Doc/whatsnew/3.9.rst

****************************
  What's New In Python 3.9
****************************

:Release: |release|
:Date: |today|

.. Rules for maintenance:

   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.

   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)

   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.

   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.

   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).

   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.

   * It's helpful to add the bug/patch number as a comment:

   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer in :issue:`12345`.)

   This saves the maintainer the effort of going through the Mercurial log
   when researching a change.

This article explains the new features in Python 3.9, compared to 3.8.

For full details, see the :ref:`changelog <changelog>`.

.. note::

   Prerelease users should be aware that this document is currently in draft
   form. It will be updated substantially as Python 3.9 moves towards release,
   so it's worth checking back even after reading earlier versions.


Summary -- Release highlights
=============================

.. This section singles out the most important changes in Python 3.9.
   Brevity is key.


.. PEP-sized items next.


You should check for DeprecationWarning in your code
====================================================

When Python 2.7 was still supported, many functions were kept for backward
compatibility with Python 2.7. With the end of Python 2.7 support, these
backward compatibility layers have been removed, or will be removed soon.
Most of them emitted a :exc:`DeprecationWarning` warning for several years. For
example, using ``collections.Mapping`` instead of ``collections.abc.Mapping``
emits a :exc:`DeprecationWarning` since Python 3.3, released in 2012.

Test your application with the :option:`-W` ``default`` command-line option to see
:exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, or even with
:option:`-W` ``error`` to treat them as errors. :ref:`Warnings Filter
<warning-filter>` can be used to ignore warnings from third-party code.

It has been decided to keep a few backward compatibility layers for one last
release, to give more time to Python projects maintainers to organize the
removal of the Python 2 support and add support for Python 3.9.

Aliases to :ref:`Abstract Base Classes <collections-abstract-base-classes>` in
the :mod:`collections` module, like ``collections.Mapping`` alias to
:class:`collections.abc.Mapping`, are kept for one last release for backward
compatibility. They will be removed from Python 3.10.

More generally, try to run your tests in the :ref:`Python Development Mode
<devmode>` which helps to prepare your code to make it compatible with the
next Python version.


New Features
============

Dictionary Merge & Update Operators
-----------------------------------

Merge (``|``) and update (``|=``) operators have been added to the built-in
:class:`dict` class.  See :pep:`584` for a full description.
(Contributed by Brandt Bucher in :issue:`36144`.)

PEP 616: New removeprefix() and removesuffix() string methods
-------------------------------------------------------------

:meth:`str.removeprefix(prefix)<str.removeprefix>` and
:meth:`str.removesuffix(suffix)<str.removesuffix>` have been added
to easily remove an unneeded prefix or a suffix from a string. Corresponding
``bytes``, ``bytearray``, and ``collections.UserString`` methods have also been
added. See :pep:`616` for a full description. (Contributed by Dennis Sweeney in
:issue:`18939`.)

PEP 585: Builtin Generic Types
------------------------------

In type annotations you can now use built-in collection types such as
``list`` and ``dict`` as generic types instead of importing the
corresponding capitalized types (e.g. ``List`` or ``Dict``) from
``typing``.  Some other types in the standard library are also now generic,
for example ``queue.Queue``.

Example:

.. code-block:: python

   def greet_all(names: list[str]) -> None:
       for name in names:
           print("Hello", name)

See :pep:`585` for more details.  (Contributed by Guido van Rossum,
Ethan Smith, and Batuhan Taşkaya in :issue:`39481`.)

PEP 617: New Parser
-------------------

Python 3.9 uses a new parser, based on `PEG
<https://en.wikipedia.org/wiki/Parsing_expression_grammar>`_ instead
of `LL(1) <https://en.wikipedia.org/wiki/LL_parser>`_.  The new
parser's performance is roughly comparable to that of the old parser,
but the PEG formalism is more flexible than LL(1) when it comes to
designing new language features.  We'll start using this flexibility
in Python 3.10 and later.

The :mod:`ast` module uses the new parser and produces the same AST as
the old parser.

In Python 3.10, the old parser will be deleted and so will all
functionality that depends on it (primarily the :mod:`parser` module,
which has long been deprecated).  In Python 3.9 *only*, you can switch
back to the LL(1) parser using a command line switch (``-X
oldparser``) or an environment variable (``PYTHONOLDPARSER=1``).

See :pep:`617` for more details.  (Contributed by Guido van Rossum,
Pablo Galindo and Lysandros Nikolau in :issue:`40334`.)


Other Language Changes
======================

* :func:`__import__` now raises :exc:`ImportError` instead of
  :exc:`ValueError`, which used to occur when a relative import went past
  its top-level package.
  (Contributed by Ngalim Siregar in :issue:`37444`.)


