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
============

.. _whatsnew310-pep563:

Parenthesized context managers
------------------------------

Using enclosing parentheses for continuation across multiple lines
in context managers is now supported. This allows formatting a long
collection of context managers in multiple lines in a similar way
as it was previously possible with import statements. For instance,
all these examples are now valid:

.. code-block:: python

    with (CtxManager() as example):
        ...

    with (
        CtxManager1(),
        CtxManager2()
    ):
        ...

    with (CtxManager1() as example,
          CtxManager2()):
        ...

    with (CtxManager1(),
          CtxManager2() as example):
        ...

    with (
        CtxManager1() as example1,
        CtxManager2() as example2
    ):
        ...

it is also possible to use a trailing comma at the end of the
enclosed group:

.. code-block:: python

    with (
        CtxManager1() as example1,
        CtxManager2() as example2,
        CtxManager3() as example3,
    ):
        ...

This new syntax uses the non LL(1) capacities of the new parser.
Check :pep:`617` for more details.

(Contributed by Guido van Rossum, Pablo Galindo and Lysandros Nikolaou
in :issue:`12782` and :issue:`40334`.)


PEP 563: Postponed Evaluation of Annotations Becomes Default
------------------------------------------------------------

In Python 3.7, postponed evaluation of annotations was added,
to be enabled with a ``from __future__ import annotations``
directive.  In 3.10 this became the default behavior, even
without that future directive.  With this being default, all
annotations stored in :attr:`__annotations__` will be strings.
If needed, annotations can be resolved at runtime using
:func:`typing.get_type_hints`.  See :pep:`563` for a full
description.  Also, the :func:`inspect.signature` will try to
resolve types from now on, and when it fails it will fall back to
showing the string annotations.  (Contributed by Batuhan Taskaya
in :issue:`38605`.)

* 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`.)

* :pep:`618`: The :func:`zip` function now has an optional ``strict`` flag, used
  to require that all the iterables have an equal length.

PEP 613: TypeAlias Annotation
-----------------------------

:pep:`484` introduced the concept of type aliases, only requiring them to be
top-level unannotated assignments. This simplicity sometimes made it difficult
for type checkers to distinguish between type aliases and ordinary assignments,
especially when forward references or invalid types were involved. Compare::

   StrCache = 'Cache[str]'  # a type alias
   LOG_PREFIX = 'LOG[DEBUG]'  # a module constant

Now the :mod:`typing` module has a special annotation :data:`TypeAlias` to
declare type aliases more explicitly::

   StrCache: TypeAlias = 'Cache[str]'  # a type alias
   LOG_PREFIX = 'LOG[DEBUG]'  # a module constant

See :pep:`613` for more details.

(Contributed by Mikhail Golubev in :issue:`41923`.)

PEP 604: New Type Union Operator
--------------------------------

A new type union operator was introduced which enables the syntax ``X | Y``.
This provides a cleaner way of expressing 'either type X or type Y' instead of
using :data:`typing.Union`, especially in type hints (annotations).

In previous versions of Python, to apply a type hint for functions accepting
arguments of multiple types, :data:`typing.Union` was used::

   def square(number: Union[int, float]) -> Union[int, float]:
       return number ** 2


Type hints can now be written in a more succinct manner::

   def square(number: int | float) -> int | float:
       return number ** 2


This new syntax is also accepted as the second argument to :func:`isinstance`
and :func:`issubclass`::

   >>> isinstance(1, int | str)
   True

See :ref:`types-union` and :pep:`604` for more details.

(Contributed by Maggie Moss and Philippe Prados in :issue:`41428`.)

PEP 612: Parameter Specification Variables
------------------------------------------

Two new options to improve the information provided to static type checkers for
:pep:`484`\ 's ``Callable`` have been added to the :mod:`typing` module.

The first is the parameter specification variable.  They are used to forward the
parameter types of one callable to another callable -- a pattern commonly
found in higher order functions and decorators.  Examples of usage can be found
in :class:`typing.ParamSpec`. Previously, there was no easy way to type annotate
dependency of parameter types in such a precise manner.

The second option is the new ``Concatenate`` operator. It's used in conjunction
with parameter specification variables to type annotate a higher order callable
which adds or removes parameters of another callable.  Examples of usage can
be found in :class:`typing.Concatenate`.

See :class:`typing.Callable`, :class:`typing.ParamSpec`,
:class:`typing.Concatenate` and :pep:`612` for more details.

(Contributed by Ken Jin in :issue:`41559`.)

PEP 634: Structural Pattern Matching
------------------------------------

Structural pattern matching has been added in the form of a *match statement*
and *case statements* of patterns with associated actions. Patterns
consist of sequences, mappings, primitive data types as well as class instances.
Pattern matching enables programs to extract information from complex data types,
branch on the structure of data, and apply specific actions based on different
forms of data.

