# What’s New In Python 3.7

This article explains the new features in Python 3.7, compared to 3.6.

For full details, see the changelog.

Note

Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.7 moves towards release, so it’s worth checking back even after reading earlier versions.

## Summary – Release highlights

## New Features

### PEP 538: Legacy C Locale Coercion

An ongoing challenge within the Python 3 series has been determining a sensible default strategy for handling the “7-bit ASCII” text encoding assumption currently implied by the use of the default C locale on non-Windows platforms.

PEP 538 updates the default interpreter command line interface to automatically coerce that locale to an available UTF-8 based locale as described in the documentation of the new `PYTHONCOERCECLOCALE` environment variable. Automatically setting `LC_CTYPE` this way means that both the core interpreter and locale-aware C extensions (such as `readline`) will assume the use of UTF-8 as the default text encoding, rather than ASCII.

The platform support definition in PEP 11 has also been updated to limit full text handling support to suitably configured non-ASCII based locales.

As part of this change, the default error handler for `stdin` and `stdout` is now `surrogateescape` (rather than `strict`) when using any of the defined coercion target locales (currently `C.UTF-8`, `C.utf8`, and `UTF-8`). The default error handler for `stderr` continues to be `backslashreplace`, regardless of locale.

Locale coercion is silent by default, but to assist in debugging potentially locale related integration problems, explicit warnings (emitted directly on `stderr` can be requested by setting `PYTHONCOERCECLOCALE=warn`. This setting will also cause the Python runtime to emit a warning if the legacy C locale remains active when the core interpreter is initialized.

See also

PEP 538 – Coercing the legacy C locale to a UTF-8 based locale
  PEP written and implemented by Nick Coghlan.

### PEP 553: Built-in breakpoint()

PEP 553 describes a new built-in called `breakpoint()` which makes it easy and consistent to enter the Python debugger. Built-in `breakpoint()` calls `sys.breakpointhook()`. By default, this latter imports `pdb` and then calls `pdb.set_trace()`, but by binding `sys.breakpointhook()` to the function of your choosing, `breakpoint()` can enter any debugger. Or, the environment variable `PYTHONBREAKPOINT` can be set to the callable of your debugger of choice. Set `PYTHONBREAKPOINT=0` to completely disable built-in `breakpoint()`.

See also

PEP 553 – Built-in breakpoint()
  PEP written and implemented by Barry Warsaw

### PEP 539: A New C-API for Thread-Local Storage in CPython

While Python provides a C API for thread-local storage support; the existing Thread Local Storage (TLS) API has used `int` to represent TLS keys across all platforms. This has not generally been a problem for officially-support platforms, but that is neither POSIX-compliant, nor portable in any practical sense.

PEP 539 changes this by providing a new Thread Specific Storage (TSS) API to CPython which supersedes use of the existing TLS API within the CPython interpreter, while deprecating the existing API. The TSS API uses a new type `Py_tss_t` instead of `int` to represent TSS keys–an opaque type the definition of which may depend on the underlying TLS implementation. Therefore, this will allow to build CPython on platforms where the native TLS key is defined in a way that cannot be safely cast to `int`.

Note that on platforms where the native TLS key is defined in a way that cannot be safely cast to `int`, all functions of the existing TLS API will be no-op and immediately return failure. This indicates clearly that the old API is not supported on platforms where it cannot be used reliably, and that no effort will be made to add such support.

See also

PEP 539 – A New C-API for Thread-Local Storage in CPython
  PEP written by Erik M. Bray; implementation by Masayuki Yamamoto.

### PEP 562: Customization of access to module attributes

It is sometimes convenient to customize or otherwise have control over access to module attributes. A typical example is managing deprecation warnings. Typical workarounds are assigning `__class__` of a module object to a custom subclass of `types.ModuleType` or replacing the `sys.modules` item with a custom wrapper instance. This procedure is now simplified by recognizing `__getattr__` defined directly in a module that would act like a normal `__getattr__()` method, except that it will be defined on module instances.

See also

PEP 562 – Module `__getattr__` and `__dir__`
  PEP written and implemented by Ivan Levkivskyi

### PEP 563: Postponed evaluation of annotations

The advent of type hints in Python uncovered two glaring usability issues with the functionality of annotations added in PEP 3107 and refined further in PEP 526:

- annotations could only use names which were already available in the current scope, in other words they didn’t support forward references of any kind; and
- annotating source code had adverse effects on startup time of Python programs.

Both of these issues are fixed by postponing the evaluation of annotations. Instead of compiling code which executes expressions in annotations at their definition time, the compiler stores the annotation in a string form equivalent to the AST of the expression in question. If needed, annotations can be resolved at runtime using `typing.get_type_hints()`. In the common case where this is not required, the annotations are cheaper to store (since short strings are interned by the interpreter) and make startup time faster.

Usability-wise, annotations now support forward references, making the following syntax valid:

    class C:
        @classmethod
        def from_string(cls, source: str) -> C:
            ...

        def validate_b(self, obj: B) -> bool:
            ...

    class B:
        ...

Since this change breaks compatibility, the new behavior can be enabled on a per-module basis in Python 3.7 using a `__future__` import, like this:

    from __future__ import annotations

It will become the default in Python 4.0.

See also

PEP 563 – Postponed evaluation of annotations
  PEP written and implemented by Łukasz Langa.