* Python now gets the absolute path of the script filename specified on
  the command line (ex: ``python3 script.py``): the ``__file__`` attribute of
  the :mod:`__main__` module became an absolute path, rather than a relative
  path. These paths now remain valid after the current directory is changed
  by :func:`os.chdir`. As a side effect, the traceback also displays the
  absolute path for :mod:`__main__` module frames in this case.
  (Contributed by Victor Stinner in :issue:`20443`.)

* In the :ref:`Python Development Mode <devmode>` and in debug build, the
  *encoding* and *errors* arguments are now checked for string encoding and
  decoding operations. Examples: :func:`open`, :meth:`str.encode` and
  :meth:`bytes.decode`.

  By default, for best performance, the *errors* argument is only checked at
  the first encoding/decoding error and the *encoding* argument is sometimes
  ignored for empty strings.
  (Contributed by Victor Stinner in :issue:`37388`.)

* ``"".replace("", s, n)`` now returns ``s`` instead of an empty string for
  all non-zero ``n``.  It is now consistent with ``"".replace("", s)``.
  There are similar changes for :class:`bytes` and :class:`bytearray` objects.
  (Contributed by Serhiy Storchaka in :issue:`28029`.)

* Any valid expression can now be used as a :term:`decorator`.  Previously, the
  grammar was much more restrictive.  See :pep:`614` for details.
  (Contributed by Brandt Bucher in :issue:`39702`.)

* Improved help for the :mod:`typing` module. Docstrings are now shown for
  all special forms and special generic aliases (like ``Union`` and ``List``).
  Using :func:`help` with generic alias like ``List[int]`` will show the help
  for the correspondent concrete type (``list`` in this case).
  (Contributed by Serhiy Storchaka in :issue:`40257`.)


New Modules
===========

* None yet.


Improved Modules
================

ast
---

Added the *indent* option to :func:`~ast.dump` which allows it to produce a
multiline indented output.
(Contributed by Serhiy Storchaka in :issue:`37995`.)

Added :func:`ast.unparse` as a function in the :mod:`ast` module that can
be used to unparse an :class:`ast.AST` object and produce a string with code
that would produce an equivalent :class:`ast.AST` object when parsed.
(Contributed by Pablo Galindo and Batuhan Taskaya in :issue:`38870`.)

Added docstrings to AST nodes that contains the ASDL signature used to
construct that node. (Contributed by Batuhan Taskaya in :issue:`39638`.)

asyncio
-------

Due to significant security concerns, the *reuse_address* parameter of
:meth:`asyncio.loop.create_datagram_endpoint` is no longer supported. This is
because of the behavior of the socket option ``SO_REUSEADDR`` in UDP. For more
details, see the documentation for ``loop.create_datagram_endpoint()``.
(Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
:issue:`37228`.)

Added a new :term:`coroutine` :meth:`~asyncio.loop.shutdown_default_executor`
that schedules a shutdown for the default executor that waits on the
:class:`~concurrent.futures.ThreadPoolExecutor` to finish closing. Also,
:func:`asyncio.run` has been updated to use the new :term:`coroutine`.
(Contributed by Kyle Stanley in :issue:`34037`.)

Added :class:`asyncio.PidfdChildWatcher`, a Linux-specific child watcher
implementation that polls process file descriptors. (:issue:`38692`)

concurrent.futures
------------------

Added a new *cancel_futures* parameter to
:meth:`concurrent.futures.Executor.shutdown` that cancels all pending futures
which have not started running, instead of waiting for them to complete before
shutting down the executor.
(Contributed by Kyle Stanley in :issue:`39349`.)

Removed daemon threads from :class:`~concurrent.futures.ThreadPoolExecutor`
and :class:`~concurrent.futures.ProcessPoolExecutor`. This improves
compatibility with subinterpreters and predictability in their shutdown
processes. (Contributed by Kyle Stanley in :issue:`39812`.)

Workers in :class:`~concurrent.futures.ProcessPoolExecutor` are now spawned on
demand, only when there are no available idle workers to reuse. This optimizes
startup overhead and reduces the amount of lost CPU time to idle workers.
(Contributed by Kyle Stanley in :issue:`39207`.)

curses
------

Add :func:`curses.get_escdelay`, :func:`curses.set_escdelay`,
:func:`curses.get_tabsize`, and :func:`curses.set_tabsize` functions.
(Contributed by Anthony Sottile in :issue:`38312`.)

fcntl
-----

Added constants :data:`~fcntl.F_OFD_GETLK`, :data:`~fcntl.F_OFD_SETLK`
and :data:`~fcntl.F_OFD_SETLKW`.
(Contributed by Dong-hee Na in :issue:`38602`.)

ftplib
-------

:class:`~ftplib.FTP` and :class:`~ftplib.FTP_TLS` now raise a :class:`ValueError`
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

functools
---------

Add the :class:`functools.TopologicalSorter` class to offer functionality to perform
topological sorting of graphs. (Contributed by Pablo Galindo, Tim Peters and Larry
Hastings in :issue:`17005`.)

gc
--

When the garbage collector makes a collection in which some objects resurrect
(they are reachable from outside the isolated cycles after the finalizers have
been executed), do not block the collection of all objects that are still
unreachable. (Contributed by Pablo Galindo and Tim Peters in :issue:`38379`.)

