Doc/whatsnew/3.10.rst

****************************
  What's New In Python 3.10
****************************

: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 3.10, compared to 3.9.

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 3.10 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 3.10.
   Brevity is key.


.. PEP-sized items next.



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

* The :class:`int` type has a new method :meth:`int.bit_count`, returning the
  number of ones in the binary expansion of a given integer, also known
  as the population count. (Contributed by Niklas Fiekas in :issue:`29882`.)

* The views returned by :meth:`dict.keys`, :meth:`dict.values` and
  :meth:`dict.items` now all have a ``mapping`` attribute that gives a
  :class:`types.MappingProxyType` object wrapping the original
  dictionary. (Contributed by Dennis Sweeney in :issue:`40890`.)


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

* Builtin and extension functions that take integer arguments no longer accept
  :class:`~decimal.Decimal`\ s, :class:`~fractions.Fraction`\ s and other
  objects that can be converted to integers only with a loss (e.g. that have
  the :meth:`~object.__int__` method but do not have the
  :meth:`~object.__index__` method).
  (Contributed by Serhiy Storchaka in :issue:`37999`.)


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

* None yet.


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

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

* The :mod:`runpy` module now imports less modules.
  The ``python3 -m module-name`` command startup time is 1.3x faster in
  average.
  (Contributed by Victor Stinner in :issue:`41006`.)


Deprecated
==========


Removed
=======


Porting to Python 3.10
======================

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



Build Changes
=============

* The C99 functions :c:func:`snprintf` and :c:func:`vsnprintf` are now required
  to build Python.
  (Contributed by Victor Stinner in :issue:`36020`.)


C API Changes
=============

New Features
------------

  The result of :c:func:`PyNumber_Index` now always has exact type :class:`int`.
  Previously, the result could have been an instance of a subclass of ``int``.
  (Contributed by Serhiy Storchaka in :issue:`40792`.)


Porting to Python 3.10
----------------------

* Since :c:func:`Py_TYPE()` is changed to the inline static function,
  ``Py_TYPE(obj) = new_type`` must be replaced with ``Py_SET_TYPE(obj, new_type)``:
  see :c:func:`Py_SET_TYPE()` (available since Python 3.9). For backward
  compatibility, this macro can be used::

      #if PY_VERSION_HEX < 0x030900A4
      #  define Py_SET_TYPE(obj, type) ((Py_TYPE(obj) = (type)), (void)0)
      #endif

  (Contributed by Dong-hee Na in :issue:`39573`.)

* Since :c:func:`Py_REFCNT()` is changed to the inline static function,
  ``Py_REFCNT(obj) = new_refcnt`` must be replaced with ``Py_SET_REFCNT(obj, new_refcnt)``:
  see :c:func:`Py_SET_REFCNT()` (available since Python 3.9). For backward
  compatibility, this macro can be used::

      #if PY_VERSION_HEX < 0x030900A4
      #  define Py_SET_REFCNT(obj, refcnt) ((Py_REFCNT(obj) = (refcnt)), (void)0)
      #endif

  (Contributed by Victor Stinner in :issue:`39573`.)

* Since :c:func:`Py_SIZE()` is changed to the inline static function,
  ``Py_SIZE(obj) = new_size`` must be replaced with ``Py_SET_SIZE(obj, new_size)``:
  see :c:func:`Py_SET_SIZE()` (available since Python 3.9). For backward
  compatibility, this macro can be used::

      #if PY_VERSION_HEX < 0x030900A4
      #  define Py_SET_SIZE(obj, size) ((Py_SIZE(obj) = (size)), (void)0)
      #endif

  (Contributed by Victor Stinner in :issue:`39573`.)

* Calling :c:func:`PyDict_GetItem` without :term:`GIL` held had been allowed
  for historical reason. It is no longer allowed.
  (Contributed by Victor Stinner in :issue:`40839`.)

Removed
-------