### PEP 564: Add new time functions with nanosecond resolution

Add six new “nanosecond” variants of existing functions to the `time` module:

- `time.clock_gettime_ns()`
- `time.clock_settime_ns()`
- `time.monotonic_ns()`
- `time.perf_counter_ns()`
- `time.process_time_ns()`
- `time.time_ns()`

While similar to the existing functions without the `_ns` suffix, they provide nanosecond resolution: they return a number of nanoseconds as a Python `int`.

The `time.time_ns()` resolution is 3 times better than the `time.time()` resolution on Linux and Windows.

See also

PEP 564 – Add new time functions with nanosecond resolution
  PEP written and implemented by Victor Stinner

### PEP 565: Show DeprecationWarning in `__main__`

The default handling of `DeprecationWarning` has been changed such that these warnings are once more shown by default, but only when the code triggering them is running directly in the `__main__` module. As a result, developers of single file scripts and those using Python interactively should once again start seeing deprecation warnings for the APIs they use, but deprecation warnings triggered by imported application, library and framework modules will continue to be hidden by default.

As a result of this change, the standard library now allows developers to choose between three different deprecation warning behaviours:

- `FutureWarning`: always displayed by default, recommended for warnings intended to be seen by application end users (e.g. for deprecated application configuration settings).
- `DeprecationWarning`: displayed by default only in `__main__` and when running tests, recommended for warnings intended to be seen by other Python developers where a version upgrade may result in changed behaviour or an error.
- `PendingDeprecationWarning`: displayed by default only when running tests, intended for cases where a future version upgrade will change the warning category to `DeprecationWarning` or `FutureWarning`.

Previously both `DeprecationWarning` and `PendingDeprecationWarning` were only visible when running tests, which meant that developers primarily writing single file scripts or using Python interactively could be surprised by breaking changes in the APIs they used.

See also

PEP 565 – Show DeprecationWarning in `__main__`
  PEP written and implemented by Nick Coghlan

### PEP 540: Add a new UTF-8 mode

Add a new UTF-8 mode to ignore the locale, use the UTF-8 encoding, and change `sys.stdin` and `sys.stdout` error handlers to `surrogateescape`. This mode is enabled by default in the POSIX locale, but otherwise disabled by default.

The new `-X` `utf8` command line option and `PYTHONUTF8` environment variable are added to control the UTF-8 mode.

See also

PEP 540 – Add a new UTF-8 mode
  PEP written and implemented by Victor Stinner

### PEP 557: Data Classes

Adds a new module `dataclasses`. It provides a class decorator `dataclass()` which inspects the class’s variable annotations (see PEP 526) and using them, adds methods such as `__init__`, `__repr__`, and `__eq__` to the class. It is similar to `typing.NamedTuple`, but also works on classes with mutable instances, among other features.

For example:

    @dataclass
    class Point:
        x: float
        y: float
        z: float = 0.0

    p = Point(1.5, 2.5)
    print(p)   # produces "Point(x=1.5, y=2.5, z=0.0)"

See also

PEP 557 – Data Classes
  PEP written and implemented by Eric V. Smith

### PEP 567: Context Variables

Adds a new module `contextvars`, that provides APIs to manage, store, and access non-local state.

Context variables are natively supported in `asyncio` and are ready to be used without any extra configuration.

The `decimal` module was updated to use contextvars to store the current decimal context. This allows decimal operations to work with the correct context in async/await code.

See also

PEP 567 – Context Variables
  PEP written and implemented by Yury Selivanov

### New Development Mode: -X dev

Add a new “development mode”: `-X` `dev` command line option and `PYTHONDEVMODE` environment variable to enable CPython’s “development mode”, introducing additional runtime checks which are too expensive to be enabled by default. See `-X` `dev` documentation for the effects of the development mode.

### Hash-based pycs

Python has traditionally checked the up-to-dateness of bytecode cache files (i.e., `.pyc` files) by comparing the source metadata (last-modified timestamp and size) with source metadata saved in the cache file header when it was generated. While effective, this invalidation method has its drawbacks. When filesystem timestamps are too coarse, Python can miss source updates, leading to user confusion. Additionally, having a timestamp in the cache file is problematic for build reproduciblity and content-based build systems.

PEP 552 extends the pyc format to allow the hash of the source file to be used for invalidation instead of the source timestamp. Such `.pyc` files are called “hash-based”. By default, Python still uses timestamp-based invalidation and does not generate hash-based `.pyc` files at runtime. Hash-based `.pyc` files may be generated with `py_compile` or `compileall`.

Hash-based `.pyc` files come in two variants: checked and unchecked. Python validates checked hash-based `.pyc` files against the corresponding source files at runtime but doesn’t do so for unchecked hash-based pycs. Unchecked hash-based `.pyc` files are a useful performance optimization for environments where a system external to Python (e.g., the build system) is responsible for keeping `.pyc` files up-to-date.

See Cached bytecode invalidation for more information.