Added a new function :func:`gc.is_finalized` to check if an object has been
finalized by the garbage collector. (Contributed by Pablo Galindo in
:issue:`39322`.)

http
----

HTTP status codes ``103 EARLY_HINTS``, ``418 IM_A_TEAPOT`` and ``425 TOO_EARLY`` are added to
:class:`http.HTTPStatus`. (Contributed by Dong-hee Na in :issue:`39509` and Ross Rhodes in :issue:`39507`.)

imaplib
-------

:class:`~imaplib.IMAP4` and :class:`~imaplib.IMAP4_SSL` now have
an optional *timeout* parameter for their constructors.
Also, the :meth:`~imaplib.IMAP4.open` method now has an optional *timeout* parameter
with this change. The overridden methods of :class:`~imaplib.IMAP4_SSL` and
:class:`~imaplib.IMAP4_stream` were applied to this change.
(Contributed by Dong-hee Na in :issue:`38615`.)

:meth:`imaplib.IMAP4.unselect` is added.
:meth:`imaplib.IMAP4.unselect` frees server's resources associated with the
selected mailbox and returns the server to the authenticated
state. This command performs the same actions as :meth:`imaplib.IMAP4.close`, except
that no messages are permanently removed from the currently
selected mailbox. (Contributed by Dong-hee Na in :issue:`40375`.)

importlib
---------

To improve consistency with import statements, :func:`importlib.util.resolve_name`
now raises :exc:`ImportError` instead of :exc:`ValueError` for invalid relative
import attempts.
(Contributed by Ngalim Siregar in :issue:`37444`.)

inspect
-------

:attr:`inspect.BoundArguments.arguments` is changed from ``OrderedDict`` to regular
dict.  (Contributed by Inada Naoki in :issue:`36350` and :issue:`39775`.)

ipaddress
---------

:mod:`ipaddress` now supports IPv6 Scoped Addresses (IPv6 address with suffix ``%<scope_id>``).

Scoped IPv6 addresses can be parsed using :class:`ipaddress.IPv6Address`.
If present, scope zone ID is available through the :attr:`~ipaddress.IPv6Address.scope_id` attribute.
(Contributed by Oleksandr Pavliuk in :issue:`34788`.)

math
----

Expanded the :func:`math.gcd` function to handle multiple arguments.
Formerly, it only supported two arguments.
(Contributed by Serhiy Storchaka in :issue:`39648`.)

Add :func:`math.lcm`: return the least common multiple of specified arguments.
(Contributed by Mark Dickinson, Ananthakrishnan and Serhiy Storchaka in
:issue:`39479` and :issue:`39648`.)

Add :func:`math.nextafter`: return the next floating-point value after *x*
towards *y*.
(Contributed by Victor Stinner in :issue:`39288`.)

Add :func:`math.ulp`: return the value of the least significant bit
of a float.
(Contributed by Victor Stinner in :issue:`39310`.)

nntplib
-------

:class:`~nntplib.NNTP` and :class:`~nntplib.NNTP_SSL` now raise a :class:`ValueError`
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

multiprocessing
---------------

The :class:`multiprocessing.SimpleQueue` class has a new
:meth:`~multiprocessing.SimpleQueue.close` method to explicitly close the
queue.
(Contributed by Victor Stinner in :issue:`30966`.)

os
--

Added :data:`~os.CLD_KILLED` and :data:`~os.CLD_STOPPED` for :attr:`si_code`.
(Contributed by Dong-hee Na in :issue:`38493`.)

Exposed the Linux-specific :func:`os.pidfd_open` (:issue:`38692`) and
:data:`os.P_PIDFD` (:issue:`38713`) for process management with file
descriptors.

The :func:`os.unsetenv` function is now also available on Windows.
(Contributed by Victor Stinner in :issue:`39413`.)

The :func:`os.putenv` and :func:`os.unsetenv` functions are now always
available.
(Contributed by Victor Stinner in :issue:`39395`.)

