input filename never starts with a
dash):

```shell
em.py -d -o $output -- $input
```

For GNU Make:

```make
EMPY ?= em.py
EMPY_OPTIONS ?= -d

%: %.em
        $(EMPY) $(EMPY_OPTIONS) -o $@ -- $<
```

For Ninja:

```ninja
empy = em.py
empy_options = -d

rule empy
    command = $empy $empy_options -o $out -- $in
```


### Context formatting

**Contexts** are objects which contain the filename, the line number,
the column number, and the character (Unicode code point) number to
record the location of an EmPy error during processing.

These are formatted into human-readable strings with a **context
format**, a string specifiable with `--context-format` (_configuration variable:_ `contextFormat`).  A few different mechanisms for formatting contexts are
available:

{#context-formatting-mechanisms-table}
| Mechanism | Description | Example
| --- | --- | --- |
| format | Use the `str.format` method | `{name}:{line}:{column}` |
| operator | Use the `%` operator | `%(name)s:%(line)d:%(column)d` |
| variable | Use `$` variables | `$NAME:$LINE:$COLUMN` |

The default context format is `%(name)s:%(line)d:%(column)d` and
uses the operator mechanism for backward compatibility.

When a context format is set, EmPy will attempt to detect which of the above mechanisms is needed:

{#context-formatting-criteria-table}
| Mechanism | Criteria |
| --- | --- |
| format | string begins with `format:` or does not contain a `%` |
| operator | string begins with `operator:` or contains a `%` | 
| variable | string begins with `variable:` |

  
### Data flow

**input ⟶ interpreter ⟶ diversions ⟶ filters ⟶ output**

Here, in summary, is how data flows through a working EmPy system:

1. Input comes from a source, such as an .em file on the command line,
   `sys.stdin`, a module import, or via an `empy.include` statement.

2. The interpreter processes this material as it comes in,
   processing EmPy expansions as it goes.

3. After expansion, data is then sent through the diversion layer,
   which may allow it directly through (if no diversion is in
   progress) or defer it temporarily.  Diversions that are recalled
   initiate from this point.

4. If output is disabled, the expansion is dropped.

5. Otherwise, any filters in place are then used to filter the data
   and produce filtered data as output.

6. Finally, any material surviving this far is sent to the output
   stream.  That stream is `sys.stdout` by default, but can be changed
   with the `-o/--output=FILENAME` or `-a/--append=FILENAME` options.

7. If an error occurs, execute the error handler (which by default
   prints an EmPy error).  If the `-r/--raw-errors` option is
   specified, then print a full Python traceback.  If
   `-k/--keep-going` is specified, continue processing rather than
   exit; otherwise halt.

8. On unsuccessful exit, if `-d/--delete-on-error` is specified, delete any
   specified output file.


### Glossary

The following terms with their definitions are used by EmPy:

*[callback](#callbacks)*

: The user-provided callback which is called when the custom markup
  `@<...>` is encountered.  _Custom markup and callbacks have been
  supplanted by [extensions](#extensions)._
  
*[command](#commands)*

: A processing step which is performed before or after main document
  processing.  Examples are `-D/--define=DEFN`, `-F/--file=FILENAME` or
  `-P/--preprocess=FILENAME`.

*[configuration](#configuration)*

: An object encapsulating all the configurable behavior of an
  interpreter which passed into interpreter on creation.
  Configurations can be shared between multiple interpreters.

*[context](#context-formatting)*

: An object which tracks the location of the parser in an EmPy file
  for tracking and error reporting purposes.

*[control markup](#control-markups)*

: A markup used to direct high-level control flow within an EmPy
  session.  Control markups are expressed with the `@[...]` notation.

*[core](#cores)*

: An interpreter core is a plugin which determines how the underlying
  language is evaluated, executed, serialized, and how the `@[def
  ...]` control markup works.  By default, the underlying language is
  Python.

*[custom markup](#callback)*

: The custom markup invokes a callback which is provided by the user,
  allowing any desired behavior.  Custom markup is `@<...>`.  _Custom
  markup and callbacks have been supplanted by
  [extensions](#extensions)._

*[diacritic](#diacritic-markup)*

: A markup which joins together a letter and one or more combining
  characters from a dictionary in the configuration and outputs it.
  Diacritic markup is `@^...`.

*[dispatcher](#error-dispatcher)*

: An error dispatcher determines whether to dispatch the error to the
  interpreter's error handler (`True`), to reraise the error to the
  caller (`False`), or something else.

