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What's New In Python 3.7

Editor:Elvis Pranskevichus <elvis@magic.io>

This article explains the new features in Python 3.7, compared to 3.6. Python 3.7 was released on June 27, 2018. For full details, see the :ref:`changelog <changelog>`.

Summary -- Release Highlights

New syntax features:

Backwards incompatible syntax changes:

New library modules:

New built-in features:

Python data model improvements:

Significant improvements in the standard library:

CPython implementation improvements:

C API improvements:

Documentation improvements:

This release features notable performance improvements in many areas. The :ref:`whatsnew37-perf` section lists them in detail.

For a list of changes that may affect compatibility with previous Python releases please refer to the :ref:`porting-to-python-37` section.

New Features

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 :func:`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 needs to be enabled on a per-module basis in Python 3.7 using a :mod:`__future__` import:

from __future__ import annotations

It will become the default in Python 4.0.

.. seealso::

    :pep:`563` -- Postponed evaluation of annotations
       PEP written and implemented by Łukasz Langa.

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 or POSIX 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 :envvar:`PYTHONCOERCECLOCALE` environment variable. Automatically setting LC_CTYPE this way means that both the core interpreter and locale-aware C extensions (such as :mod:`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 :data:`~sys.stdin` and :data:`~sys.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 :data:`~sys.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 :data:`~sys.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.

While PEP 538's locale coercion has the benefit of also affecting extension modules (such as GNU readline), as well as child processes (including those running non-Python applications and older versions of Python), it has the downside of requiring that a suitable target locale be present on the running system. To better handle the case where no suitable target locale is available (as occurs on RHEL/CentOS 7, for example), Python 3.7 also implements :ref:`whatsnew37-pep540`.

.. seealso::

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

PEP 540: Forced UTF-8 Runtime Mode

The new :option:`-X` utf8 command line option and :envvar:`PYTHONUTF8` environment variable can be used to enable the CPython UTF-8 mode.

When in UTF-8 mode, CPython ignores the locale settings, and uses the UTF-8 encoding by default. The error handlers for :data:`sys.stdin` and :data:`sys.stdout` streams are set to surrogateescape.

The forced UTF-8 mode can be used to change the text handling behavior in an embedded Python interpreter without changing the locale settings of an embedding application.

While PEP 540's UTF-8 mode has the benefit of working regardless of which locales are available on the running system, it has the downside of having no effect on extension modules (such as GNU readline), child processes running non-Python applications, and child processes running older versions of Python. To reduce the risk of corrupting text data when communicating with such components, Python 3.7 also implements :ref:`whatsnew37-pep540`).

The UTF-8 mode is enabled by default when the locale is C or POSIX, and the PEP 538 locale coercion feature fails to change it to a UTF-8 based alternative (whether that failure is due to PYTHONCOERCECLOCALE=0 being set, LC_ALL being set, or the lack of a suitable target locale).

.. seealso::

    :pep:`540` -- Add a new UTF-8 mode
       PEP written and implemented by Victor Stinner

PEP 553: Built-in breakpoint()

Python 3.7 includes the new built-in :func:`breakpoint` function as an easy and consistent way to enter the Python debugger.

Built-in breakpoint() calls :func:`sys.breakpointhook`. By default, the latter imports :mod:`pdb` and then calls pdb.set_trace(), but by binding sys.breakpointhook() to the function of your choosing, breakpoint() can enter any debugger. Additionally, the environment variable :envvar:`PYTHONBREAKPOINT` can be set to the callable of your debugger of choice. Set PYTHONBREAKPOINT=0 to completely disable built-in breakpoint().

.. seealso::

    :pep:`553` -- Built-in breakpoint()
      PEP written and implemented by Barry Warsaw

PEP 539: New C API for Thread-Local Storage

While Python provides a C API for thread-local storage support; the existing :ref:`Thread Local Storage (TLS) API <thread-local-storage-api>` has used :c:type:`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 :ref:`Thread Specific Storage (TSS) API <thread-specific-storage-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 :c:type:`Py_tss_t` instead of :c:type:`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 :c:type:`int`.

Note that on platforms where the native TLS key is defined in a way that cannot be safely cast to :c:type:`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.

.. seealso::

    :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

Python 3.7 allows defining :meth:`__getattr__` on modules and will call it whenever a module attribute is otherwise not found. Defining :meth:`__dir__` on modules is now also allowed.

A typical example of where this may be useful is module attribute deprecation and lazy loading.

.. seealso::

    :pep:`562` -- Module ``__getattr__`` and ``__dir__``
       PEP written and implemented by Ivan Levkivskyi

PEP 564: New Time Functions With Nanosecond Resolution

The resolution of clocks in modern systems can exceed the limited precision of a floating point number returned by the :func:`time.time` function and its variants. To avoid loss of precision, PEP 564 adds six new "nanosecond" variants of the existing timer functions to the :mod:`time` module:

The new functions return the number of nanoseconds as an integer value.

Measurements show that on Linux and Windows the resolution of :func:`time.time_ns` is approximately 3 times better than that of :func:`time.time`.

.. seealso::

    :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 :exc:`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 :mod:`__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:

Previously both :exc:`DeprecationWarning` and :exc:`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.

.. seealso::

    :pep:`565` -- Show DeprecationWarning in ``__main__``
      PEP written and implemented by Nick Coghlan

PEP 560: Core Support for typing module and Generic Types

Initially PEP 484 was designed in such way that it would not introduce any changes to the core CPython interpreter. Now type hints and the :mod:`typing` module are extensively used by the community, so this restriction is removed. The PEP introduces two special methods :meth:`__class_getitem__` and __mro_entries__, these methods are now used by most classes and special constructs in :mod:`typing`. As a result, the speed of various operations with types increased up to 7 times, the generic types can be used without metaclass conflicts, and several long standing bugs in :mod:`typing` module are fixed.

.. seealso::

   :pep:`560` -- Core support for typing module and generic types
      PEP written and implemented by Ivan Levkivskyi

PEP 552: Hash-based .pyc Files

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 reproducibility 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 :mod:`py_compile` or :mod:`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 :ref:`pyc-invalidation` for more information.

.. seealso::

   :pep:`552` -- Deterministic pycs
      PEP written and implemented by Benjamin Peterson

PEP 545: Python Documentation Translations

PEP 545 describes the process of creating and maintaining Python documentation translations.

Three new translations have been added:

.. seealso::

   :pep:`545` -- Python Documentation Translations
      PEP written and implemented by Julien Palard, Inada Naoki, and
      Victor Stinner.

Development Runtime Mode: -X dev

The new :option:`-X` dev command line option or the new :envvar:`PYTHONDEVMODE` environment variable can be used to enable CPython's development mode. When in development mode, CPython performs additional runtime checks that are too expensive to be enabled by default. See :option:`-X` dev documentation for the full description of the effects of this mode.

