# Creator-Defined Statements

Creator-Defined Statements (CDS) allow you to add your own statements to Ren'Py's scripting language. This makes it possible to add functionality that is not supported by the current syntax.

CDS can be more flexible than the direct Python code equivalent.

For example, picking a line of dialogue at random:

    label introduction:
        python:
            greetings = ['Hello.', 'Welcome.', 'Can I help you?']
            greeting = renpy.random.choice(greetings)

        "[greeting]"

Ren'Py's parser does not know ahead of time what happens in the python block or how it should be executed. It does not do anything with this code until execution and triggers an error if an exception occurs.

Using a CDS allows you to:

- Check the correctness of the parsed syntax (For example, check that the items in the list sent to renpy.random.choice have valid text)

- Ignore incorrect data at execution (For non-critical functions, it is often better to skip the execution than to throw an exception)

- Predict Displayables (If the function uses them)

- Give you addition information during Lint (If at runtime an error was ignored you can have a report here).

For example, the above behaviour, but written as a CDS:

    python early:
        def parse_random(lexer):
            subblock_lexer = lexer.subblock_lexer()
            choices = []

            while subblock_lexer.advance():
                with subblock_lexer.catch_error():
                    statement = subblock_lexer.renpy_statement()
                    choices.append(statement)

            return choices

        def next_random(choices):
            return renpy.random.choice(choices)

        def lint_random(parsed_object):
            for i in parsed_object:
                renpy.error(renpy.check_text_tags(i.what))

        renpy.register_statement(
            name="random",
            block=True,
            parse=parse_random,
            next=next_random,
            lint=lint_random,
        )

`random` is now available as a statement:

    label introduction:
        random:
            "Hello."
            "Welcome."
            "Can I help you?"

Using a CDS does not guarantee that the execution will be successful, but the better you code your statement, the better Ren'Py can "understand" what you expect from it.

## Usage

Creator-Defined Statements (CDS) must conform to the following rules:

- They must be defined in a `python early` block.

- They cannot be used in the same file in which they are defined.

- The file containing the CDS must be loaded earlier than any file that uses it. (Since Ren'Py loads files in the Unicode sort order of their path, it generally makes sense to prefix the name of any file containing a CDS with 01 or some other small number.)

Creator-Defined Statements are registered using the `renpy.register_statement()` function. This functions takes other functions that perform operations on the content of the CDS.

For example, a new statement named `line` that allows lines of text to be specified without quotes.

    line e "These quotes will show up," Eileen said, "and don't need to be backslashed."

The parse function will be sent the lexed content for parsing. The execute function should run an operation on the parsed content. The lint function should report any errors in the parsed content.

    python early:
        def parse_smartline(lexer):
            who = lexer.simple_expression()
            what = lexer.rest()
            return (who, what)

        def execute_smartline(parsed_object):
            who, what = parsed_object
            renpy.say(eval(who), what)

        def lint_smartline(parsed_object):
            who, what = parsed_object
            try:
                eval(who)
            except Exception:
                renpy.error("Character not defined: {}".format(who))

            tte = renpy.check_text_tags(what)
            if tte:
                renpy.error(tte)

        renpy.register_statement(
            "line",
            parse=parse_smartline,
            execute=execute_smartline,
            lint=lint_smartline,
        )

## API Reference

### Lexer object

A custom statement's parse function takes an instance of a Lexer object.

class Lexer
  error(msg)
    Parameters:

      msg (str) -- Message to add to the list of detected parsing errors.

    Add msg (with the current position) to the list of detected parsing errors. This interrupts the parsing of the current statement, but does not prevent further parsing.

  require(thing, name=None)

    Try to parse thing and report an error if it cannot be done.

    If thing is a string, try to parse it using `match()`.

    Otherwise, thing must be another method on this lexer object which is called without arguments.

    If name is not specified, the name of the method will be used in the message (or thing if it's a string), otherwise name will be used.

  eol()
    Returns:

      True if the lexer is at the end of the line, else False.

    Return type:

      bool

  expect_eol()

    If not at the end of the line, raise an error.

  expect_noblock(stmt)

    Called to indicate this statement does not expect a block. If a block is found, raise an error. stmt should be a string, it will be added to the message with an error.

  expect_block(stmt)

    Called to indicate that the statement requires that a non-empty block is present. stmt should be a string, it will be added to the message with an error.

  has_block()
    Returns:

      True if the current line has a non-empty block, else False.