*[diversion](#diversions)*

: A process by which output is deferred, and can be recalled later on
  demand, multiple times if desired.

*document*

: An EmPy file containing EmPy markup to expand.

*[embedding](#embedding-empy)*

: Using an EmPy system by importing the `em` module and using the API
  to create and manipulate interpreters programmatically, as opposed
  to standalone.

*[emoji](#emoji-markup)*

: A markup which looks up a Unicode code point by name via a
  customizable set of installable emoji modules, or via a dictionary
  in the configuration.  Emoji markup is `@:...:`.

*[error](#error-handling)*

: An exception thrown by a running EmPy system.  When these occur,
  they are dispatched by an error dispatcher and then (possibly)
  passed to an error handler.

*[escape](#escape-markup)*

: A markup designed to expand to a single (often non-printable)
  character, similar to escape sequences in C or other languages.
  Escape markup is `@\...`.

*executable*

: The Python executable for an EmPy system, which by default is em.py.

*expansion*

: The process of processing EmPy markups and producing output.

*[expression](#expression-markup)*

: An expression markup represents a Python expression to be evaluated,
  and replaced with the `str` of its value.  Expression markup is
  `@(...)`.

*[extension](#extensions)*

: An interpreter extension is a plugin which defines user-specifiable
  custom markups.

*file*

: An object which exhibits a file-like interface (methods such as
  `write` and `close`).

*[filter](#filters)*

: A file-like object which can be chained to other filters or the
  final stream, and can buffer, alter, or manipulate in any way the
  data sent.  Filters can be chained together in arbitrary order.

*[finalizer](#finalizers)*

: A function which is called when an interpreter exits.  Multiple
  finalizers can be added to each interpreter.

*finder*

: The `importlib` architecture for importng custom modules uses the
  meta path (`sys.meta_path`) which consists of a list of module
  finders to use.  EmPy installs its own finder in this meta path for
  EmPy module support.

*globals*

: The dictionary (or dictionary-like object) which resides inside the
  interpreter and holds the currently-defined variables.
  
*[handler](#error-handlers)*

: An error handler which is called whenever an error occurs in the
  EmPy system.  The default error handler prints details about the
  error to `sys.stderr`.

*[hook](#hooks)*

: A callable object that can be registered in a dictionary, and which
  will be invoked before, during, or after certain internal
  operations, identified by name with a string.  Some types of hooks
  can override the behavior of the EmPy interpreter.

*[icon](#icon-markup)*

: A markup which looks up a variable-length abbreviation for a string
  from a lookup table in the configuration.  Icon markup is `@|...`.

*[interpreter](#pseudomodule-interpreter)*

: The application (or class instance) which processes EmPy markup.

*locals*

: Along with the globals, a locals dictionary can be passed into
  individual EmPy API calls.

*[markup](#markup)*

: EmPy substitutions set off with a prefix (by default `@`) and
  appropriate delimiters.

*[module](#modules)*

: An EmPy module, imported with the native Python `import` statement.

*[named escape](#named-escape-markup)*

: A control character referenced by name in an escape markup,
  `@\^{...}`.

*output*

: The final destination of the result of processing an EmPy file.

*[plugin](#plugins)*

: An object which can be attached to an interpreter for custom
  functionality and implicitly retains a reference to it.  Examples
  are cores and extensions.

*[prefix](#prefix)*

: The Unicode code point (character) used to set off an expansions.
  By default, the prefix is `@`.  If set to `None`, no markup will be
  processed.

*processor*

: An extensible system which processes a group of EmPy files, usually
  arranged in a filesystem, and scans them for significators.

*proxy*

: An object which replaces the `sys.stdout` file object and allows the
  EmPy system to intercept any indirect output to `sys.stdout` (say,
  by the `print` function).

*[pseudomodule](#pseudomodule-interpreter)*

: The module-like object named `empy` (by default) which is exposed as
  a global inside every EmPy system.  The pseudomodule and the
  interpreter are in fact the same object, an instance of the
  `Interpreter` class.

*recode*

: Converting reference values representing strings (contained in the
  dictionaries corresponding to `emojis`,
  `icons`, `diacritics` or
  `controls`) into native strings for expansion.

*[shortcut](#import-shortcuts-table)*

: An abbreviation that can be used in the `-I/--import=MODULES`
  command line option; _e.g._, `--import sys:version=ver` for `from
  sys import version as ver`.

*[significator](#significator-markup)*

: A special form of an assignment markup in EmPy which can be easily
  parsed externally, primarily designed for representing uniform
  assignment across a collection of files.  Significator markup is
  `@%[!]... NL` and `@%%[!]...%% NL`.