Other Language Changes

New Modules

contextvars

The new :mod:`contextvars` module and a set of :ref:`new C APIs <contextvarsobjects>` introduce support for context variables. Context variables are conceptually similar to thread-local variables. Unlike TLS, context variables support asynchronous code correctly.

The :mod:`asyncio` and :mod:`decimal` modules have been updated to use and support context variables out of the box. Particularly the active decimal context is now stored in a context variable, which allows decimal operations to work with the correct context in asynchronous code.

.. seealso::

    :pep:`567` -- Context Variables
       PEP written and implemented by Yury Selivanov

dataclasses

The new :func:`~dataclasses.dataclass` decorator provides a way to declare data classes. A data class describes its attributes using class variable annotations. Its constructor and other magic methods, such as :meth:`~object.__repr__`, :meth:`~object.__eq__`, and :meth:`~object.__hash__` are generated automatically.

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)"
.. seealso::

    :pep:`557` -- Data Classes
       PEP written and implemented by Eric V. Smith

importlib.resources

The new :mod:`importlib.resources` module provides several new APIs and one new ABC for access to, opening, and reading resources inside packages. Resources are roughly similar to files inside packages, but they needn't be actual files on the physical file system. Module loaders can provide a :meth:`get_resource_reader()` function which returns a :class:`importlib.abc.ResourceReader` instance to support this new API. Built-in file path loaders and zip file loaders both support this.

Contributed by Barry Warsaw and Brett Cannon in :issue:`32248`.

.. seealso::

    `importlib_resources <http://importlib-resources.readthedocs.io/en/latest/>`_
    -- a PyPI backport for earlier Python versions.

Improved Modules

argparse

The new :meth:`ArgumentParser.parse_intermixed_args() <argparse.ArgumentParser.parse_intermixed_args>` method allows intermixing options and positional arguments. (Contributed by paul.j3 in :issue:`14191`.)

asyncio

The :mod:`asyncio` module has received many new features, usability and :ref:`performance improvements <whatsnew37-asyncio-perf>`. Notable changes include:

Several asyncio APIs have been :ref:`deprecated <whatsnew37-asyncio-deprecated>`.

binascii

The :func:`~binascii.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 :issue:`30103`.)

calendar

The :class:`~calendar.HTMLCalendar` class has new class attributes which ease the customization of CSS classes in the produced HTML calendar. (Contributed by Oz Tiram in :issue:`30095`.)

collections

collections.namedtuple() now supports default values. (Contributed by Raymond Hettinger in :issue:`32320`.)

compileall

:func:`compileall.compile_dir` learned the new invalidation_mode parameter, which can be used to enable :ref:`hash-based .pyc invalidation <whatsnew37-pep552>`. The invalidation mode can also be specified on the command line using the new --invalidation-mode argument. (Contributed by Benjamin Peterson in :issue:`31650`.)

concurrent.futures

:class:`ProcessPoolExecutor <concurrent.futures.ProcessPoolExecutor>` and :class:`ThreadPoolExecutor <concurrent.futures.ThreadPoolExecutor>` now support the new initializer and initargs constructor arguments. (Contributed by Antoine Pitrou in :issue:`21423`.)

The :class:`ProcessPoolExecutor <concurrent.futures.ProcessPoolExecutor>` can now take the multiprocessing context via the new mp_context argument. (Contributed by Thomas Moreau in :issue:`31540`.)

contextlib

The new :func:`~contextlib.nullcontext` is a simpler and faster no-op context manager than :class:`~contextlib.ExitStack`. (Contributed by Jesse-Bakker in :issue:`10049`.)

The new :func:`~contextlib.asynccontextmanager`, :class:`~contextlib.AbstractAsyncContextManager`, and :class:`~contextlib.AsyncExitStack` have been added to complement their synchronous counterparts. (Contributed by Jelle Zijlstra in :issue:`29679` and :issue:`30241`, and by Alexander Mohr and Ilya Kulakov in :issue:`29302`.)

cProfile

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

crypt

The :mod:`crypt` module now supports the Blowfish hashing method. (Contributed by Serhiy Storchaka in :issue:`31664`.)

The :func:`~crypt.mksalt` function now allows specifying the number of rounds for hashing. (Contributed by Serhiy Storchaka in :issue:`31702`.)

datetime

The new :meth:`datetime.fromisoformat() <datetime.datetime.fromisoformat>` method constructs a :class:`~datetime.datetime` object from a string in one of the formats output by :meth:`datetime.isoformat() <datetime.datetime.isoformat>`. (Contributed by Paul Ganssle in :issue:`15873`.)

The :class:`tzinfo <datetime.tzinfo>` class now supports sub-minute offsets. (Contributed by Alexander Belopolsky in :issue:`5288`.)

dbm

:mod:`dbm.dumb` now supports reading read-only files and no longer writes the index file when it is not changed.

decimal

The :mod:`decimal` module now uses :ref:`context variables <whatsnew37-pep567>` to store the decimal context. (Contributed by Yury Selivanov in :issue:`32630`.)

dis

The :func:`~dis.dis` function is now able to disassemble nested code objects (the code of comprehensions, generator expressions and nested functions, and the code used for building nested classes). The maximum depth of disassembly recursion is controlled by the new depth parameter. (Contributed by Serhiy Storchaka in :issue:`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 :issue:`11913`.)

enum

The :class:`Enum <enum.Enum>` learned the new _ignore_ class property, which allows listing the names of properties which should not become enum members. (Contributed by Ethan Furman in :issue:`31801`.)

In Python 3.8, attempting to check for non-Enum objects in :class:`Enum` classes will raise a :exc:`TypeError` (e.g. 1 in Color); similarly, attempting to check for non-Flag objects in a :class:`Flag` member will raise :exc:`TypeError` (e.g. 1 in Perm.RW); currently, both operations return :const:`False` instead and are deprecated. (Contributed by Ethan Furman in :issue:`33217`.)

functools

:func:`functools.singledispatch` now supports registering implementations using type annotations. (Contributed by Łukasz Langa in :issue:`32227`.)

gc

The new :func:`gc.freeze` function allows freezing all objects tracked by the garbage collector and excluding them from future collections. This can be used before a POSIX fork() call to make the GC copy-on-write friendly or to speed up collection. The new :func:`gc.unfreeze` functions reverses this operation. Additionally, :func:`gc.get_freeze_count` can be used to obtain the number of frozen objects. (Contributed by Li Zekun in :issue:`31558`.)

hmac

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

http.client

:class:`~http.client.HTTPConnection` and :class:`~http.client.HTTPSConnection` now support the new blocksize argument for improved upload throughput. (Contributed by Nir Soffer in :issue:`31945`.)

http.server

:class:`~http.server.SimpleHTTPRequestHandler` now 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 :issue:`29654`.)