Add :func:`os.waitstatus_to_exitcode` function:
convert a wait status to an exit code.
(Contributed by Victor Stinner in :issue:`40094`.)

pathlib
-------

Added :meth:`pathlib.Path.readlink()` which acts similarly to
:func:`os.readlink`.
(Contributed by Girts Folkmanis in :issue:`30618`)

poplib
------

:class:`~poplib.POP3` and :class:`~poplib.POP3_SSL` now raise a :class:`ValueError`
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

pprint
------

:mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`.
(Contributed by Carl Bordum Hansen in :issue:`37376`.)

pydoc
-----

The documentation string is now shown not only for class, function,
method etc, but for any object that has its own ``__doc__`` attribute.
(Contributed by Serhiy Storchaka in :issue:`40257`.)

random
------

Add a new :attr:`random.Random.randbytes` method: generate random bytes.
(Contributed by Victor Stinner in :issue:`40286`.)

signal
------

Exposed the Linux-specific :func:`signal.pidfd_send_signal` for sending to
signals to a process using a file descriptor instead of a pid. (:issue:`38712`)

smtplib
-------

:class:`~smtplib.SMTP` and :class:`~smtplib.SMTP_SSL` now raise a :class:`ValueError`
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

:class:`~smtplib.LMTP` constructor  now has an optional *timeout* parameter.
(Contributed by Dong-hee Na in :issue:`39329`.)

socket
------

The :mod:`socket` module now exports the :data:`~socket.CAN_RAW_JOIN_FILTERS`
constant on Linux 4.1 and greater.
(Contributed by Stefan Tatschner and Zackery Spytz in :issue:`25780`.)

sys
---

Add a new :attr:`sys.platlibdir` attribute: name of the platform-specific
library directory. It is used to build the path of platform-specific dynamic
libraries and the path of the standard library. It is equal to ``"lib"`` on
most platforms.  On Fedora and SuSE, it is equal to ``"lib64"`` on 64-bit
platforms.
(Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)


typing
------

:pep:`593` introduced an :data:`typing.Annotated` type to decorate existing
types with context-specific metadata and new ``include_extras`` parameter to
:func:`typing.get_type_hints` to access the metadata at runtime. (Contributed
by Till Varoquaux and Konstantin Kashin.)

unicodedata
-----------

The Unicode database has been updated to version 13.0.0. (:issue:`39926`).

venv
----

The activation scripts provided by :mod:`venv` now all specify their prompt
customization consistently by always using the value specified by
``__VENV_PROMPT__``. Previously some scripts unconditionally used
``__VENV_PROMPT__``, others only if it happened to be set (which was the default
case), and one used ``__VENV_NAME__`` instead.
(Contributed by Brett Cannon in :issue:`37663`.)

xml
---

White space characters within attributes are now preserved when serializing
:mod:`xml.etree.ElementTree` to XML file. EOLNs are no longer normalized
to "\n". This is the result of discussion about how to interpret
section 2.11 of XML spec.
(Contributed by Mefistotelis in :issue:`39011`.)


Optimizations
=============

* Optimized the idiom for assignment a temporary variable in comprehensions.
  Now ``for y in [expr]`` in comprehensions is as fast as a simple assignment
  ``y = expr``.  For example:

     sums = [s for s in [0] for x in data for s in [s + x]]

  Unlike to the ``:=`` operator this idiom does not leak a variable to the
  outer scope.

  (Contributed by Serhiy Storchaka in :issue:`32856`.)

* Optimize signal handling in multithreaded applications. If a thread different
  than the main thread gets a signal, the bytecode evaluation loop is no longer
  interrupted at each bytecode instruction to check for pending signals which
  cannot be handled. Only the main thread of the main interpreter can handle
  signals.

  Previously, the bytecode evaluation loop was interrupted at each instruction
  until the main thread handles signals.
  (Contributed by Victor Stinner in :issue:`40010`.)

* Optimize the :mod:`subprocess` module on FreeBSD using ``closefrom()``.
  (Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak and Victor
  Stinner in :issue:`38061`.)


Build and C API Changes
=======================

* New :c:func:`PyFrame_GetCode` function: get a frame code.
  (Contributed by Victor Stinner in :issue:`40421`.)

* Add :c:func:`PyFrame_GetLineNumber` to the limited C API.
  (Contributed by Victor Stinner in :issue:`40421`.)

* New :c:func:`PyThreadState_GetInterpreter` and
  :c:func:`PyInterpreterState_Get` functions to get the interpreter.
  New :c:func:`PyThreadState_GetFrame` function to get the current frame of a
  Python thread state.
  New :c:func:`PyThreadState_GetID` function: get the unique identifier of a
  Python thread state.
  (Contributed by Victor Stinner in :issue:`39947`.)