*standalone*

: Using the EmPy system by running the `em.py` executable from the
  command line.

*[statement](#statement-markup)*

: A line of code that needs to be executed; statements do not have
  return values.  Statement markup is `@{...}`.

*stream*

: A file-like object which manages diversion and filtering.  A stack
  of these is used by the interpreter with the top one being active.
  
*system*

: A running EmPy environment.
  
*token*

: An element of EmPy parsing.  Tokens are parsed and then processed
  one at a time.

### Statistics

% wc setup.py bench.py emdoc.py emhelp.py timeline.py emlib.py em.py test.sh LICENSE.md README.md README.md.em
    75    307   2551 setup.py
   102    330   3084 bench.py
   551   1519  18487 emdoc.py
  1080   4944  47843 emhelp.py
   291    932   8404 timeline.py
  1345   4013  43224 emlib.py
  7032  24918 262309 em.py
   923   3139  23555 test.sh
    14    230   1520 LICENSE.md
  8244  35329 247737 README.md
  7605  35614 250472 README.md.em
 27262 111275 909186 total

% sha1sum setup.py bench.py emdoc.py emhelp.py timeline.py emlib.py em.py test.sh LICENSE.md README.md README.md.em
5e451dd6da2d96bc1cab865d470fb0b2f9ce0ff7  setup.py
1918abd38533653ddde61f93043f2d5a7d0aa277  bench.py
7821817cbb6d55a5e14a5d250913d5a7f813ddf9  emdoc.py
1e18843ff150236f4d0a22d85814440c6257a83d  emhelp.py
eda8b74776467e9e160722c9be198770d13ea8c5  timeline.py
c8cc5e34c26beab4c438d3b9878b9da44d0added  emlib.py
a43d49fd854f68a2047e77013384f1139775d12f  em.py
4a54d29cf0a87d2d350d6c44d05d00c0c2eed3a2  test.sh
b6db0b81a7c250442f29dcf8530c032b33662a95  LICENSE.md
eb4b9c8d8fa1349197a8fbc7825f3e0c5b8966ca  README.md
28f1f1dee6d3f31d9c2a71702be53188b25532c0  README.md.em

## End notes
### Author's notes

I originally conceived EmPy as a replacement for my [Web templating
system](http://www.alcyone.com/max/info/m4.html) which uses
[m4](https://www.gnu.org/software/m4/), a general macroprocessing
system for Unix.

Most of my Web sites use a variety of m4 files, some of which are
dynamically generated from databases, which are then scanned by a
cataloging tool to organize them hierarchically (so that, say, a
particular m4 file can understand where it is in the hierarchy, or
what the titles of files related to it are without duplicating
information); the results of the catalog are then written in database
form as an m4 file (which every other m4 file implicitly includes),
and then GNU Make converts each m4 to an HTML file by processing it.

As the Web sites got more complicated, the use of m4 (which I had
originally enjoyed for the challenge and abstractness) really started
to become an impediment to serious work; while I was very
knowledgeable about m4 -- having used it for so many years -- getting
even simple things done with it is awkward and often difficult.  Worse
yet, as I started to use Python more and more over the years, the
cataloging programs which scanned the m4 and built m4 databases were
migrated to Python and made almost trivial, but writing out huge
awkward tables of m4 definitions simply to make them accessible in
other m4 scripts started to become almost farcical.

It occurred to me what I really wanted was an all-Python solution.
But replacing what used to be the m4 files with standalone Python
programs would result in somewhat awkward programs normally consisting
mostly of unprocessed text punctuated by small portions where
variables and small amounts of code need to be substituted.  Thus the
idea was a sort of inverse of a Python interpreter: a program that
normally would just pass text through unmolested, but when it found a
special signifier would execute Python code in a normal environment.
I looked at existing Python templating systems, and didn't find
anything that appealed to me -- I wanted something where the desired
markups were simple and unobtrusive.  After considering choices of
prefixes, I settled on `@` and EmPy was born.

As I developed the tool, I realized it could have general appeal, even
to those with widely varying problems to solve, provided the core tool
they needed was an interpreter that could embed Python code inside
templated text.  As I continue to use the tool, I have been adding
features as unobtrusively as possible as I see areas that can be
improved.