:class:`~http.server.SimpleHTTPRequestHandler` accepts the new directory argument, in addition to the new --directory command line argument. 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 :issue:`28707`.)

The new :class:`ThreadingHTTPServer <http.server.ThreadingHTTPServer>` class uses threads to handle requests using :class:`~socketserver.ThreadingMixin`. It is used when http.server is run with -m. (Contributed by Julien Palard in :issue:`31639`.)

idlelib and IDLE

Multiple fixes for autocompletion. (Contributed by Louie Lu in :issue:`15786`.)

Module Browser (on the File menu, formerly called Class Browser), now displays nested functions and classes in addition to top-level functions and classes. (Contributed by Guilherme Polo, Cheryl Sabella, and Terry Jan Reedy in :issue:`1612262`.)

The Settings dialog (Options, Configure IDLE) has been partly rewritten to improve both appearance and function. (Contributed by Cheryl Sabella and Terry Jan Reedy in multiple issues.)

The font sample now includes a selection of non-Latin characters so that users can better see the effect of selecting a particular font. (Contributed by Terry Jan Reedy in :issue:`13802`.) The sample can be edited to include other characters. (Contributed by Serhiy Storchaka in :issue:`31860`.)

The IDLE features formerly implemented as extensions have been reimplemented as normal features. Their settings have been moved from the Extensions tab to other dialog tabs. (Contributed by Charles Wohlganger and Terry Jan Reedy in :issue:`27099`.)

Editor code context option revised. Box displays all context lines up to maxlines. Clicking on a context line jumps the editor to that line. Context colors for custom themes is added to Highlights tab of Settings dialog. (Contributed by Cheryl Sabella and Terry Jan Reedy in :issue:`33642`, :issue:`33768`, and :issue:`33679`.)

On Windows, a new API call tells Windows that tk scales for DPI. On Windows 8.1+ or 10, with DPI compatibility properties of the Python binary unchanged, and a monitor resolution greater than 96 DPI, this should make text and lines sharper. It should otherwise have no effect. (Contributed by Terry Jan Reedy in :issue:`33656`.)

New in 3.7.1:

Output over N lines (50 by default) is squeezed down to a button. N can be changed in the PyShell section of the General page of the Settings dialog. Fewer, but possibly extra long, lines can be squeezed by right clicking on the output. Squeezed output can be expanded in place by double-clicking the button or into the clipboard or a separate window by right-clicking the button. (Contributed by Tal Einat in :issue:`1529353`.)

The changes above have been backported to 3.6 maintenance releases.

importlib

The :class:`importlib.abc.ResourceReader` ABC was introduced to support the loading of resources from packages. See also :ref:`whatsnew37_importlib_resources`. (Contributed by Barry Warsaw, Brett Cannon in :issue:`32248`.)

:func:`importlib.reload` now raises :exc:`ModuleNotFoundError` if the module lacks a spec. (Contributed by Garvit Khatri in :issue:`29851`.)

:func:`importlib.find_spec` now raises :exc:`ModuleNotFoundError` instead of :exc:`AttributeError` if the specified parent module is not a package (i.e. lacks a __path__ attribute). (Contributed by Milan Oberkirch in :issue:`30436`.)

The new :func:`importlib.source_hash` can be used to compute the hash of the passed source. A :ref:`hash-based .pyc file <whatsnew37-pep552>` embeds the value returned by this function.

io

The new :meth:`TextIOWrapper.reconfigure() <io.TextIOWrapper.reconfigure>` method can be used to reconfigure the text stream with the new settings. (Contributed by Antoine Pitrou in :issue:`30526` and INADA Naoki in :issue:`15216`.)

ipaddress

The new subnet_of() and supernet_of() methods of :class:`ipaddress.IPv6Network` and :class:`ipaddress.IPv4Network` can be used for network containment tests. (Contributed by Michel Albert and Cheryl Sabella in :issue:`20825`.)

itertools

:func:`itertools.islice` now accepts :meth:`integer-like objects <object.__index__>` as start, stop, and slice arguments. (Contributed by Will Roberts in :issue:`30537`.)

locale

The new monetary argument to :func:`locale.format_string` can be used to make the conversion use monetary thousands separators and grouping strings. (Contributed by Garvit in :issue:`10379`.)

The :func:`locale.getpreferredencoding` function now always returns 'UTF-8' on Android or when in the :ref:`forced UTF-8 mode <whatsnew37-pep540>`.

logging

:class:`~logging.Logger` instances can now be pickled. (Contributed by Vinay Sajip in :issue:`30520`.)

The new :meth:`StreamHandler.setStream() <logging.StreamHandler.setStream>` method can be used to replace the logger stream after handler creation. (Contributed by Vinay Sajip in :issue:`30522`.)

It is now possible to specify keyword arguments to handler constructors in configuration passed to :func:`logging.config.fileConfig`. (Contributed by Preston Landers in :issue:`31080`.)

math

The new :func:`math.remainder` function implements the IEEE 754-style remainder operation. (Contributed by Mark Dickinson in :issue:`29962`.)

mimetypes

The MIME type of .bmp has been changed from 'image/x-ms-bmp' to 'image/bmp'. (Contributed by Nitish Chandra in :issue:`22589`.)

msilib

The new :meth:`Database.Close() <msilib.Database.Close>` method can be used to close the :abbr:`MSI` database. (Contributed by Berker Peksag in :issue:`20486`.)

multiprocessing

The new :meth:`Process.close() <multiprocessing.Process.close>` method explicitly closes the process object and releases all resources associated with it. :exc:`ValueError` is raised if the underlying process is still running. (Contributed by Antoine Pitrou in :issue:`30596`.)

The new :meth:`Process.kill() <multiprocessing.Process.kill>` method can be used to terminate the process using the :data:`SIGKILL` signal on Unix. (Contributed by Vitor Pereira in :issue:`30794`.)

Non-daemonic threads created by :class:`~multiprocessing.Process` are now joined on process exit. (Contributed by Antoine Pitrou in :issue:`18966`.)

os

:func:`os.fwalk` now accepts the path argument as :class:`bytes`. (Contributed by Serhiy Storchaka in :issue:`28682`.)

:func:`os.scandir` gained support for :ref:`file descriptors <path_fd>`. (Contributed by Serhiy Storchaka in :issue:`25996`.)

The new :func:`~os.register_at_fork` function allows registering Python callbacks to be executed at process fork. (Contributed by Antoine Pitrou in :issue:`16500`.)

Added :func:`os.preadv` (combine the functionality of :func:`os.readv` and :func:`os.pread`) and :func:`os.pwritev` functions (combine the functionality of :func:`os.writev` and :func:`os.pwrite`). (Contributed by Pablo Galindo in :issue:`31368`.)