* Add ``--with-platlibdir`` option to the ``configure`` script: name of the
  platform-specific library directory, stored in the new :attr:`sys.platlibdir`
  attribute. See :attr:`sys.platlibdir` attribute for more information.
  (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)

* Add a new public :c:func:`PyObject_CallNoArgs` function to the C API, which
  calls a callable Python object without any arguments. It is the most efficient
  way to call a callable Python object without any argument.
  (Contributed by Victor Stinner in :issue:`37194`.)

* The global variable :c:data:`PyStructSequence_UnnamedField` is now a constant
  and refers to a constant string.
  (Contributed by Serhiy Storchaka in :issue:`38650`.)

* Exclude ``PyFPE_START_PROTECT()`` and ``PyFPE_END_PROTECT()`` macros of
  ``pyfpe.h`` from ``Py_LIMITED_API`` (stable API).
  (Contributed by Victor Stinner in :issue:`38835`.)

* Remove ``PyMethod_ClearFreeList()`` and ``PyCFunction_ClearFreeList()``
  functions: the free lists of bound method objects have been removed.
  (Contributed by Inada Naoki and Victor Stinner in :issue:`37340`.)

* Remove ``PyUnicode_ClearFreeList()`` function: the Unicode free list has been
  removed in Python 3.3.
  (Contributed by Victor Stinner in :issue:`38896`.)

* The ``tp_print`` slot of :ref:`PyTypeObject <type-structs>` has been removed.
  It was used for printing objects to files in Python 2.7 and before. Since
  Python 3.0, it has been ignored and unused.
  (Contributed by Jeroen Demeyer in :issue:`36974`.)

* On non-Windows platforms, the :c:func:`setenv` and :c:func:`unsetenv`
  functions are now required to build Python.
  (Contributed by Victor Stinner in :issue:`39395`.)

* The ``COUNT_ALLOCS`` special build macro has been removed.
  (Contributed by Victor Stinner in :issue:`39489`.)

* Changes in the limited C API (if ``Py_LIMITED_API`` macro is defined):

  * Provide :c:func:`Py_EnterRecursiveCall` and :c:func:`Py_LeaveRecursiveCall`
    as regular functions for the limited API. Previously, there were defined as
    macros, but these macros didn't compile with the limited C API which cannot
    access ``PyThreadState.recursion_depth`` field (the structure is opaque in
    the limited C API).

  * Exclude the following functions from the limited C API:

    * ``_Py_CheckRecursionLimit``
    * ``_Py_NewReference()``
    * ``_Py_ForgetReference()``
    * ``_PyTraceMalloc_NewReference()``
    * ``_Py_GetRefTotal()``
    * The trashcan mechanism which never worked in the limited C API.
    * ``PyTrash_UNWIND_LEVEL``
    * ``Py_TRASHCAN_BEGIN_CONDITION``
    * ``Py_TRASHCAN_BEGIN``
    * ``Py_TRASHCAN_END``
    * ``Py_TRASHCAN_SAFE_BEGIN``
    * ``Py_TRASHCAN_SAFE_END``

  * The following static inline functions or macros become regular "opaque"
    function to hide implementation details:

    * ``_Py_NewReference()``
    * ``PyObject_INIT()`` and ``PyObject_INIT_VAR()``  become aliases to
      :c:func:`PyObject_Init` and :c:func:`PyObject_InitVar` in the limited C
      API, but are overriden with static inline function otherwise. Thanks to
      that, it was possible to exclude ``_Py_NewReference()`` from the limited
      C API.

  * Move following functions and definitions to the internal C API:

    * ``_PyDebug_PrintTotalRefs()``
    * ``_Py_PrintReferences()``
    * ``_Py_PrintReferenceAddresses()``
    * ``_Py_tracemalloc_config``
    * ``_Py_AddToAllObjects()`` (specific to ``Py_TRACE_REFS`` build)

  (Contributed by Victor Stinner in :issue:`38644` and :issue:`39542`.)

* ``PyInterpreterState.eval_frame`` (:pep:`523`) now requires a new mandatory
  *tstate* parameter (``PyThreadState*``).
  (Contributed by Victor Stinner in :issue:`38500`.)

* Extension modules: :c:member:`~PyModuleDef.m_traverse`,
  :c:member:`~PyModuleDef.m_clear` and :c:member:`~PyModuleDef.m_free`
  functions of :c:type:`PyModuleDef` are no longer called if the module state
  was requested but is not allocated yet. This is the case immediately after
  the module is created and before the module is executed
  (:c:data:`Py_mod_exec` function). More precisely, these functions are not called
  if :c:member:`~PyModuleDef.m_size` is greater than 0 and the module state (as
  returned by :c:func:`PyModule_GetState`) is ``NULL``.