A design goal of EmPy is that its feature set should work on several
levels; at any given level, if the user does not wish or need to use
features from another level, they are under no obligation to do so --
in fact, they wouldn't even need to know they exist.  If you have no
need of diversions, for instance, you are under no obligation to use
them or even to know anything about them.  If significators will not
help you organize a set of EmPy scripts globally, then you can ignore
them.  New features that are being added are whenever feasible
transparently backward compatible (except for major version releases);
if you do not need them, their introduction should not affect you in
any way.  Finally, the use of unknown prefix and escape sequences
results in errors, ensuring that they are reserved for future use.


### Acknowledgements

Questions, suggestions, bug reports, evangelism, and even complaints
from many people over the years have helped make EmPy what it is
today.  Some, but by no means all, of these people are (in
alphabetical order by surname):

- Biswapesh Chattopadhyay
- Beni Cherniavsky
- Dr. S. Candelaria de Ram
- Eric Eide
- Dinu Gherman
- Grzegorz Adam Hankiewicz
- Robert Kroeger
- Bohdan Kushnir
- Kouichi Takahashi
- Ville Vainio


### Known issues and caveats

{#security}
- A running EmPy system is just an alternate form of a Python
  interpreter; EmPy code is just as powerful as any Python code.  Thus
  it is vitally important that an EmPy system not expand EmPy markup
  from an untrusted source; this is just as unsafe and potentially
  dangerous as executing untrusted Python code.

{#speed}
- As the EmPy parser is written in Python, it is not designed for
  speed.  A compiled version designed for speed may be added in the
  future.

- To function properly, EmPy must override `sys.stdout` with a proxy
  file object, so that it can capture output of side effects and
  support diversions for each interpreter instance.  It is important
  that code executed in an environment _not_ rebind `sys.stdout`,
  although it is perfectly legal to reference it explicitly (_e.g._,
  `@sys.stdout.write("Hello world\n")`).  If one really needs to
  access the "true" stdout, then use `sys.__stdout__` instead (which
  should also not be rebound).  EmPy uses the standard Python error
  handlers when exceptions are raised in EmPy code, which print to
  `sys.stderr`.  `sys.stderr`, `sys.__stdout__`, and `sys.__stderr__`
  are never overridden by the interpreter; only `sys.stdout` is.

- If you are using multiple interpreters with distinct output files
  and are using the low-level interpreter methods (the ones not
  documented here) to perform expansion and output, the `sys.stdout`
  proxy will not be reliable.  Only the high-level interpreter methods
  (`evaluate`, `execute`, `string`, `expand`) properly use the
  protected stream stack on the `sys.stdout` proxy to guarantee valid
  output.  Either only use a single interpreter instance at a time
  (creating and shutting it down with its `shutdown` method), use the
  `-n/--no-proxy` option and only perform output with the
  `write` method on the interpreter (_i.e._, do not use any `print`
  statements in your code), or only use the high-level interpreter
  methods documented here.

- The `empy` "module" exposed through the EmPy interface (_i.e._,
  `@empy`) is an artificial module.  It is automatically exposed in
  the globals of a running interpreter and it cannot be manually
  imported with the `import` statement (nor should it be -- it is an
  artifact of the EmPy processing system and does not correspond
  directly to any .py file).

- For an EmPy statement expansion all alone on a line, _e.g._, `@{a =
  1}`, will include a blank line due to the newline following the
  closing curly brace.  To suppress this blank line, use the symmetric
  convention `@{a = 1}@`, where the final `@` markup precedes the
  newline, making it whitespace markup and thus consumed.  For
  instance:

  ````empy
  @{a = 1}
  There will be an extra newline above (following the closing brace).
  Compare this to:
  @{a = 1}@
  There will be no extra newline above.
  ````

  See [here](#idiom) for more details.

- Errors generated from within nested control structures (_e.g._,
  `@[for ...]@[if ...]...@[end if]@[end for]` will report a context
  of the start of the top-level control structure markup, not the
  innermost markup, which would be much more helpful.  This issue is
  not new to 4.0 and will be addressed in a future release.

- Errors are very literal and could be made more useful to find the
  underlying cause.

- Contexts (such as `empy.identify`) track the context of executed
  _EmPy_ code, not Python code.  This means, for instance, that blocks
  of code delimited with `@{` and `}` will identify themselves as
  appearing on the line at which the `@{` appears.  If you're
  tracking errors and want more information about the location of the
  errors from the Python code, use the `-r/--raw-errors` option, which
  will provide you with the full Python traceback.