The mode argument of :func:`os.makedirs` no longer affects the file permission bits of newly-created intermediate-level directories. (Contributed by Serhiy Storchaka in :issue:`19930`.)

:func:`os.dup2` now returns the new file descriptor. Previously, None was always returned. (Contributed by Benjamin Peterson in :issue:`32441`.)

The structure returned by :func:`os.stat` now contains the :attr:`~os.stat_result.st_fstype` attribute on Solaris and its derivatives. (Contributed by Jesús Cea Avión in :issue:`32659`.)

pathlib

The new :meth:`Path.is_mount() <pathlib.Path.is_mount>` method is now available on POSIX systems and can be used to determine whether a path is a mount point. (Contributed by Cooper Ry Lees in :issue:`30897`.)

pdb

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

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

py_compile

:func:`py_compile.compile` -- and by extension, :mod:`compileall` -- now respects the :envvar:`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 :issue:`29708`.)

pydoc

The pydoc server can now bind to an arbitrary hostname specified by the new -n command-line argument. (Contributed by Feanil Patel in :issue:`31128`.)

queue

The new :class:`~queue.SimpleQueue` class is an unbounded :abbr:`FIFO` queue. (Contributed by Antoine Pitrou in :issue:`14976`.)

re

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

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

Regular expressions compiled with the :const:`re.LOCALE` flag no longer depend on the locale at compile time. Locale settings are applied only when the compiled regular expression is used. (Contributed by Serhiy Storchaka in :issue:`30215`.)

:exc:`FutureWarning` is now emitted if a regular expression contains character set constructs that will change semantically in the future, such as nested sets and set operations. (Contributed by Serhiy Storchaka in :issue:`30349`.)

Compiled regular expression and match objects can now be copied using :func:`copy.copy` and :func:`copy.deepcopy`. (Contributed by Serhiy Storchaka in :issue:`10076`.)

signal

The new warn_on_full_buffer argument to the :func:`signal.set_wakeup_fd` function makes it possible to specify whether Python prints a warning on stderr when the wakeup buffer overflows. (Contributed by Nathaniel J. Smith in :issue:`30050`.)

socket

The new :func:`socket.getblocking() <socket.socket.getblocking>` method returns True if the socket is in blocking mode and False otherwise. (Contributed by Yury Selivanov in :issue:`32373`.)

The new :func:`socket.close` function closes the passed socket file descriptor. This function should be used instead of :func:`os.close` for better compatibility across platforms. (Contributed by Christian Heimes in :issue:`32454`.)

The :mod:`socket` module now exposes the :data:`socket.TCP_CONGESTION` (Linux 2.6.13), :data:`socket.TCP_USER_TIMEOUT` (Linux 2.6.37), and :data:`socket.TCP_NOTSENT_LOWAT` (Linux 3.12) constants. (Contributed by Omar Sandoval in :issue:`26273` and Nathaniel J. Smith in :issue:`29728`.)

Support for :data:`socket.AF_VSOCK` sockets has been added to allow communication between virtual machines and their hosts. (Contributed by Cathy Avery in :issue:`27584`.)

Sockets now auto-detect family, type and protocol from file descriptor by default. (Contributed by Christian Heimes in :issue:`28134`.)

socketserver

:meth:`socketserver.ThreadingMixIn.server_close` now waits until all non-daemon threads complete. :meth:`socketserver.ForkingMixIn.server_close` now waits until all child processes complete.

Add a new :attr:`socketserver.ForkingMixIn.block_on_close` class attribute to :class:`socketserver.ForkingMixIn` and :class:`socketserver.ThreadingMixIn` classes. Set the class attribute to False to get the pre-3.7 behaviour.

sqlite3

:class:`sqlite3.Connection` now exposes the :meth:`~sqlite3.Connection.backup` method when the underlying SQLite library is at version 3.6.11 or higher. (Contributed by Lele Gaifax in :issue:`27645`.)

The database argument of :func:`sqlite3.connect` now accepts any :term:`path-like object`, instead of just a string. (Contributed by Anders Lorentsen in :issue:`31843`.)

ssl

The :mod:`ssl` module now uses OpenSSL's builtin API instead of :func:`~ssl.match_hostname` to check a host name or an IP address. Values are validated during TLS handshake. Any certificate validation error including failing the host name check now raises :exc:`~ssl.SSLCertVerificationError` and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with :attr:`SSLContext.host_flags <ssl.SSLContext.host_flags>`. (Contributed by Christian Heimes in :issue:`31399`.)

Note

The improved host name check requires a libssl implementation compatible with OpenSSL 1.0.2 or 1.1. Consequently, OpenSSL 0.9.8 and 1.0.1 are no longer supported (see :ref:`37-platform-support-removals` for more details). The ssl module is mostly compatible with LibreSSL 2.7.2 and newer.

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

:func:`~ssl.match_hostname` no longer supports partial wildcards like www*.example.org. :attr:`SSLContext.host_flags <ssl.SSLContext.host_flags>` has partial wildcard matching disabled by default. (Contributed by Mandeep Singh in :issue:`23033` and Christian Heimes in :issue:`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 updates. Default cipher suite selection can be configured at compile time. (Contributed by Christian Heimes in :issue:`31429`.)

Validation of server certificates containing internationalized domain names (IDNs) is now supported. As part of this change, the :attr:`SSLSocket.server_hostname <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 :issue:`28414`.)

The ssl module has preliminary and experimental support for TLS 1.3 and OpenSSL 1.1.1. At the time of Python 3.7.0 release, OpenSSL 1.1.1 is still under development and TLS 1.3 hasn't been finalized yet. The TLS 1.3 handshake and protocol behaves slightly differently than TLS 1.2 and earlier, see :ref:`ssl-tlsv1_3`. (Contributed by Christian Heimes in :issue:`32947`, :issue:`20995`, :issue:`29136`, :issue:`30622` and :issue:`33618`)

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

OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are available as :attr:`SSLContext.minimum_version <ssl.SSLContext.minimum_version>` and :attr:`SSLContext.maximum_version <ssl.SSLContext.maximum_version>`. Supported protocols are indicated by several new flags, such as :data:`~ssl.HAS_TLSv1_1`. (Contributed by Christian Heimes in :issue:`32609`.)

string

:class:`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 :issue:`1198569`.)

subprocess

The :func:`subprocess.run` function accepts the new capture_output keyword argument. When true, stdout and stderr will be captured. This is equivalent to passing :data:`subprocess.PIPE` as stdout and stderr arguments. (Contributed by Bo Bayles in :issue:`32102`.)

The subprocess.run function and the :class:`subprocess.Popen` constructor now accept the text keyword argument as an alias to universal_newlines. (Contributed by Andrew Clegg in :issue:`31756`.)