## Other Language Changes

- More than 255 arguments can now be passed to a function, and a function can now have more than 255 parameters. (Contributed by Serhiy Storchaka in bpo-12844 and bpo-18896.)
- `bytes.fromhex()` and `bytearray.fromhex()` now ignore all ASCII whitespace, not only spaces. (Contributed by Robert Xiao in bpo-28927.)
- `ImportError` now displays module name and module `__file__` path when `from ... import ...` fails. (Contributed by Matthias Bussonnier in bpo-29546.)
- Circular imports involving absolute imports with binding a submodule to a name are now supported. (Contributed by Serhiy Storchaka in bpo-30024.)
- `object.__format__(x, '')` is now equivalent to `str(x)` rather than `format(str(self), '')`. (Contributed by Serhiy Storchaka in bpo-28974.)
- In order to better support dynamic creation of stack traces, `types.TracebackType` can now be instantiated from Python code, and the `tb_next` attribute on tracebacks is now writable. (Contributed by Nathaniel J. Smith in bpo-30579.)
- When using the `-m` switch, `sys.path[0]` is now eagerly expanded to the full starting directory path, rather than being left as the empty directory (which allows imports from the current working directory at the time when an import occurs) (Contributed by Nick Coghlan in bpo-33053.)

## New Modules

### importlib.resources

This module provides several new APIs and one new ABC for access to, opening, and reading resources inside packages. Resources are roughly akin to files inside of packages, but they needn’t be actual files on the physical file system. Module loaders can provide a `get_resource_reader()` function which returns a `importlib.abc.ResourceReader` instance to support this new API. Built-in file path loaders and zip file loaders both support this. (see the PyPI package importlib_resources as a compatible back port for older Python versions).

## Improved Modules

### argparse

The `parse_intermixed_args()` supports letting the user intermix options and positional arguments on the command line, as is possible in many unix commands. It supports most but not all argparse features. (Contributed by paul.j3 in bpo-14191.)

### binascii

The `b2a_uu()` function now accepts an optional backtick keyword argument. When it’s true, zeros are represented by `'`'` instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)

### calendar

The class `HTMLCalendar` has new class attributes which ease the customisation of the CSS classes in the produced HTML calendar. (Contributed by Oz Tiram in bpo-30095.)

### contextlib

`asynccontextmanager()` and `AbstractAsyncContextManager` have been added. (Contributed by Jelle Zijlstra in bpo-29679 and bpo-30241.)

`contextlib.AsyncExitStack` has been added. (Contributed by Alexander Mohr and Ilya Kulakov in bpo-29302.)

### cProfile

`cProfile` command line now accepts `-m module_name` as an alternative to script path. (Contributed by Sanyam Khurana in bpo-21862.)

### crypt

Added support for the Blowfish method. (Contributed by Serhiy Storchaka in bpo-31664.)

The `mksalt()` function now allows to specify the number of rounds for hashing. (Contributed by Serhiy Storchaka in bpo-31702.)

### datetime

Added the `datetime.fromisoformat` method, which constructs a `datetime` object from a string in one of the formats output by `datetime.isoformat`. (Contributed by Paul Ganssle in bpo-15873.)

### dis

The `dis()` function now is able to disassemble nested code objects (the code of comprehensions, generator expressions and nested functions, and the code used for building nested classes). (Contributed by Serhiy Storchaka in bpo-11822.)

### distutils

`README.rst` is now included in the list of distutils standard READMEs and therefore included in source distributions. (Contributed by Ryan Gonzalez in bpo-11913.)

`distutils.core.setup` now warns if the `classifiers`, `keywords` and `platforms` fields are not specified as a list or a string. (Contributed by Berker Peksag in bpo-19610.)

The `upload` command no longer tries to change CR end-of-line characters to CRLF. This fixes a corruption issue with sdists that ended with a byte equivalent to CR. (Contributed by Bo Bayles in bpo-32304.)

### http.client

Add configurable blocksize to `HTTPConnection` and `HTTPSConnection` for improved upload throughput. (Contributed by Nir Soffer in bpo-31945.)

### http.server

`SimpleHTTPRequestHandler` supports the HTTP `If-Modified-Since` header. The server returns the 304 response status if the target file was not modified after the time specified in the header. (Contributed by Pierre Quentel in bpo-29654.)

Add the parameter directory to the `SimpleHTTPRequestHandler` and the `--directory` to the command line of the module `http.server`. With this parameter, the server serves the specified directory, by default it uses the current working directory. (Contributed by Stéphane Wirtel and Julien Palard in bpo-28707.)

### hmac

The `hmac` module now has an optimized one-shot `digest()` function, which is up to three times faster than `HMAC()`. (Contributed by Christian Heimes in bpo-32433.)

### importlib

The `importlib.abc.ResourceReader` ABC was introduced to support the loading of resource from packages.

### locale

Added another argument monetary in `format_string()` of `locale`. If monetary is true, the conversion uses monetary thousands separator and grouping strings. (Contributed by Garvit in bpo-10379.)

The `getpreferredencoding()` function now always returns `'UTF-8'` on Android or in the UTF-8 mode (`-X` `utf8` option), the locale and the do_setlocale argument are ignored.

### math

New `remainder()` function, implementing the IEEE 754-style remainder operation. (Contributed by Mark Dickinson in bpo-29962.)

### os

Added support for `bytes` paths in `fwalk()`. (Contributed by Serhiy Storchaka in bpo-28682.)

Added support for file descriptors in `scandir()` on Unix. (Contributed by Serhiy Storchaka in bpo-25996.)

New function `register_at_fork()` allows registering Python callbacks to be executed on a process fork. (Contributed by Antoine Pitrou in bpo-16500.)

### pdb

`set_trace()` now takes an optional header keyword-only argument. If given, this is printed to the console just before debugging begins. (Contributed by Barry Warsaw in bpo-31389.)

`pdb` command line now accepts `-m module_name` as an alternative to script file. (Contributed by Mario Corchero in bpo-32206.)