- The `@[for]` variable specification supports tuples for tuple
  unpacking, even recursive tuples.  However, it is limited in that
  the names included may only be valid Python identifiers, not
  arbitrary Python "lvalues."  Since this is something of an
  accidental Python feature that is very unlikely to be relied on in
  practice, this is not thought to be a significant limitation.  As a
  concrete example:

  ```python
  a = [None]
  for a[0] in range(5):
      print(a)
  ```

  is valid (but strange) Python code, but the EmPy equivalent with
  `@[for a[0] in range(5)]...` is invalid.

- The `:=` assignment expression syntax ("walrus operator") for
  `while` loops and `if` statements, introduced in Python 3.8, is not
  supported in the EmPy equivalent control markups `@[while]` and
  `@[if]`.  This may be supported in the future.

- As of Python 3.10, the `with` control structure supports specifying
  multiple context managers separated by commas.  This is not yet
  supported by EmPy, but may be in a future version.  For now, just
  use nested `@[with]` control markups.


### For package maintainers

EmPy is available as a system package in most major Linux
distributions, though some have not updated to EmPy 4._x_ yet.

EmPy can be made available as an operating system distribution package
in several different ways.  Regardless of the high-level organization,
the installed .py Python files must be made available as importable
Python modules, with the additional requirement that em.py must be
made available as an executable in the default `PATH`.  If necessary,
this executable may also be named `empy`, but `em.py` is preferred --
and either way it is still important that the em.py file be available
for importing as a Python module (`em`).

:::{important}

Since EmPy 4._x_ is not fully compatible with EmPy 3._x_, I suggest
making both EmPy 3._x_ and 4._x_ packages available side by side until
4._x_ becomes more fully adopted by the community.

:::

Here is a breakdown of the contents of a release tarball:

{#release-tarball-contents-table}
| File | Description |
| --- | --- |
| em.py | Main EmPy module and executable |
| emhelp.py | Help subsystem module |
| emlib.py | Supplementary EmPy facilities module |
| emdoc.py | Documentation subsystem module |
| setup.py | `setuptools` installation script |
| ANNOUNCE.md | EmPy 4._x_ release announcement |
| HELP.md | Help topic summaries |
| LEGACY.md | Legacy user's guide (3.3.4) |
| LICENSE.md | Software license |
| README.md | README (this file) |
| README.md.em | README source file |
| doc | HTML documentation directory hierarchy |
| test.sh | Test shell script |
| tests | Tests directory hierarchy |
| suites | Test suites directory hierarchy |

They can either be bundled up into a single, monolithic package, or
divided into a series of subpackages.  Here's a suggestion for a
fleshed-out series of EmPy subpackages:

`empy-minimal`

: Just the em.py file, available as a Python module as well as an
  executable.  Note that this will not allow the use of the EmPy help
  subsystem, unless the module emhelp.py is also included.

`empy-basic`

: The .md files, all the .py files (em.py,
  emhelp.py, emlib.py, emdoc.py) available as Python modules, with the
  em.py file also available as an executable.

`empy-doc`

: The docs directory hierarchy, the top-level .md files (README.md,
  LICENSE.md, etc.) and the README EmPy source file README.md.em.
  
`empy-test`

: The test script test.sh, the tests directory, and the suites
  directory.

`empy`

: All of the above.


### Reporting bugs

If you find a bug in EmPy, please follow these steps:

1. Whittle a reproducible test case down to the smallest standalone
   example which demonstrates the issue, the smaller the better;
   
2. Collect the output of `em.py -Z` (this will provide detailed
   diagnostic details about your environment), or at least `em.py -W`
   (which provides only basic details);
   
3. [Send me an email](mailto:software@alcyone.com) with _EmPy_ in the
   Subject line including both files and a description of the problem.

Thank you!


### Release history

{#latest-release}
4.2.1 (2026 Feb 8)

: `codecs.open` is deprecated as of Python 3.14, so use `open` instead
  in binary mode; better proxy and module finder management using
  `sys` module; uniform attachment and detachment of plugins;
  converted and expanded documentation to Furo theme; better Java
  exception printing under Jython; add preinitializers,
  postinitialiers, and requirements for testing; additions to named
  escapes; add `SimpleToken` token factory.

4.2 (2024 Aug 25)

: Add module support; add support for disabling output and switch
  markup; add support for reconfiguring stdin/stdout; support repeated
  curly braces with functional expression; add backward-compatible
  `Case` abstraction for match markup; add more preprocessing and
  postprocessing commands via command line options
  (`-K/--postexecute=STATEMENT`, `-X/--expand=MARKUP`, `-Y/--postexpand=MARKUP`).