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 :class:`subprocess.Popen`. This means that close_fds now defaults to True on all supported platforms. (Contributed by Segev Finer in :issue:`19764`.)

The subprocess module is now more graceful when handling :exc:`KeyboardInterrupt` during :func:`subprocess.call`, :func:`subprocess.run`, or in a :class:`~subprocess.Popen` context manager. It now waits a short amount of time for the child to exit, before continuing the handling of the KeyboardInterrupt exception. (Contributed by Gregory P. Smith in :issue:`25942`.)

sys

The new :func:`sys.breakpointhook` hook function is called by the built-in :func:`breakpoint`. (Contributed by Barry Warsaw in :issue:`31353`.)

On Android, the new :func:`sys.getandroidapilevel` returns the build-time Android API version. (Contributed by Victor Stinner in :issue:`28740`.)

The new :func:`sys.get_coroutine_origin_tracking_depth` function returns the current coroutine origin tracking depth, as set by the new :func:`sys.set_coroutine_origin_tracking_depth`. :mod:`asyncio` has been converted to use this new API instead of the deprecated :func:`sys.set_coroutine_wrapper`. (Contributed by Nathaniel J. Smith in :issue:`32591`.)

time

PEP 564 adds six new functions with nanosecond resolution to the :mod:`time` module:

New clock identifiers have been added:

The new :func:`time.thread_time` and :func:`time.thread_time_ns` functions can be used to get per-thread CPU time measurements. (Contributed by Antoine Pitrou in :issue:`32025`.)

The new :func:`time.pthread_getcpuclockid` function returns the clock ID of the thread-specific CPU-time clock.

tkinter

The new :class:`tkinter.ttk.Spinbox` class is now available. (Contributed by Alan Moore in :issue:`32585`.)

tracemalloc

:class:`tracemalloc.Traceback` behaves more like regular tracebacks, sorting the frames from oldest to most recent. :meth:`Traceback.format() <tracemalloc.Traceback.format>` now accepts negative limit, truncating the result to the abs(limit) oldest frames. To get the old behaviour, use the new most_recent_first argument to Traceback.format(). (Contributed by Jesse Bakker in :issue:`32121`.)

types

The new :class:`~types.WrapperDescriptorType`, :class:`~types.MethodWrapperType`, :class:`~types.MethodDescriptorType`, and :class:`~types.ClassMethodDescriptorType` classes are now available. (Contributed by Manuel Krebber and Guido van Rossum in :issue:`29377`, and Serhiy Storchaka in :issue:`32265`.)

The new :func:`types.resolve_bases` function resolves MRO entries dynamically as specified by PEP 560. (Contributed by Ivan Levkivskyi in :issue:`32717`.)

unicodedata

The internal :mod:`unicodedata` database has been upgraded to use Unicode 11. (Contributed by Benjamin Peterson.)

unittest

The new -k command-line option allows filtering tests by a name substring or a Unix shell-like pattern. For example, python -m unittest -k foo runs foo_tests.SomeTest.test_something, bar_tests.SomeTest.test_foo, but not bar_tests.FooTest.test_something. (Contributed by Jonas Haag in :issue:`32071`.)

unittest.mock

The :const:`~unittest.mock.sentinel` attributes now preserve their identity when they are :mod:`copied <copy>` or :mod:`pickled <pickle>`. (Contributed by Serhiy Storchaka in :issue:`20804`.)

The new :func:`~unittest.mock.seal` function allows sealing :class:`~unittest.mock.Mock` instances, which will disallow further creation of attribute mocks. The seal is applied recursively to all attributes that are themselves mocks. (Contributed by Mario Corchero in :issue:`30541`.)

urllib.parse

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

uu

The :func:`uu.encode` function now accepts an optional backtick keyword argument. When it's true, zeros are represented by '`' instead of spaces. (Contributed by Xiang Zhang in :issue:`30103`.)

uuid

The new :attr:`UUID.is_safe <uuid.UUID.is_safe>` attribute relays information from the platform about whether generated UUIDs are generated with a multiprocessing-safe method. (Contributed by Barry Warsaw in :issue:`22807`.)

:func:`uuid.getnode` now prefers universally administered MAC addresses over locally administered MAC addresses. This makes a better guarantee for global uniqueness of UUIDs returned from :func:`uuid.uuid1`. If only locally administered MAC addresses are available, the first such one found is returned. (Contributed by Barry Warsaw in :issue:`32107`.)

warnings

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

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

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

    • the BytesWarning filter for :option:`-b` (or -bb)
    • any filters specified with the :option:`-W` option
    • any filters specified with the :envvar:`PYTHONWARNINGS` environment variable
    • 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 :issue:`20361`, :issue:`32043`, and :issue:`32230`.)

Deprecation warnings are once again shown by default in single-file scripts and at the interactive prompt. See :ref:`whatsnew37-pep565` for details. (Contributed by Nick Coghlan in :issue:`31975`.)

xml.etree

:ref:`ElementPath <elementtree-xpath>` predicates in the :meth:`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 :issue:`31648`.)

xmlrpc.server

:meth:`SimpleXMLRPCDispatcher.register_function <xmlrpc.server.SimpleXMLRPCDispatcher>` can now be used as a decorator. (Contributed by Xiang Zhang in :issue:`7769`.)

zipapp

Function :func:`~zipapp.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 :issue:`31072`.)

Function :func:`~zipapp.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 :issue:`31638`.)

zipfile

:class:`~zipfile.ZipFile` now accepts the new compresslevel parameter to control the compression level. (Contributed by Bo Bayles in :issue:`21417`.)

Subdirectories in archives created by ZipFile are now stored in alphabetical order. (Contributed by Bernhard M. Wiedemann in :issue:`30693`.)

C API Changes

A new API for thread-local storage has been implemented. See :ref:`whatsnew37-pep539` for an overview and :ref:`thread-specific-storage-api` for a complete reference. (Contributed by Masayuki Yamamoto in :issue:`25658`.)

The new :ref:`context variables <whatsnew37-pep567>` functionality exposes a number of :ref:`new C APIs <contextvarsobjects>`.

The new :c:func:`PyImport_GetModule` function returns the previously imported module with the given name. (Contributed by Eric Snow in :issue:`28411`.)

The new :c:macro:`Py_RETURN_RICHCOMPARE` macro eases writing rich comparison functions. (Contributed by Petr Victorin in :issue:`23699`.)

The new :c:macro:`Py_UNREACHABLE` macro can be used to mark unreachable code paths. (Contributed by Barry Warsaw in :issue:`31338`.)

The :mod:`tracemalloc` now exposes a C API through the new :c:func:`PyTraceMalloc_Track` and :c:func:`PyTraceMalloc_Untrack` functions. (Contributed by Victor Stinner in :issue:`30054`.)