### py_compile

`py_compile.compile()` – and by extension, `compileall` – now respects the `SOURCE_DATE_EPOCH` environment variable by unconditionally creating `.pyc` files for hash-based validation. This allows for guaranteeing reproducible builds of `.pyc` files when they are created eagerly. (Contributed by Bernhard M. Wiedemann in bpo-29708.)

### re

The flags `re.ASCII`, `re.LOCALE` and `re.UNICODE` can be set within the scope of a group. (Contributed by Serhiy Storchaka in bpo-31690.)

`re.split()` now supports splitting on a pattern like `r'\b'`, `'^$'` or `(?=-)` that matches an empty string. (Contributed by Serhiy Storchaka in bpo-25054.)

### sqlite3

`sqlite3.Connection` now exposes a `backup` method, if the underlying SQLite library is at version 3.6.11 or higher. (Contributed by Lele Gaifax in bpo-27645.)

### ssl

The `ssl` module now uses OpenSSL’s builtin API instead of `match_hostname()` to check host name or IP address. Values are validated during TLS handshake. Any cert validation error including a failing host name match now raises `SSLCertVerificationError` and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with `host_flags`. (Contributed by Christian Heimes in bpo-31399.)

Note

The improved host name check requires an OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 0.9.8 and 1.0.1 are no longer supported. LibreSSL is temporarily not supported until it gains the necessary OpenSSL 1.0.2 APIs.

The ssl module no longer sends IP addresses in SNI TLS extension. (Contributed by Christian Heimes in bpo-32185.)

`match_hostname()` no longer supports partial wildcards like `www*.example.org`. `host_flags` has partial wildcard matching disabled by default. (Contributed by Mandeep Singh in bpo-23033 and Christian Heimes in bpo-31399.)

The default cipher suite selection of the ssl module now uses a blacklist approach rather than a hard-coded whitelist. Python no longer re-enables ciphers that have been blocked by OpenSSL security update. Default cipher suite selection can be configured on compile time. (Contributed by Christian Heimes in bpo-31429.)

Added support for validating server certificates containing internationalized domain names (IDNs). As part of this change, the `ssl.SSLSocket.server_hostname` attribute now stores the expected hostname in A-label form (`"xn--pythn-mua.org"`), rather than the U-label form (`"pythön.org"`). (Contributed by Nathaniel J. Smith and Christian Heimes in bpo-28414.)

The ssl module has preliminary and experimental support for TLS 1.3 and OpenSSL 1.1.1. (Contributed by Christian Heimes in bpo-32947, bpo-20995, bpo-29136, and bpo-30622)

`SSLSocket` and `SSLObject` no longer have a public constructor. Direct instantiation was never a documented and supported feature. Instances must be created with `SSLContext` methods `wrap_socket()` and `wrap_bio()`. (Contributed by Christian Heimes in bpo-32951)

OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are available as `minimum_version` and `maximum_version`. Supported protocols are indicated by new flags like `HAS_TLSv1_1`. (Contributed by Christian Heimes in bpo-32609.)

### string

`string.Template` now lets you to optionally modify the regular expression pattern for braced placeholders and non-braced placeholders separately. (Contributed by Barry Warsaw in bpo-1198569.)

### subprocess

On Windows the default for close_fds was changed from `False` to `True` when redirecting the standard handles. It’s now possible to set close_fds to `True` when redirecting the standard handles. See `subprocess.Popen`.

This means that close_fds now defaults to `True` on all supported platforms. (Contributed by Segev Finer in bpo-19764.)

### sys

Added `sys.flags.dev_mode` flag for the new development mode.

### tkinter

Added `tkinter.ttk.Spinbox`. (Contributed by Alan Moore in bpo-32585.)

### time

The PEP 564 added six new functions with nanosecond resolution:

- `time.clock_gettime_ns()`
- `time.clock_settime_ns()`
- `time.monotonic_ns()`
- `time.perf_counter_ns()`
- `time.process_time_ns()`
- `time.time_ns()`

Add new clock identifiers:

- `time.CLOCK_BOOTTIME` (Linux): Identical to `time.CLOCK_MONOTONIC`, except it also includes any time that the system is suspended.
- `time.CLOCK_PROF` (FreeBSD, NetBSD and OpenBSD): High-resolution per-process timer from the CPU.
- `time.CLOCK_UPTIME` (FreeBSD, OpenBSD): Time whose absolute value is the time the system has been running and not suspended, providing accurate uptime measurement, both absolute and interval.

Added functions `time.thread_time()` and `time.thread_time_ns()` to get per-thread CPU time measurements. (Contributed by Antoine Pitrou in bpo-32025.)

### unicodedata

The internal `unicodedata` database has been upgraded to use Unicode 10. (Contributed by Benjamin Peterson.)

### unittest

Added new command-line option `-k` to filter tests to run with a substring or Unix shell-like pattern. For example, `python -m unittest -k foo` runs the tests `foo_tests.SomeTest.test_something`, `bar_tests.SomeTest.test_foo`, but not `bar_tests.FooTest.test_something`. (Contributed by Jonas Haag in bpo-32071.)

### unittest.mock

The `sentinel` attributes now preserve their identity when they are `copied` or `pickled`. (Contributed by Serhiy Storchaka in bpo-20804.)

New function `seal()` will disable the creation of mock children by preventing to get or set any new attribute on the sealed mock. The sealing process is performed recursively. (Contributed by Mario Corchero in bpo-30541.)