{#last-release}
4.1 (2024 Mar 24)

: Add support for extension markup `@((...))`, `@[[...]]`,
  `@{{...}}`, `@<...>`, etc., with custom callbacks retained for
  backward compatibility; add `@[match]` control support; add
  interpreter cores for overriding interpreter behavior; add more
  command line option toggles; add notion of verbose/brief errors;
  more uniform error message formatting; various documentation
  updates.

{#last-minor-release}
4.0.1 (2023 Dec 24)

: Add root context argument, serializers, and idents to interpreter;
  fix `setContext...`  methods so they also modify the currents stack;
  better backward compatibility for `expand` function and
  `CompatibilityError`; fix inconsistent stack usage with `expand`
  method; add error dispatchers, cleaner error handling and
  `ignoreErrors`; have `expand` method/function raise
  exceptions to caller; eliminate need for `FullContext` class
  distinct from `Context`; support comments in "clean" controls; add
  `--no-none-symbol` option; add clearer errors for removed literal
  markup; add `Container` support class in `emlib`; hide non-standard
  proxy attributes and methods; support string errors (why not);
  update and expand tests; help subsystem and documentation updates.

{#last-major-release}
4.0 (2023 Nov 29)

: A major revamp, refresh, and modernization.  Major new features
  include inline comments `@*...*`; backquote literals `` @`...` ``;
  chained if-then-else expressions; functional expressions `@f{...}`;
  full support for `@[try]`, `@[while ...]` and `@[with ...]`
  control markup; `@[defined ...]` control markup; stringized and
  multiline significators; named escapes `@\^{...}`; diacritics
  `@^...`; icons `@|...`; emojis `@:...:`; configurations; full
  Unicode and file buffering support; proxy now reference counted;
  hooks can override behavior; many bug fixes; an extensive builtin
  help system (`emhelp`); and rewritten and expanded documentation in
  addition to a dedicated module (`emdoc`).  Changes include
  relicensing to BSD, interpreter constructor now requires keyword
  arguments, `-d/--delete-on-error` instead of "fully buffered files";
  cleaned up environment variables; "repr" markup replaced with emoji
  markup; remove literal markups `@)`, `@]`, `@}`; context line
  markup `@!...` no longer pre-adjusts line; custom markup `@<...>`
  now parsed more sensibly; filter shortcuts removed; context now
  track column and character count; auxiliary classes moved to `emlib`
  module; use `argv` instead of `argc` for interpreter arguments.  See
  [Full list of changes between EmPy 3._x_ and
  4.0](ANNOUNCE.md#all-changes) for a more comprehensive list.

{#prior-major-release}
3.3.4a (2021 Nov 19)

: Fix an error in setup.py in the release tarball (did not affect PIP
  downloads).

3.3.4 (2019 Feb 26)

: Minor fix for a Python 3._x_ compatibility issue.

3.3.3 (2017 Feb 12)

: Fix for `empy.defined` interpreter method.

3.3.2 (2014 Jan 24)

: Additional fix for source compatibility between 2._x_ and 3.0.

3.3.1 (2014 Jan 22)

: Source compatibility for 2._x_ and 3.0; 1._x_ compatibility dropped.

3.3 (2003 Oct 27)

: Custom markup `@<...>`; remove separate pseudomodule instance for
  greater transparency; deprecate `Interpreter` attribute of
  pseudomodule; deprecate auxiliary class name attributes associated
  with pseudomodule in preparation for separate support library in
  4.0; add `--no-callback-error` [defunct] and
  `--no-bangpath-processing` [now
  `--no-ignore-bangpaths`] command line options; add
  `atToken` hook.

3.2 (2003 Oct 7)

: Reengineer hooks support to use hook instances; add `-v/--verbose`
  and `-l/--relative-path` option; reversed PEP 317 style;
  modify Unicode support to give less confusing errors in the case of
  unknown encodings and error handlers; relicensed under LGPL.

3.1.1 (2003 Sep 20)

: Add string literal `@"..."` markup; add
  `-w/--pause-at-end` option; fix improper globals collision
  error via the `sys.stdout` proxy.

3.1 (2003 Aug 8)

: Unicode support (Python 2.0 and above); add Document and Processor
  helper classes for processing significators [later moved to
  `emlib`]; add `--no-prefix` option for suppressing all
  markups.

3.0.4 (2003 Aug 7)

: Implement somewhat more robust "lvalue" parsing for `@[for]`
  construct.