The new :c:func:`import__find__load__start` and :c:func:`import__find__load__done` static markers can be used to trace module imports. (Contributed by Christian Heimes in :issue:`31574`.)

The fields :c:member:`name` and :c:member:`doc` of structures :c:type:`PyMemberDef`, :c:type:`PyGetSetDef`, :c:type:`PyStructSequence_Field`, :c:type:`PyStructSequence_Desc`, and :c:type:`wrapperbase` are now of type const char * rather of char *. (Contributed by Serhiy Storchaka in :issue:`28761`.)

The result of :c:func:`PyUnicode_AsUTF8AndSize` and :c:func:`PyUnicode_AsUTF8` is now of type const char * rather of char *. (Contributed by Serhiy Storchaka in :issue:`28769`.)

The result of :c:func:`PyMapping_Keys`, :c:func:`PyMapping_Values` and :c:func:`PyMapping_Items` is now always a list, rather than a list or a tuple. (Contributed by Oren Milman in :issue:`28280`.)

Added functions :c:func:`PySlice_Unpack` and :c:func:`PySlice_AdjustIndices`. (Contributed by Serhiy Storchaka in :issue:`27867`.)

:c:func:`PyOS_AfterFork` is deprecated in favour of the new functions :c:func:`PyOS_BeforeFork`, :c:func:`PyOS_AfterFork_Parent` and :c:func:`PyOS_AfterFork_Child`. (Contributed by Antoine Pitrou in :issue:`16500`.)

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 :issue:`22898` and :issue:`30697`.

Added C API support for timezones with timezone constructors :c:func:`PyTimeZone_FromOffset` and :c:func:`PyTimeZone_FromOffsetAndName`, and access to the UTC singleton with :c:data:`PyDateTime_TimeZone_UTC`. Contributed by Paul Ganssle in :issue:`10381`.

The type of results of :c:func:`PyThread_start_new_thread` and :c:func:`PyThread_get_thread_ident`, and the id parameter of :c:func:`PyThreadState_SetAsyncExc` changed from :c:type:`long` to :c:type:`unsigned long`. (Contributed by Serhiy Storchaka in :issue:`6532`.)

:c:func:`PyUnicode_AsWideCharString` now raises a :exc:`ValueError` if the second argument is NULL and the :c:type:`wchar_t*` string contains null characters. (Contributed by Serhiy Storchaka in :issue:`30708`.)

Changes to the startup sequence and the management of dynamic memory allocators mean that the long documented requirement to call :c:func:`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 :ref:`porting-to-python-37` section in this document and the :ref:`pre-init-safe` section in the C API documentation for more details.

The new :c:func:`PyInterpreterState_GetID` returns the unique ID for a given interpreter. (Contributed by Eric Snow in :issue:`29102`.)

:c:func:`Py_DecodeLocale`, :c:func:`Py_EncodeLocale` now use the UTF-8 encoding when the :ref:`UTF-8 mode <whatsnew37-pep540>` is enabled. (Contributed by Victor Stinner in :issue:`29240`.)

:c:func:`PyUnicode_DecodeLocaleAndSize` and :c:func:`PyUnicode_EncodeLocale` now use the current locale encoding for surrogateescape error handler. (Contributed by Victor Stinner in :issue:`29240`.)

The start and end parameters of :c:func:`PyUnicode_FindChar` are now adjusted to behave like string slices. (Contributed by Xiang Zhang in :issue:`28822`.)

Build Changes

Support for building --without-threads has been removed. The :mod:`threading` module is now always available. (Contributed by Antoine Pitrou in :issue:`31370`.).

A full copy of libffi is no longer bundled for use when building the :mod:`_ctypes <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 :issue:`27979`.)

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 :issue:`30450`.)

The :mod:`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.

Optimizations

The overhead of calling many methods of various standard library classes implemented in C has been significantly reduced by porting more code to use the METH_FASTCALL convention. (Contributed by Victor Stinner in :issue:`29300`, :issue:`29507`, :issue:`29452`, and :issue:`29286`.)

Various optimizations have reduced Python startup time by 10% on Linux and up to 30% on macOS. (Contributed by Victor Stinner, INADA Naoki in :issue:`29585`, and Ivan Levkivskyi in :issue:`31333`.)

Method calls are now up to 20% faster due to the bytecode changes which avoid creating bound method instances. (Contributed by Yury Selivanov and INADA Naoki in :issue:`26110`.)

The :mod:`asyncio` module received a number of notable optimizations for commonly used functions:

As a result of :ref:`PEP 560 work <whatsnew37-pep560>`, the import time of :mod:`typing` has been reduced by a factor of 7, and many typing operations are now faster. (Contributed by Ivan Levkivskyi in :issue:`32226`.)

:func:`sorted` and :meth:`list.sort` have been optimized for common cases to be up to 40-75% faster. (Contributed by Elliot Gorokhovsky in :issue:`28685`.)

:meth:`dict.copy` is now up to 5.5 times faster. (Contributed by Yury Selivanov in :issue:`31179`.)

:func:`hasattr` and :func:`getattr` are now about 4 times faster when name is not found and obj does not override :meth:`object.__getattr__` or :meth:`object.__getattribute__`. (Contributed by INADA Naoki in :issue:`32544`.)

Searching for certain Unicode characters (like Ukrainian capital "Є") in a string was up to 25 times slower than searching for other characters. It is now only 3 times slower in the worst case. (Contributed by Serhiy Storchaka in :issue:`24821`.)

The :func:`collections.namedtuple` factory has been reimplemented to make the creation of named tuples 4 to 6 times faster. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in :issue:`28638`.)

:meth:`date.fromordinal` and :meth:`date.fromtimestamp` are now up to 30% faster in the common case. (Contributed by Paul Ganssle in :issue:`32403`.)

The :func:`os.fwalk` function is now up to 2 times faster thanks to the use of :func:`os.scandir`. (Contributed by Serhiy Storchaka in :issue:`25996`.)

The speed of the :func:`shutil.rmtree` function has been improved by 20--40% thanks to the use of the :func:`os.scandir` function. (Contributed by Serhiy Storchaka in :issue:`28564`.)

Optimized case-insensitive matching and searching of :mod:`regular expressions <re>`. Searching some patterns can now be up to 20 times faster. (Contributed by Serhiy Storchaka in :issue:`30285`.)

:func:`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 :issue:`31671`.)

The :meth:`~selectors.BaseSelector.modify` methods of classes :class:`selectors.EpollSelector`, :class:`selectors.PollSelector` and :class:`selectors.DevpollSelector` may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola' in :issue:`30014`)

Constant folding has been moved from the peephole optimizer to the new AST optimizer, which is able perform optimizations more consistently. (Contributed by Eugene Toder and INADA Naoki in :issue:`29469` and :issue:`11549`.)