### urllib.parse

`urllib.parse.quote()` has been updated from RFC 2396 to RFC 3986, adding `~` to the set of characters that is never quoted by default. (Contributed by Christian Theune and Ratnadeep Debnath in bpo-16285.)

### uu

Function `encode()` now accepts an optional backtick keyword argument. When it’s true, zeros are represented by `'`'` instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)

### warnings

The initialization of the default warnings filters has changed as follows:

- warnings enabled via command line options (including those for `-b` and the new CPython-specific `-X dev` option) are always passed to the warnings machinery via the `sys.warnoptions` attribute.

- warnings filters enabled via the command line or the environment now have the following precedence order:

  > - the `BytesWarning` filter for `-b` (or `-bb`)
  > - any filters specified with `-W`
  > - any filters specified with `PYTHONWARNINGS`
  > - any other CPython specific filters (e.g. the `default` filter added for the new `-X dev` mode)
  > - any implicit filters defined directly by the warnings machinery

- in CPython debug builds, all warnings are now displayed by default (the implicit filter list is empty)

(Contributed by Nick Coghlan and Victor Stinner in bpo-20361, bpo-32043, and bpo-32230)

### xml.etree

ElementPath predicates in the `find()` methods can now compare text of the current node with `[. = "text"]`, not only text in children. Predicates also allow adding spaces for better readability. (Contributed by Stefan Behnel in bpo-31648.)

### xmlrpc.server

`register_function()` of `SimpleXMLRPCDispatcher` and its subclasses can be used as a decorator. (Contributed by Xiang Zhang in bpo-7769.)

### zipapp

Function `create_archive()` now accepts an optional filter argument to allow the user to select which files should be included in the archive. (Contributed by Irmen de Jong in bpo-31072.)

Function `create_archive()` now accepts an optional compressed argument to generate a compressed archive. A command line option `--compress` has also been added to support compression. (Contributed by Zhiming Wang in bpo-31638.)

## Optimizations

- Added two new opcodes: `LOAD_METHOD` and `CALL_METHOD` to avoid instantiation of bound method objects for method calls, which results in method calls being faster up to 20%. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)
- Searching some unlucky Unicode characters (like Ukrainian capital “Є”) in a string was up to 25 times slower than searching other characters. Now it is slower only by 3 times in the worst case. (Contributed by Serhiy Storchaka in bpo-24821.)
- Fast implementation from standard C library is now used for functions `erf()` and `erfc()` in the `math` module. (Contributed by Serhiy Storchaka in bpo-26121.)
- The `os.fwalk()` function has been sped up by 2 times. This was done using the `os.scandir()` function. (Contributed by Serhiy Storchaka in bpo-25996.)
- The `shutil.rmtree()` function has been sped up to 20–40%. This was done using the `os.scandir()` function. (Contributed by Serhiy Storchaka in bpo-28564.)
- Optimized case-insensitive matching and searching of `regular expressions`. Searching some patterns can now be up to 20 times faster. (Contributed by Serhiy Storchaka in bpo-30285.)
- `re.compile()` now converts `flags` parameter to int object if it is `RegexFlag`. It is now as fast as Python 3.5, and faster than Python 3.6 by about 10% depending on the pattern. (Contributed by INADA Naoki in bpo-31671.)
- `modify()` methods of classes `selectors.EpollSelector`, `selectors.PollSelector` and `selectors.DevpollSelector` may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola’ in bpo-30014)
- Constant folding is moved from peephole optimizer to new AST optimizer. (Contributed by Eugene Toder and INADA Naoki in bpo-29469)
- Most functions and methods in `abc` have been rewritten in C. This makes creation of abstract base classes, and calling `isinstance()` and `issubclass()` on them 1.5x faster. This also reduces Python start-up time by up to 10%. (Contributed by Ivan Levkivskyi and INADA Naoki in bpo-31333)
- Significant speed improvements to alternate constructors for `datetime.date` and `datetime.datetime` by using fast-path constructors when not constructing subclasses. (Contributed by Paul Ganssle in bpo-32403)

## Build and C API Changes

- `py_compile` and `compileall` now support the `SOURCE_DATE_EPOCH` environment variable by unconditionally building `.pyc` files for hash verification instead of potentially timestamp-based `.pyc` files. See the notes for the py_compile improvement notes for more details.
- A full copy of libffi is no longer bundled for use when building the `_ctypes` module on non-OSX UNIX platforms. An installed copy of libffi is now required when building `_ctypes` on such platforms. (Contributed by Zachary Ware in bpo-27979.)
- The fields `name` and `doc` of structures `PyMemberDef`, `PyGetSetDef`, `PyStructSequence_Field`, `PyStructSequence_Desc`, and `wrapperbase` are now of type `const char *` rather of `char *`. (Contributed by Serhiy Storchaka in bpo-28761.)
- The result of `PyUnicode_AsUTF8AndSize()` and `PyUnicode_AsUTF8()` is now of type `const char *` rather of `char *`. (Contributed by Serhiy Storchaka in bpo-28769.)
- The result of `PyMapping_Keys()`, `PyMapping_Values()` and `PyMapping_Items()` is now always a list, rather than a list or a tuple. (Contributed by Oren Milman in bpo-28280.)
- Added functions `PySlice_Unpack()` and `PySlice_AdjustIndices()`. (Contributed by Serhiy Storchaka in bpo-27867.)
- `PyOS_AfterFork()` is deprecated in favour of the new functions `PyOS_BeforeFork()`, `PyOS_AfterFork_Parent()` and `PyOS_AfterFork_Child()`. (Contributed by Antoine Pitrou in bpo-16500.)
- The Windows build process no longer depends on Subversion to pull in external sources, a Python script is used to download zipfiles from GitHub instead. If Python 3.6 is not found on the system (via `py -3.6`), NuGet is used to download a copy of 32-bit Python for this purpose. (Contributed by Zachary Ware in bpo-30450.)
- The `PyExc_RecursionErrorInst` singleton that was part of the public API has been removed as its members being never cleared may cause a segfault during finalization of the interpreter. Contributed by Xavier de Gaye in bpo-22898 and bpo-30697.
- Support for building `--without-threads` is removed. (Contributed by Antoine Pitrou in bpo-31370.).
- Added C API support for timezones with timezone constructors `PyTimeZone_FromOffset()` and `PyTimeZone_FromOffsetAndName()`, and access to the UTC singleton with `PyDateTime_TimeZone_UTC`. Contributed by Paul Ganssle in bpo-10381.