3.0.3 (2003 Jul 9)

: Fix bug regarding recursive tuple unpacking using `@[for]`; add
  `empy.saveGlobals`, `empy.restoreGlobals`, and `empy.defined`
  functions.

3.0.2 (2003 Jun 19)

: `@?` and `@!` markups for changing the current context name and
  line, respectively; add `update` method to interpreter; new and
  renamed context operations, `empy.setContextName`,
  `empy.setContextLine`, `empy.pushContext`, `empy.popContext`.

3.0.1 (2003 Jun 9)

: Fix simple bug preventing command line preprocessing directives
  (`-I/--import=MODULES`, `-D/--define=DEFN`, `-E/--execute=STATEMENT`,
  `-F/--file=FILENAME`, `-P/--preprocess=FILENAME`) from executing properly;
  defensive PEP 317 compliance [defunct].

3.0 (2003 Jun 1)

: Replace substitution markup with control markup `@[...]`; support
  `@(...?...!...)` for conditional expressions; add acknowledgements
  and glossary sections to documentation; rename buffering option back
  to `-b/--buffering`; add `-m/--pseudomodule=NAME` and
  `-n/--no-proxy` for suppressing `sys.stdout` proxy; rename
  main error class to `Error`; add standalone `expand` function; add
  `--binary` and `--chunk-size` options [defunct]; reengineer parsing
  system to use tokens for easy extensibility; safeguard curly braces
  in simple expressions [now used by functional expressions]; fix bug
  involving custom `Interpreter` instances ignoring globals argument;
  `distutils` [now `setuptools`] support.

2.3 (2003 Feb 20)

: Proper and full support for concurrent and recursive interpreters;
  protection from closing the true stdout file object; detect edge
  cases of interpreter globals or `sys.stdout` proxy collisions; add
  globals manipulation functions `empy.getGlobals`, `empy.setGlobals`,
  and `empy.updateGlobals` which properly preserve the `empy`
  pseudomodule; separate usage info out into easily accessible lists
  for easier presentation; have `-h` option show simple usage and `-H`
  show extended usage [defunct]; add `NullFile` utility class.

2.2.6 (2003 Jan 30)

: Fix a bug in the `Filter.detach` method (which would not normally be
  called anyway).

2.2.5 (2003 Jan 9)

: Strip carriage returns out of executed code blocks for DOS/Windows
  compatibility.

2.2.4 (2002 Dec 23)

: Abstract Filter interface to use methods only; add `@[noop: ...]`
  substitution for completeness and block commenting [defunct].

2.2.3 (2002 Dec 16)

: Support compatibility with Jython by working around a minor
  difference between CPython and Jython in string splitting.

2.2.2 (2002 Dec 14)

: Include better docstrings for pseudomodule functions; segue to a
  dictionary-based options system for interpreters; add
  `empy.clearAllHooks` and `empy.clearGlobals`; include a short
  documentation section on embedding interpreters; fix a bug in
  significator regular expression.

2.2.1 (2002 Nov 30)

: Tweak test script to avoid writing unnecessary temporary file; add
  `Interpreter.single` method; expose `evaluate`, `execute`,
  `substitute` [defunct], and `single` methods to the pseudomodule;
  add (rather obvious) `EMPY_OPTIONS` environment variable support;
  add `empy.enableHooks` and `empy.disableHooks`; include optimization
  to transparently disable hooks until they are actually used.

2.2 (2002 Nov 21)

: Switched to `-V/--version` option for version information;
  `empy.createDiversion` for creating initially empty diversion;
  direct access to diversion objects with `empy.retrieveDiversion`;
  environment variable support; removed `--raw` long argument (use
  `-r/--raw-errors` instead); added quaternary escape code
  (well, why not).

2.1 (2002 Oct 18)

: Finalizers now separate from hooks; include a benchmark sample and
  test.sh verification script; expose `empy.string` directly;
  `-D/--define=DEFN` option for explicit defines on command line;
  remove ill-conceived support for `@else:` separator in `@[if ...]`
  substitution [defunct]; handle nested substitutions properly
  [defunct]; `@[macro ...]` substitution for creating recallable
  expansions [defunct]; add support for finalizers with `empy.atExit`
  [now `empy.appendFinalizer`].

2.0.1 (2002 Oct 8)

: Fix missing usage information; fix `after_evaluate` hook not getting
  called [defunct].

2.0 (2002 Sep 30)

: Parsing system completely revamped and simplified, eliminating a
  whole class of context-related bugs; builtin support for buffered
  filters; support for registering hooks; support for command line
  arguments; interactive mode with `-i/--interactive`; significator
  value extended to be any valid Python expression.