  Extension modules without module state (``m_size <= 0``) are not affected.

* If :c:func:`Py_AddPendingCall` is called in a subinterpreter, the function is
  now scheduled to be called from the subinterpreter, rather than being called
  from the main interpreter. Each subinterpreter now has its own list of
  scheduled calls.
  (Contributed by Victor Stinner in :issue:`39984`.)

* Remove ``_PyRuntime.getframe`` hook and remove ``_PyThreadState_GetFrame``
  macro which was an alias to ``_PyRuntime.getframe``. They were only exposed
  by the internal C API. Remove also ``PyThreadFrameGetter`` type.
  (Contributed by Victor Stinner in :issue:`39946`.)

* The :c:func:`PyModule_AddType` function is added to help adding a type to a module.
  (Contributed by Dong-hee Na in :issue:`40024`.)

* The Windows registry is no longer used to initialize :data:`sys.path` when
  the ``-E`` option is used. This is significant when embedding Python on
  Windows.
  (Contributed by Zackery Spytz in :issue:`8901`.)

* Add the functions :c:func:`PyObject_GC_IsTracked` and
  :c:func:`PyObject_GC_IsFinalized` to the public API to allow to query if
  Python objects are being currently tracked or have been already finalized by
  the garbage collector respectively. (Contributed by Pablo Galindo in
  :issue:`40241`.)


Deprecated
==========

* The distutils ``bdist_msi`` command is now deprecated, use
  ``bdist_wheel`` (wheel packages) instead.
  (Contributed by Hugo van Kemenade in :issue:`39586`.)

* Currently :func:`math.factorial` accepts :class:`float` instances with
  non-negative integer values (like ``5.0``).  It raises a :exc:`ValueError`
  for non-integral and negative floats.  It is now deprecated.  In future
  Python versions it will raise a :exc:`TypeError` for all floats.
  (Contributed by Serhiy Storchaka in :issue:`37315`.)

* The :mod:`parser` module is deprecated and will be removed in future versions
  of Python. For the majority of use cases, users can leverage the Abstract Syntax
  Tree (AST) generation and compilation stage, using the :mod:`ast` module.

* Using :data:`NotImplemented` in a boolean context has been deprecated,
  as it is almost exclusively the result of incorrect rich comparator
  implementations. It will be made a :exc:`TypeError` in a future version
  of Python.
  (Contributed by Josh Rosenberg in :issue:`35712`.)

* The :mod:`random` module currently accepts any hashable type as a
  possible seed value.  Unfortunately, some of those types are not
  guaranteed to have a deterministic hash value.  After Python 3.9,
  the module will restrict its seeds to :const:`None`, :class:`int`,
  :class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`.

* Opening the :class:`~gzip.GzipFile` file for writing without specifying
  the *mode* argument is deprecated.  In future Python versions it will always
  be opened for reading by default.  Specify the *mode* argument for opening
  it for writing and silencing a warning.
  (Contributed by Serhiy Storchaka in :issue:`28286`.)

* Deprecated the ``split()`` method of :class:`_tkinter.TkappType` in
  favour of the ``splitlist()`` method which has more consistent and
  predicable behavior.
  (Contributed by Serhiy Storchaka in :issue:`38371`.)

* The explicit passing of coroutine objects to :func:`asyncio.wait` has been
  deprecated and will be removed in version 3.11.
  (Contributed by Yury Selivanov and Kyle Stanley in :issue:`34790`.)

* binhex4 and hexbin4 standards are now deprecated. The :`binhex` module
  and the following :mod:`binascii` functions are now deprecated:

  * :func:`~binascii.b2a_hqx`, :func:`~binascii.a2b_hqx`
  * :func:`~binascii.rlecode_hqx`, :func:`~binascii.rledecode_hqx`

  (Contributed by Victor Stinner in :issue:`39353`.)

* :mod:`ast` classes ``slice``, ``Index`` and ``ExtSlice`` are considered deprecated
  and will be removed in future Python versions.  ``value`` itself should be
  used instead of ``Index(value)``.  ``Tuple(slices, Load())`` should be
  used instead of ``ExtSlice(slices)``.
  (Contributed by Serhiy Storchaka in :issue:`32892`.)

* :mod:`ast` classes ``Suite``, ``Param``, ``AugLoad`` and ``AugStore``
  are considered deprecated and will be removed in future Python versions.
  They were not generated by the parser and not accepted by the code
  generator in Python 3.
  (Contributed by Batuhan Taskaya in :issue:`39639` and :issue:`39969`
  and Serhiy Storchaka in :issue:`39988`.)