- The type of results of `PyThread_start_new_thread()` and `PyThread_get_thread_ident()`, and the id parameter of `PyThreadState_SetAsyncExc()` changed from `long` to `unsigned long`. (Contributed by Serhiy Storchaka in bpo-6532.)
- `PyUnicode_AsWideCharString()` now raises a `ValueError` if the second argument is NULL and the `wchar_t*` string contains null characters. (Contributed by Serhiy Storchaka in bpo-30708.)
- Changes to the startup sequence and the management of dynamic memory allocators mean that the long documented requirement to call `Py_Initialize()` before calling most C API functions is now relied on more heavily, and failing to abide by it may lead to segfaults in embedding applications. See the Porting to Python 3.7 section in this document and the Before Python Initialization section in the C API documentation for more details.

## Other CPython Implementation Changes

- Trace hooks may now opt out of receiving `line` events from the interpreter by setting the new `f_trace_lines` attribute to `False` on the frame being traced. (Contributed by Nick Coghlan in bpo-31344.)
- Trace hooks may now opt in to receiving `opcode` events from the interpreter by setting the new `f_trace_opcodes` attribute to `True` on the frame being traced. (Contributed by Nick Coghlan in bpo-31344.)
- Fixed some consistency problems with namespace package module attributes. Namespace module objects now have an `__file__` that is set to `None` (previously unset), and their `__spec__.origin` is also set to `None` (previously the string `"namespace"`). See bpo-32305. Also, the namespace module object’s `__spec__.loader` is set to the same value as `__loader__` (previously, the former was set to `None`). See bpo-32303.

## Deprecated

- In Python 3.8, the abstract base classes in `collections.abc` will no longer be exposed in the regular `collections` module. This will help create a clearer distinction between the concrete classes and the abstract base classes.

- Yield expressions (both `yield` and `yield from` clauses) are now deprecated in comprehensions and generator expressions (aside from the iterable expression in the leftmost `for` clause). This ensures that comprehensions always immediately return a container of the appropriate type (rather than potentially returning a generator iterator object), while generator expressions won’t attempt to interleave their implicit output with the output from any explicit yield expressions.

  In Python 3.7, such expressions emit `DeprecationWarning` when compiled, in Python 3.8+ they will emit `SyntaxError`. (Contributed by Serhiy Storchaka in bpo-10544.)

- Function `PySlice_GetIndicesEx()` is deprecated and replaced with a macro if `Py_LIMITED_API` is not set or set to the value between `0x03050400` and `0x03060000` (not including) or `0x03060100` or higher. (Contributed by Serhiy Storchaka in bpo-27867.)
- Deprecated `format()` from `locale`, use the `format_string()` instead. (Contributed by Garvit in bpo-10379.)
- Methods `MetaPathFinder.find_module()` (replaced by `MetaPathFinder.find_spec()`) and `PathEntryFinder.find_loader()` (replaced by `PathEntryFinder.find_spec()`) both deprecated in Python 3.4 now emit `DeprecationWarning`. (Contributed by Matthias Bussonnier in bpo-29576)
- Using non-integer value for selecting a plural form in `gettext` is now deprecated. It never correctly worked. (Contributed by Serhiy Storchaka in bpo-28692.)
- The `macpath` is now deprecated and will be removed in Python 3.8.
- The `importlib.abc.ResourceLoader` ABC has been deprecated in favour of `importlib.abc.ResourceReader`.
- Deprecated `sys.set_coroutine_wrapper()` and `sys.get_coroutine_wrapper()`.
- `ssl.wrap_socket()` is deprecated. Use `ssl.SSLContext.wrap_socket()` instead. (Contributed by Christian Heimes in bpo-28124.)

### Windows Only

- The python launcher, (py.exe), can accept 32 & 64 bit specifiers without having to specify a minor version as well. So `py -3-32` and `py -3-64` become valid as well as `py -3.7-32`, also the -m-64 and -m.n-64 forms are now accepted to force 64 bit python even if 32 bit would have otherwise been used. If the specified version is not available py.exe will error exit. (Contributed by Steve Barnes in bpo-30291.)
- The launcher can be run as `py -0` to produce a list of the installed pythons, with default marked with an asterisk. Running `py -0p` will include the paths. If py is run with a version specifier that cannot be matched it will also print the short form list of available specifiers. (Contributed by Steve Barnes in bpo-30362.)