    Return type:

      bool

  match(re)

    Match an arbitrary regexp string.

    All of the statements in the lexer that match things are implemented in terms of this function. They first skip whitespace, then attempt to match against the line. If the match succeeds, the matched text is returned. Otherwise, None is returned, and the state of the lexer is unchanged.

  keyword(s)

    Match s as a keyword.

  name()

    Match a name. This does not match built-in keywords.

  word()
    Returns:

      The text of the matched word.

    Return type:

      str

    Match any word, including keywords.

  image_name_component()

    Match an image name component. Unlike a word, an image name component can begin with a number.

  string()

    Match a Ren'Py string.

  integer()
    Returns:

      String containing the found integer.

    Return type:

      str

    Match an integer.

  float()
    Returns:

      String containing the found floating point number.

    Return type:

      str

    Match a floating point number.

  label_name(declare=False)

    Match a label name, either absolute or relative. If declare is true, then the global label name is set. (Note that this does not actually declare the label - the statement is required to do that by returning it from the label function.)

  simple_expression()

    Match a simple Python expression, returns it as a string. This is often used when you expect a variable name. It is not recommended to change the result. The correct action is to evaluate the result in the future.

  delimited_python(delim)

    Match a Python expression that ends in a delim, for example ':'. This is often used when you expect a condition until the delimiter. It is not recommended to change the result. The correct action is to evaluate the result in the future. This raises an error if end of line is reached before the delimiter.

  arguments()

    This must be called before the parentheses with the arguments list, if they are not specified returns None, otherwise returns an object representing the arguments to a function call. This object has an `evaluate` method on it that takes an optional scope dictionary, and returns a tuple in which the first component is a tuple of positional arguments, and the second component is a dictionary of keyword arguments.

  rest()

    Skip whitespace, then return the rest of the line.

  checkpoint()

    Return an opaque object representing the current state of the lexer.

  revert(o)

    When o is the object returned from checkpoint(), reverts the state of the lexer to what it was when checkpoint() was called. (This is used for backtracking.)

  subblock_lexer()
    Returns:

      A Lexer for the block associated with the current line.

  advance()

    In a subblock lexer, advance to the next line. This must be called before the first line, so the first line can be parsed. Return True if we've successfully advanced to a line in the block, or False if we have advanced beyond all lines in the block.

  renpy_statement()

    When called, this parses the current line as a Ren'Py script statement, generating an error if this is not possible. This method returns an opaque object that can be returned from the next function passed to `renpy.register_statement()`, or passed to `renpy.jump()` or `renpy.call()`. This object should not be stored except as part of the parse result of the statement.

    When the statement returned from this completes, control is transferred to the statement after the creator-defined statement. (Which might be the statement created using post_execute).

  renpy_block(empty=False)

    Parse all of the remaining lines in the current block as Ren'Py script, and return a SubParse corresponding to the first statement in the block. The block is chained together such that all statements in the block are run, and then control is transferred to the statement after this creator-defined statement.

    Note that this parses the current block. In the more likely case that you'd like to parse the subblock of the current statement, the correct way to do that is:

        def mystatement_parse(l):

            l.require(':')
            l.expect_eol()
            l.expect_block("mystatement")

            child = l.subblock_lexer().renpy_block()

            return { "child" : child }

    empty

      If True, allows an empty block to be parsed. (An empty block is equivalent to a block with a single `pass` statement.)

      If False, an empty block triggers an error.

  catch_error()

    This is a context decorator, used in conjunction with the with statement, that catches and reports lexer errors inside its context block, then continues after the block.

    Here's an example of how it can be used to report multiple errors in a single subblock.

        def mystatement_parse(l):

            l.require(':')
            l.expect_eol()
            l.expect_block("mystatement")

            strings = [ ]
            ll = l.subblock_lexer()

            while ll.advance():
                with ll.catch_error():
                    strings.append(ll.require(ll.string))
                    ll.expect_noblock("string inside mystatement")
                    ll.expect_eol()

            return { "strings" : strings }

### Lint Utility Functions

These functions are useful when writing lint functions.