* The :c:func:`PyEval_InitThreads` and :c:func:`PyEval_ThreadsInitialized`
  functions are now deprecated and will be removed in Python 3.11. Calling
  :c:func:`PyEval_InitThreads` now does nothing. The :term:`GIL` is initialized
  by :c:func:`Py_Initialize()` since Python 3.7.
  (Contributed by Victor Stinner in :issue:`39877`.)

* Passing ``None`` as the first argument to the :func:`shlex.split` function
  has been deprecated.  (Contributed by Zackery Spytz in :issue:`33262`.)


Removed
=======

* The erroneous version at :data:`unittest.mock.__version__` has been removed.

* :class:`nntplib.NNTP`: ``xpath()`` and ``xgtitle()`` methods have been removed.
  These methods are deprecated since Python 3.3. Generally, these extensions
  are not supported or not enabled by NNTP server administrators.
  For ``xgtitle()``, please use :meth:`nntplib.NNTP.descriptions` or
  :meth:`nntplib.NNTP.description` instead.
  (Contributed by Dong-hee Na in :issue:`39366`.)

* :class:`array.array`: ``tostring()`` and ``fromstring()`` methods have been
  removed. They were aliases to ``tobytes()`` and ``frombytes()``, deprecated
  since Python 3.2.
  (Contributed by Victor Stinner in :issue:`38916`.)

* The undocumented ``sys.callstats()`` function has been removed. Since Python
  3.7, it was deprecated and always returned :const:`None`. It required a special
  build option ``CALL_PROFILE`` which was already removed in Python 3.7.
  (Contributed by Victor Stinner in :issue:`37414`.)

* The ``sys.getcheckinterval()`` and ``sys.setcheckinterval()`` functions have
  been removed. They were deprecated since Python 3.2. Use
  :func:`sys.getswitchinterval` and :func:`sys.setswitchinterval` instead.
  (Contributed by Victor Stinner in :issue:`37392`.)

* The C function ``PyImport_Cleanup()`` has been removed. It was documented as:
  "Empty the module table.  For internal use only."
  (Contributed by Victor Stinner in :issue:`36710`.)

* ``_dummy_thread`` and ``dummy_threading`` modules have been removed. These
  modules were deprecated since Python 3.7 which requires threading support.
  (Contributed by Victor Stinner in :issue:`37312`.)

* ``aifc.openfp()`` alias to ``aifc.open()``, ``sunau.openfp()`` alias to
  ``sunau.open()``, and ``wave.openfp()`` alias to :func:`wave.open()` have been
  removed. They were deprecated since Python 3.7.
  (Contributed by Victor Stinner in :issue:`37320`.)

* The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread`
  has been removed. It was deprecated since Python 3.8.
  Use :meth:`~threading.Thread.is_alive()` instead.
  (Contributed by Dong-hee Na in :issue:`37804`.)

* Methods ``getchildren()`` and ``getiterator()`` of classes
  :class:`~xml.etree.ElementTree.ElementTree` and
  :class:`~xml.etree.ElementTree.Element` in the :mod:`~xml.etree.ElementTree`
  module have been removed.  They were deprecated in Python 3.2.
  Use ``iter(x)`` or ``list(x)`` instead of ``x.getchildren()`` and
  ``x.iter()`` or ``list(x.iter())`` instead of ``x.getiterator()``.
  The ``xml.etree.cElementTree`` module has been removed,
  use the :mod:`xml.etree.ElementTree` module instead.
  Since Python 3.3 the ``xml.etree.cElementTree`` module has been deprecated,
  the ``xml.etree.ElementTree`` module uses a fast implementation whenever
  available.
  (Contributed by Serhiy Storchaka in :issue:`36543`.)

* The old :mod:`plistlib` API has been removed, it was deprecated since Python
  3.4. Use the :func:`~plistlib.load`, :func:`~plistlib.loads`, :func:`~plistlib.dump`, and
  :func:`~plistlib.dumps` functions. Additionally, the *use_builtin_types* parameter was
  removed, standard :class:`bytes` objects are always used instead.
  (Contributed by Jon Janzen in :issue:`36409`.)

* The C function ``PyThreadState_DeleteCurrent()`` has been removed. It was not documented.
  (Contributed by Joannah Nanjekye in :issue:`37878`.)

* The C function ``PyGen_NeedsFinalizing`` has been removed. It was not
  documented, tested, or used anywhere within CPython after the implementation
  of :pep:`442`. Patch by Joannah Nanjekye.
  (Contributed by Joannah Nanjekye in :issue:`15088`)

* ``base64.encodestring()`` and ``base64.decodestring()``, aliases deprecated
  since Python 3.1, have been removed: use :func:`base64.encodebytes` and
  :func:`base64.decodebytes` instead.
  (Contributed by Victor Stinner in :issue:`39351`.)