## Removed

### Platform Support Removals

- FreeBSD 9 and older are no longer supported.

### API and Feature Removals

- The `os.stat_float_times()` function has been removed. It was introduced in Python 2.3 for backward compatibility with Python 2.2, and was deprecated since Python 3.1.
- Unknown escapes consisting of `'\'` and an ASCII letter in replacement templates for `re.sub()` were deprecated in Python 3.5, and will now cause an error.
- Removed support of the exclude argument in `tarfile.TarFile.add()`. It was deprecated in Python 2.7 and 3.2. Use the filter argument instead.
- The `splitunc()` function in the `ntpath` module was deprecated in Python 3.1, and has now been removed. Use the `splitdrive()` function instead.
- `collections.namedtuple()` no longer supports the verbose parameter or `_source` attribute which showed the generated source code for the named tuple class. This was part of an optimization designed to speed-up class creation. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in bpo-28638.)
- Functions `bool()`, `float()`, `list()` and `tuple()` no longer take keyword arguments. The first argument of `int()` can now be passed only as positional argument.
- Removed previously deprecated in Python 2.4 classes `Plist`, `Dict` and `_InternalDict` in the `plistlib` module. Dict values in the result of functions `readPlist()` and `readPlistFromBytes()` are now normal dicts. You no longer can use attribute access to access items of these dictionaries.
- The `asyncio.windows_utils.socketpair()` function has been removed: use directly `socket.socketpair()` which is available on all platforms since Python 3.5 (before, it wasn’t available on Windows). `asyncio.windows_utils.socketpair` was just an alias to `socket.socketpair` on Python 3.5 and newer.
- `asyncio`: The module doesn’t export `selectors` and `_overlapped` modules as `asyncio.selectors` and `asyncio._overlapped`. Replace `from asyncio import selectors` with `import selectors` for example.

## Porting to Python 3.7

This section lists previously described changes and other bugfixes that may require changes to your code.

### Changes in Python behavior

- PEP 479 is enabled for all code in Python 3.7, meaning that `StopIteration` exceptions raised directly or indirectly in coroutines and generators are transformed into `RuntimeError` exceptions. (Contributed by Yury Selivanov in bpo-32670.)

- Due to an oversight, earlier Python versions erroneously accepted the following syntax:

      f(1 for x in [1],)

      class C(1 for x in [1]):
          pass

  Python 3.7 now correctly raises a `SyntaxError`, as a generator expression always needs to be directly inside a set of parentheses and cannot have a comma on either side, and the duplication of the parentheses can be omitted only on calls. (Contributed by Serhiy Storchaka in bpo-32012 and bpo-32023.)

- When using the `-m` switch, the starting directory is now added to sys.path, rather than the current working directory. Any programs that are found to be relying on the previous behaviour will need to be updated to manipulate `sys.path` appropriately.

### Changes in the Python API

- `socketserver.ThreadingMixIn.server_close()` now waits until all non-daemon threads complete. Use daemonic threads by setting `socketserver.ThreadingMixIn.daemon_threads` to `True` to not wait until threads complete. (Contributed by Victor Stinner in bpo-31233.)

- `socketserver.ForkingMixIn.server_close()` now waits until all child processes complete. (Contributed by Victor Stinner in bpo-31151.)

- The `locale.localeconv()` function now sets temporarily the `LC_CTYPE` locale to the `LC_NUMERIC` locale in some cases. (Contributed by Victor Stinner in bpo-31900.)

- `pkgutil.walk_packages()` now raises `ValueError` if path is a string. Previously an empty list was returned. (Contributed by Sanyam Khurana in bpo-24744.)

- A format string argument for `string.Formatter.format()` is now positional-only. Passing it as a keyword argument was deprecated in Python 3.5. (Contributed by Serhiy Storchaka in bpo-29193.)

- Attributes `key`, `value` and `coded_value` of class `http.cookies.Morsel` are now read-only. Assigning to them was deprecated in Python 3.5. Use the `set()` method for setting them. (Contributed by Serhiy Storchaka in bpo-29192.)

- `Module`, `FunctionDef`, `AsyncFunctionDef`, and `ClassDef` AST nodes now have a new `docstring` field. The first statement in their body is not considered as a docstring anymore. `co_firstlineno` and `co_lnotab` of code object for class and module are affected by this change. (Contributed by INADA Naoki and Eugene Toder in bpo-29463.)

- The mode argument of `os.makedirs()` no longer affects the file permission bits of newly-created intermediate-level directories. To set their file permission bits you can set the umask before invoking `makedirs()`. (Contributed by Serhiy Storchaka in bpo-19930.)

- The `struct.Struct.format` type is now `str` instead of `bytes`. (Contributed by Victor Stinner in bpo-21071.)

- `parse_multipart()` returns now the same results as `FieldStorage`: for non-file fields, the value associated to a key is a list of strings, not bytes. (Contributed by Pierre Quentel in bpo-29979.)

- Due to internal changes in `socket` you won’t be able to `socket.fromshare()` a socket `share()`-ed in older Python versions.

- `repr` for `datetime.timedelta` has changed to include keyword arguments in the output. (Contributed by Utkarsh Upadhyay in bpo-30302.)