1.5.1 (2002 Sep 24)

: Allow `@]` to represent unbalanced close brackets in `@[...]`
  markups [defunct].

1.5 (2002 Sep 18)

: Escape codes (`@\...`); conditional and repeated expansion
  substitutions [defunct; replaced with control markups]; fix a few
  bugs involving files which do not end in newlines.

1.4 (2002 Sep 7)

: Add in-place markup `@:...:...:` [now `@$...$...$`]; fix bug with
  triple quotes; collapse conditional and protected expression
  syntaxes into the single generalized `@(...)` notation;
  `empy.setName` and `empy.setLine` functions [now
  `empy.setContextName` and `empy.setContextLine`]; true support for
  multiple concurrent interpreters with improved `sys.stdout` proxy;
  proper support for `empy.expand` to return a string evaluated in a
  subinterpreter as intended; reorganized parser class hierarchy.

1.3 (2002 Aug 24)

: Pseudomodule as true instance; move toward more verbose (and clear)
  pseudomodule function names; fleshed out diversions model; filters;
  conditional expressions; except expressions; preprocessing with
  `-P/--preprocess=FILENAME`.

1.2 (2002 Aug 16)

: Treat bangpaths as comments; `empy.quote` for the opposite process
  of `empy.expand`; significators (`@%...`  sequences); add
  `-I/--import=MODULES` and `-f/--flatten` options; much improved
  documentation.

1.1.5 (2002 Aug 15)

: Add a separate `invoke` function that can be called multiple times
  with arguments to execute multiple runs.

1.1.4 (2002 Aug 12)

: Handle strings thrown as exceptions properly; use `getopt` to
  process command line arguments; cleanup file buffering with
  `AbstractFile` [defunct]; very slight documentation and code
  cleanup.

1.1.3 (2002 Aug 9)

: Support for changing the prefix from within the `empy` pseudomodule
  [defunct; now in configuration].

1.1.2 (2002 Aug 5)

: Renamed buffering option [defunct], added `-F/--file=FILENAME` option
  for interpreting Python files from the command line, fixed improper
  handling of exceptions from commands (`-E/--execute=STATEMENT`,
  `-F/--file=FILENAME`).

1.1.1 (2002 Aug 4)

: Typo bugfixes; documentation clarification.

1.1 (2002 Aug 4)

: Added option for fully buffering output [defunct; use
  `-d/--delete-on-error` instead], executing commands through the command
  line; some documentation errors fixed.

{#first-major-release}
1.0 (2002 Jul 23)

: Renamed project to EmPy.  Documentation and sample tweaks; added
  `empy.flatten` [now `empy.flattenGlobals`]; added `-a/--append=FILENAME`
  option.  First official release.

0.3 (2002 Apr 14)

: Extended "simple expression" syntax, interpreter abstraction, proper
  context handling, better error handling, explicit file inclusion,
  extended samples.

0.2 (2002 Apr 13)

: Bugfixes, support non-expansion of `None`s, allow choice of alternate
  prefix.

0.1.1 (2002 Apr 12)

: Bugfixes, support for Python 1.5._x_ [defunct], add
  `-r/--raw-errors` option.

{#first-early-release}
0.1 (2002 Apr 12)

: Initial early access release.


### Timelines


[![Early development](dynamic/early.png)](dynamic/early.png)

---

[![Stable maintenance](dynamic/mid.png)](dynamic/mid.png)

---

[![Recent development](dynamic/recent.png)](dynamic/recent.png)


### Contact

This software was written by [Erik Max
Francis](http://www.alcyone.com/max/).  If you use this software, have
suggestions for future releases, or bug reports or problems with this
documentation, [I'd love to hear about
it](mailto:software@alcyone.com).

Even if you try out EmPy for a project and find it unsuitable, I'd
like to know what stumbling blocks you ran into so they can
potentially be addressed in a future version.

I hope you enjoy using EmPy! ℰ


### About this document

This document was generated with EmPy itself using the `emdoc` module.
Both the source (README.md.em) and the resulting Markdown text
(README.md) are included in the release tarball, as is the HTML
directory hierarchy generated with Sphinx (doc).

_This documentation for EmPy version 4.2.1 was generated from README.md.em (SHA1 `28f1f1dee6d3f31d9c2a71702be53188b25532c0`, 250472 bytes) at 2026-02-08 16:22:56 using EmPy version 4.2.1, in CPython/3.10.12, on Linux (POSIX), with x86_64, under GCC/11.4.0._