* ``fractions.gcd()`` function has been removed, it was deprecated since Python
  3.5 (:issue:`22486`): use :func:`math.gcd` instead.
  (Contributed by Victor Stinner in :issue:`39350`.)

* The *buffering* parameter of :class:`bz2.BZ2File` has been removed. Since
  Python 3.0, it was ignored and using it emitted a :exc:`DeprecationWarning`.
  Pass an open file object to control how the file is opened.
  (Contributed by Victor Stinner in :issue:`39357`.)

* The *encoding* parameter of :func:`json.loads` has been removed.
  As of Python 3.1, it was deprecated and ignored; using it has emitted a
  :exc:`DeprecationWarning` since Python 3.8.
  (Contributed by Inada Naoki in :issue:`39377`)

* ``with (await asyncio.lock):`` and ``with (yield from asyncio.lock):`` statements are
  not longer supported, use ``async with lock`` instead.  The same is correct for
  ``asyncio.Condition`` and ``asyncio.Semaphore``.
  (Contributed by Andrew Svetlov in :issue:`34793`.)

* The :func:`sys.getcounts` function, the ``-X showalloccount`` command line
  option and the ``show_alloc_count`` field of the C structure
  :c:type:`PyConfig` have been removed. They required a special Python build by
  defining ``COUNT_ALLOCS`` macro.
  (Contributed by Victor Stinner in :issue:`39489`.)

* The ``_field_types`` attribute of the :class:`typing.NamedTuple` class
  has been removed.  It was deprecated deprecated since Python 3.8.  Use
  the ``__annotations__`` attribute instead.
  (Contributed by Serhiy Storchaka in :issue:`40182`.)

* The :meth:`symtable.SymbolTable.has_exec` method has been removed. It was
  deprecated since 2006, and only returning ``False`` when it's called.
  (Contributed by Batuhan Taskaya in :issue:`40208`)


Porting to Python 3.9
=====================

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


Changes in the Python API
-------------------------

* :func:`__import__` and :func:`importlib.util.resolve_name` now raise
  :exc:`ImportError` where it previously raised :exc:`ValueError`. Callers
  catching the specific exception type and supporting both Python 3.9 and
  earlier versions will need to catch both using ``except (ImportError, ValueError):``.

* The :mod:`venv` activation scripts no longer special-case when
  ``__VENV_PROMPT__`` is set to ``""``.

* The :meth:`select.epoll.unregister` method no longer ignores the
  :data:`~errno.EBADF` error.
  (Contributed by Victor Stinner in :issue:`39239`.)

* The *compresslevel* parameter of :class:`bz2.BZ2File` became keyword-only,
  since the *buffering* parameter has been removed.
  (Contributed by Victor Stinner in :issue:`39357`.)

* Simplified AST for subscription. Simple indices will be represented by
  their value, extended slices will be represented as tuples.
  ``Index(value)`` will return a ``value`` itself, ``ExtSlice(slices)``
  will return ``Tuple(slices, Load())``.
  (Contributed by Serhiy Storchaka in :issue:`34822`.)

* The :mod:`importlib` module now ignores the :envvar:`PYTHONCASEOK`
  environment variable when the :option:`-E` or :option:`-I` command line
  options are being used.

* The *encoding* parameter has been added to the classes :class:`ftplib.FTP` and
  :class:`ftplib.FTP_TLS` as a keyword-only parameter, and the default encoding
  is changed from Latin-1 to UTF-8 to follow :rfc:`2640`.

* :func:`inspect.getdoc` no longer returns docstring inherited from the type
  of the object or from parent class if it is a class if it is not defined
  in the object itself.
  (Contributed by Serhiy Storchaka in :issue:`40257`.)

* :meth:`asyncio.loop.shutdown_default_executor` has been added to
  :class:`~asyncio.AbstractEventLoop`, meaning alternative event loops that
  inherit from it should have this method defined.
  (Contributed by Kyle Stanley in :issue:`34037`.)

* The constant values of future flags in the :mod:`__future__` module
  is updated in order to prevent collision with compiler flags. Previously
  ``PyCF_ALLOW_TOP_LEVEL_AWAIT`` was clashing with ``CO_FUTURE_DIVISION``.
  (Contributed by Batuhan Taskaya in :issue:`39562`)

CPython bytecode changes
------------------------

* The :opcode:`LOAD_ASSERTION_ERROR` opcode was added for handling the
  :keyword:`assert` statement. Previously, the assert statement would not work
  correctly if the :exc:`AssertionError` exception was being shadowed.
  (Contributed by Zackery Spytz in :issue:`34880`.)