- Because `shutil.rmtree()` is now implemented using the `os.scandir()` function, the user specified handler onerror is now called with the first argument `os.scandir` instead of `os.listdir` when listing the direcory is failed.

- Support of nested sets and set operations in regular expressions as in Unicode Technical Standard #18 might be added in the future. This would change the syntax, so to facilitate this change a `FutureWarning` will be raised in ambiguous cases for the time being. That include sets starting with a literal `'['` or containing literal character sequences `'--'`, `'&&'`, `'~~'`, and `'||'`. To avoid a warning escape them with a backslash. (Contributed by Serhiy Storchaka in bpo-30349.)

- The result of splitting a string on a `regular expression` that could match an empty string has been changed. For example splitting on `r'\s*'` will now split not only on whitespaces as it did previously, but also on empty strings before all non-whitespace characters and just before the end of the string. The previous behavior can be restored by changing the pattern to `r'\s+'`. A `FutureWarning` was emitted for such patterns since Python 3.5.

  For patterns that match both empty and non-empty strings, the result of searching for all matches may also be changed in other cases. For example in the string `'a\n\n'`, the pattern `r'(?m)^\s*?$'` will not only match empty strings at positions 2 and 3, but also the string `'\n'` at positions 2–3. To match only blank lines, the pattern should be rewritten as `r'(?m)^[^\S\n]*$'`.

  `re.sub()` now replaces empty matches adjacent to a previous non-empty match. For example `re.sub('x*', '-', 'abxd')` returns now `'-a-b--d-'` instead of `'-a-b-d-'` (the first minus between ‘b’ and ‘d’ replaces ‘x’, and the second minus replaces an empty string between ‘x’ and ‘d’).

  (Contributed by Serhiy Storchaka in bpo-25054 and bpo-32308.)

- Change `re.escape()` to only escape regex special characters instead of escaping all characters other than ASCII letters, numbers, and `'_'`. (Contributed by Serhiy Storchaka in bpo-29995.)

- `tracemalloc.Traceback` frames are now sorted from oldest to most recent to be more consistent with `traceback`. (Contributed by Jesse Bakker in bpo-32121.)

- On OSes that support `socket.SOCK_NONBLOCK` or `socket.SOCK_CLOEXEC` bit flags, the `socket.type` no longer has them applied. Therefore, checks like `if sock.type == socket.SOCK_STREAM` work as expected on all platforms. (Contributed by Yury Selivanov in bpo-32331.)

- On Windows the default for the close_fds argument of `subprocess.Popen` was changed from `False` to `True` when redirecting the standard handles. If you previously depended on handles being inherited when using `subprocess.Popen` with standard io redirection, you will have to pass `close_fds=False` to preserve the previous behaviour, or use `STARTUPINFO.lpAttributeList`.

### Changes in the C API

- The function `PySlice_GetIndicesEx()` is considered not safe for resizable sequences. If the slice indices are not instances of `int`, but objects that implement the `__index__()` method, the sequence can be resized after passing its length to `PySlice_GetIndicesEx()`. This can lead to returning indices out of the length of the sequence. For avoiding possible problems use new functions `PySlice_Unpack()` and `PySlice_AdjustIndices()`. (Contributed by Serhiy Storchaka in bpo-27867.)

### CPython bytecode changes

- Added two new opcodes: `LOAD_METHOD` and `CALL_METHOD`. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)
- Removed the `STORE_ANNOTATION` opcode. (Contributed by Mark Shannon in bpo-32550.)

### Other CPython implementation changes

- In preparation for potential future changes to the public CPython runtime initialization API (see PEP 432 for an initial, but somewhat outdated, draft), CPython’s internal startup and configuration management logic has been significantly refactored. While these updates are intended to be entirely transparent to both embedding applications and users of the regular CPython CLI, they’re being mentioned here as the refactoring changes the internal order of various operations during interpreter startup, and hence may uncover previously latent defects, either in embedding applications, or in CPython itself. (Initially contributed by Nick Coghlan and Eric Snow as part of bpo-22257, and further updated by Nick, Eric, and Victor Stinner in a number of other issues). Some known details affected:

  > - `PySys_AddWarnOptionUnicode()` is not currently usable by embedding applications due to the requirement to create a Unicode object prior to calling Py_Initialize. Use `PySys_AddWarnOption()` instead.
  > - warnings filters added by an embedding application with `PySys_AddWarnOption()` should now more consistently take precedence over the default filters set by the interpreter

- Due to changes in the way the default warnings filters are configured, setting `Py_BytesWarningFlag` to a value greater than one is no longer sufficient to both emit `BytesWarning` messages and have them converted to exceptions. Instead, the flag must be set (to cause the warnings to be emitted in the first place), and an explicit `error::BytesWarning` warnings filter added to convert them to exceptions.

- CPython’ `ssl` module requires OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 1.0.1 has reached end of lifetime on 2016-12-31 and is no longer supported. LibreSSL is temporarily not supported as well. LibreSSL releases up to version 2.6.4 are missing required OpenSSL 1.0.2 APIs.

## Documentation

### PEP 545: Python Documentation Translations

PEP 545 describes the process to translate Python documentation, and two translations have been added:

- Japanese: https://docs.python.org/ja/ and associated GitHub repository: https://github.com/python/python-docs-ja
- French: https://docs.python.org/fr/ and associated GitHub repository: https://github.com/python/python-docs-fr

(Contributed by Julien Palard, Inada Naoki, and Victor Stinner in bpo-26546.)