Most functions and methods in :mod:`abc` have been rewritten in C. This makes creation of abstract base classes, and calling :func:`isinstance` and :func:`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 :issue:`31333`)

Significant speed improvements to alternate constructors for :class:`datetime.date` and :class:`datetime.datetime` by using fast-path constructors when not constructing subclasses. (Contributed by Paul Ganssle in :issue:`32403`)

The speed of comparison of :class:`array.array` instances has been improved considerably in certain cases. It is now from 10x to 70x faster when comparing arrays holding values of the same integer type. (Contributed by Adrian Wielgosik in :issue:`24700`.)

The :func:`math.erf` and :func:`math.erfc` functions now use the (faster) C library implementation on most platforms. (Contributed by Serhiy Storchaka in :issue:`26121`.)

Other CPython Implementation Changes

  • Trace hooks may now opt out of receiving the line and opt into receiving the opcode events from the interpreter by setting the corresponding new f_trace_lines and f_trace_opcodes attributes on the frame being traced. (Contributed by Nick Coghlan in :issue:`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 :issue:`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 :issue:`32303`.
  • The :func:`locals` dictionary now displays in the lexical order that variables were defined. Previously, the order was undefined. (Contributed by Raymond Hettinger in :issue:`32690`.)
  • The :mod:`distutils` 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 :issue:`32304`.)

Deprecated Python Behavior

Yield expressions (both yield and yield from clauses) are now deprecated in comprehensions and generator expressions (aside from the iterable expression in the leftmost :keyword:`!for` clause). This ensures that comprehensions always immediately return a container of the appropriate type (rather than potentially returning a :term:`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 :exc:`DeprecationWarning` when compiled, in Python 3.8 this will be a :exc:`SyntaxError`. (Contributed by Serhiy Storchaka in :issue:`10544`.)

Returning a subclass of :class:`complex` from :meth:`object.__complex__` is deprecated and will be an error in future Python versions. This makes __complex__() consistent with :meth:`object.__int__` and :meth:`object.__float__`. (Contributed by Serhiy Storchaka in :issue:`28894`.)

Deprecated Python modules, functions and methods

aifc

:func:`aifc.openfp` has been deprecated and will be removed in Python 3.9. Use :func:`aifc.open` instead. (Contributed by Brian Curtin in :issue:`31985`.)

asyncio

Support for directly await-ing instances of :class:`asyncio.Lock` and other asyncio synchronization primitives has been deprecated. An asynchronous context manager must be used in order to acquire and release the synchronization resource. (Contributed by Andrew Svetlov in :issue:`32253`.)

The :meth:`asyncio.Task.current_task` and :meth:`asyncio.Task.all_tasks` methods have been deprecated. (Contributed by Andrew Svetlov in :issue:`32250`.)

collections

In Python 3.8, the abstract base classes in :mod:`collections.abc` will no longer be exposed in the regular :mod:`collections` module. This will help create a clearer distinction between the concrete classes and the abstract base classes. (Contributed by Serhiy Storchaka in :issue:`25988`.)

dbm

:mod:`dbm.dumb` now supports reading read-only files and no longer writes the index file when it is not changed. A deprecation warning is now emitted if the index file is missing and recreated in the 'r' and 'w' modes (this will be an error in future Python releases). (Contributed by Serhiy Storchaka in :issue:`28847`.)

enum

In Python 3.8, attempting to check for non-Enum objects in :class:`Enum` classes will raise a :exc:`TypeError` (e.g. 1 in Color); similarly, attempting to check for non-Flag objects in a :class:`Flag` member will raise :exc:`TypeError` (e.g. 1 in Perm.RW); currently, both operations return :const:`False` instead. (Contributed by Ethan Furman in :issue:`33217`.)

gettext

Using non-integer value for selecting a plural form in :mod:`gettext` is now deprecated. It never correctly worked. (Contributed by Serhiy Storchaka in :issue:`28692`.)

importlib

Methods :meth:`MetaPathFinder.find_module() <importlib.abc.MetaPathFinder.find_module>` (replaced by :meth:`MetaPathFinder.find_spec() <importlib.abc.MetaPathFinder.find_spec>`) and :meth:`PathEntryFinder.find_loader() <importlib.abc.PathEntryFinder.find_loader>` (replaced by :meth:`PathEntryFinder.find_spec() <importlib.abc.PathEntryFinder.find_spec>`) both deprecated in Python 3.4 now emit :exc:`DeprecationWarning`. (Contributed by Matthias Bussonnier in :issue:`29576`)

The :class:`importlib.abc.ResourceLoader` ABC has been deprecated in favour of :class:`importlib.abc.ResourceReader`.

locale

:func:`locale.format` has been deprecated, use :meth:`locale.format_string` instead. (Contributed by Garvit in :issue:`10379`.)

macpath

The :mod:`macpath` is now deprecated and will be removed in Python 3.8. (Contributed by Chi Hsuan Yen in :issue:`9850`.)

threading

:mod:`dummy_threading` and :mod:`_dummy_thread` have been deprecated. It is no longer possible to build Python with threading disabled. Use :mod:`threading` instead. (Contributed by Antoine Pitrou in :issue:`31370`.)

socket

The silent argument value truncation in :func:`socket.htons` and :func:`socket.ntohs` has been deprecated. In future versions of Python, if the passed argument is larger than 16 bits, an exception will be raised. (Contributed by Oren Milman in :issue:`28332`.)

ssl

:func:`ssl.wrap_socket` is deprecated. Use :meth:`ssl.SSLContext.wrap_socket` instead. (Contributed by Christian Heimes in :issue:`28124`.)

sunau

:func:`sunau.openfp` has been deprecated and will be removed in Python 3.9. Use :func:`sunau.open` instead. (Contributed by Brian Curtin in :issue:`31985`.)

sys

Deprecated :func:`sys.set_coroutine_wrapper` and :func:`sys.get_coroutine_wrapper`.

The undocumented sys.callstats() function has been deprecated and will be removed in a future Python version. (Contributed by Victor Stinner in :issue:`28799`.)

wave

:func:`wave.openfp` has been deprecated and will be removed in Python 3.9. Use :func:`wave.open` instead. (Contributed by Brian Curtin in :issue:`31985`.)

Deprecated functions and types of the C API

Function :c:func:`PySlice_GetIndicesEx` is deprecated and replaced with a macro if Py_LIMITED_API is not set or set to a value in the range between 0x03050400 and 0x03060000 (not inclusive), or is 0x03060100 or higher. (Contributed by Serhiy Storchaka in :issue:`27867`.)

:c:func:`PyOS_AfterFork` has been deprecated. Use :c:func:`PyOS_BeforeFork`, :c:func:`PyOS_AfterFork_Parent` or :c:func:`PyOS_AfterFork_Child()` instead. (Contributed by Antoine Pitrou in :issue:`16500`.)

Platform Support Removals

  • FreeBSD 9 and older are no longer officially supported.

  • For full Unicode support, including within extension modules, *nix platforms are now expected to provide at least one of C.UTF-8 (full locale), C.utf8 (full locale) or UTF-8 (LC_CTYPE-only locale) as an alternative to the legacy ASCII-based C locale.

  • OpenSSL 0.9.8 and 1.0.1 are no longer supported, which means building CPython 3.7 with SSL/TLS support on older platforms still using these versions requires custom build options that link to a more recent version of OpenSSL.

    Notably, this issue affects the Debian 8 (aka "jessie") and Ubuntu 14.04 (aka "Trusty") LTS Linux distributions, as they still use OpenSSL 1.0.1 by default.

    Debian 9 ("stretch") and Ubuntu 16.04 ("xenial"), as well as recent releases of other LTS Linux releases (e.g. RHEL/CentOS 7.5, SLES 12-SP3), use OpenSSL 1.0.2 or later, and remain supported in the default build configuration.

    CPython's own :source:`CI configuration file <.travis.yml>` provides an example of using the SSL :source:`compatibility testing infrastructure <Tools/ssl/multissltests.py>` in CPython's test suite to build and link against OpenSSL 1.1.0 rather than an outdated system provided OpenSSL.

API and Feature Removals

The following features and APIs have been removed from Python 3.7:

  • 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 :func:`re.sub` were deprecated in Python 3.5, and will now cause an error.
  • Removed support of the exclude argument in :meth:`tarfile.TarFile.add`. It was deprecated in Python 2.7 and 3.2. Use the filter argument instead.
  • The splitunc() function in the :mod:`ntpath` module was deprecated in Python 3.1, and has now been removed. Use the :func:`~os.path.splitdrive` function instead.
  • :func:`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 :issue:`28638`.)
  • Functions :func:`bool`, :func:`float`, :func:`list` and :func:`tuple` no longer take keyword arguments. The first argument of :func:`int` can now be passed only as positional argument.
  • Removed previously deprecated in Python 2.4 classes Plist, Dict and _InternalDict in the :mod:`plistlib` module. Dict values in the result of functions :func:`~plistlib.readPlist` and :func:`~plistlib.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 the :func:`socket.socketpair` function instead, it is available on all platforms since Python 3.5. asyncio.windows_utils.socketpair was just an alias to socket.socketpair on Python 3.5 and newer.
  • :mod:`asyncio` no longer exports the :mod:`selectors` and :mod:`_overlapped` modules as asyncio.selectors and asyncio._overlapped. Replace from asyncio import selectors with import selectors.
  • Direct instantiation of :class:`ssl.SSLSocket` and :class:`ssl.SSLObject` objects is now prohibited. The constructors were never documented, tested, or designed as public constructors. Users were supposed to use :func:`ssl.wrap_socket` or :class:`ssl.SSLContext`. (Contributed by Christian Heimes in :issue:`32951`.)
  • The unused :mod:`distutils` install_misc command has been removed. (Contributed by Eric N. Vander Weele in :issue:`29218`.)

Module Removals

The fpectl module has been removed. It was never enabled by default, never worked correctly on x86-64, and it changed the Python ABI in ways that caused unexpected breakage of C extensions. (Contributed by Nathaniel J. Smith in :issue:`29137`.)

Windows-only Changes

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 :issue:`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 :issue:`30362`.)

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

  • :keyword:`async` and :keyword:`await` names are now reserved keywords. Code using these names as identifiers will now raise a :exc:`SyntaxError`. (Contributed by Jelle Zijlstra in :issue:`30406`.)

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

  • :meth:`object.__aiter__` methods can no longer be declared as asynchronous. (Contributed by Yury Selivanov in :issue:`31709`.)

  • 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 :exc:`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 :issue:`32012` and :issue:`32023`.)

  • When using the :option:`-m` switch, the initial working directory is now added to :data:`sys.path`, rather than an empty string (which dynamically denoted the current working directory at the time of each import). Any programs that are checking for the empty string, or otherwise relying on the previous behaviour, will need to be updated accordingly (e.g. by also checking for os.getcwd() or os.path.dirname(__main__.__file__), depending on why the code was checking for the empty string in the first place).

Changes in the Python API

Changes in the C API

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

CPython bytecode changes

There are two new opcodes: :opcode:`LOAD_METHOD` and :opcode:`CALL_METHOD`. (Contributed by Yury Selivanov and INADA Naoki in :issue:`26110`.)

The :opcode:`STORE_ANNOTATION` opcode has been removed. (Contributed by Mark Shannon in :issue:`32550`.)

Windows-only Changes

The file used to override :data:`sys.path` is now called <python-executable>._pth instead of 'sys.path'. See :ref:`finding_modules` for more information. (Contributed by Steve Dower in :issue:`28137`.)

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 :issue:`22257`, and further updated by Nick, Eric, and Victor Stinner in a number of other issues). Some known details affected:

Due to changes in the way the default warnings filters are configured, setting :c:data:`Py_BytesWarningFlag` to a value greater than one is no longer sufficient to both emit :exc:`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.

Due to a change in the way docstrings are handled by the compiler, the implicit return None in a function body consisting solely of a docstring is now marked as occurring on the same line as the docstring, not on the function's header line.

The current exception state has been moved from the frame object to the co-routine. This simplified the interpreter and fixed a couple of obscure bugs caused by having swap exception state when entering or exiting a generator. (Contributed by Mark Shannon in :issue:`25612`.)

Notable changes in Python 3.7.1

Starting in 3.7.1, :c:func:`Py_Initialize` now consistently reads and respects all of the same environment settings as :c:func:`Py_Main` (in earlier Python versions, it respected an ill-defined subset of those environment variables, while in Python 3.7.0 it didn't read any of them due to :issue:`34247`). If this behavior is unwanted, set :c:data:`Py_IgnoreEnvironmentFlag` to 1 before calling :c:func:`Py_Initialize`.

In 3.7.1 the C API for Context Variables :ref:`was updated <contextvarsobjects_pointertype_change>` to use :c:type:`PyObject` pointers. See also :issue:`34762`.

In 3.7.1 the :mod:`tokenize` module now implicitly emits a NEWLINE token when provided with input that does not have a trailing new line. This behavior now matches what the C tokenizer does internally. (Contributed by Ammar Askar in :issue:`33899`.)

Notable changes in Python 3.7.2

In 3.7.2, :mod:`venv` on Windows no longer copies the original binaries, but creates redirector scripts named python.exe and pythonw.exe instead. This resolves a long standing issue where all virtual environments would have to be upgraded or recreated with each Python update. However, note that this release will still require recreation of virtual environments in order to get the new scripts.

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