Syntax and operations
~~~~~~~~~~~~~~~~~~~~~

The generic syntax of pattern matching is::

      match subject:
          case <pattern_1>:
              <action_1>
          case <pattern_2>:
              <action_2>
          case <pattern_3>:
              <action_3>
          case _:
              <action_wildcard>

A match statement takes an expression and compares its value to successive
patterns given as one or more case blocks.  Specifically, pattern matching
operates by:

    1. using data with type and shape (the ``subject``)
    2. evaluating the ``subject`` in the ``match`` statement
    3. comparing the subject with each pattern in a ``case`` statement
       from top to bottom until a match is confirmed.
    4. executing the action associated with the pattern of the confirmed
       match
    5. If an exact match is not confirmed, the last case, a wildcard ``_``,
       if provided, will be used as the matching case. If an exact match is
       not confirmed and a wildcard case does not exists, the entire match
       block is a no-op.

Declarative approach
~~~~~~~~~~~~~~~~~~~~

Readers may be aware of pattern matching through the simple example of matching
a subject (data object) to a literal (pattern) with the switch statement found
in C, Java or JavaScript (and many other languages). Often the switch statement
is used for comparison of an object/expression with case statements containing
literals.

More powerful examples of pattern matching can be found in languages, such as
Scala and Elixir. With structural pattern matching, the approach is "declarative" and
explicitly states the conditions (the patterns) for data to match.

While an "imperative" series of instructions using nested "if" statements
could be used to accomplish something similar to structural pattern matching,
it is less clear than the "declarative" approach. Instead the "declarative"
approach states the conditions to meet for a match and is more readable through
its explicit patterns. While structural pattern matching can be used in its
simplest form comparing a variable to a literal in a case statement, its
true value for Python lies in its handling of the subject's type and shape.

Simple pattern: match to a literal
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's look at this example as pattern matching in its simplest form: a value,
the subject, being matched to several literals, the patterns. In the example
below, ``status`` is the subject of the match statement. The patterns are
each of the case statements, where literals represent request status codes.
The associated action to the case is executed after a match::

    def http_error(status):
        match status:
            case 400:
                return "Bad request"
            case 404:
                return "Not found"
            case 418:
                return "I'm a teapot"
            case _:
                return "Something's wrong with the Internet"

If the above function is passed a ``status`` of 418, "I'm a teapot" is returned.
If the above function is passed a ``status`` of 500, the case statement with
``_`` will match as a wildcard, and "Something's wrong with the Internet" is
returned.
Note the last block: the variable name, ``_``, acts as a *wildcard* and insures
the subject will always match. The use of ``_`` is optional.

You can combine several literals in a single pattern using ``|`` ("or")::

            case 401 | 403 | 404:
                return "Not allowed"

Behavior without the wildcard
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If we modify the above example by removing the last case block, the example
becomes::

    def http_error(status):
        match status:
            case 400:
                return "Bad request"
            case 404:
                return "Not found"
            case 418:
                return "I'm a teapot"

Without the use of ``_`` in a case statement, a match may not exist. If no
match exists, the behavior is a no-op. For example, if ``status`` of 500 is
passed, a no-op occurs.

Pattterns with a literal and variable
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Patterns can look like unpacking assignments, and a pattern may be used to bind
variables. In this example, a data point can be unpacked to its x-coordinate
and y-coordinate::

    # point is an (x, y) tuple
    match point:
        case (0, 0):
            print("Origin")
        case (0, y):
            print(f"Y={y}")
        case (x, 0):
            print(f"X={x}")
        case (x, y):
            print(f"X={x}, Y={y}")
        case _:
            raise ValueError("Not a point")

The first pattern has two literals, ``(0, 0)``, and may be thought of as an
extension of the literal pattern shown above. The next two patterns combine a
literal and a variable, and the variable *binds* a value from the subject
(``point``).  The fourth pattern captures two values, which makes it
conceptually similar to the unpacking assignment ``(x, y) = point``.

Patterns and classes
~~~~~~~~~~~~~~~~~~~~

If you are using classes to structure your data, you can use as a pattern
the class name followed by an argument list resembling a constructor. This
pattern has the ability to capture class attributes into variables::

    class Point:
        x: int
        y: int

    def location(point):
        match point:
            case Point(x=0, y=0):
                print("Origin is the point's location.")
            case Point(x=0, y=y):
                print(f"Y={y} and the point is on the y-axis.")
            case Point(x=x, y=0):
                print(f"X={x} and the point is on the x-axis.")
            case Point():
                print("The point is located somewhere else on the plane.")
            case _:
                print("Not a point")

Patterns with positional parameters
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can use positional parameters with some builtin classes that provide an
ordering for their attributes (e.g. dataclasses). You can also define a specific
position for attributes in patterns by setting the ``__match_args__`` special
attribute in your classes. If it's set to ("x", "y"), the following patterns
are all equivalent (and all bind the ``y`` attribute to the ``var`` variable)::

    Point(1, var)
    Point(1, y=var)
    Point(x=1, y=var)
    Point(y=var, x=1)

Nested patterns
~~~~~~~~~~~~~~~

Patterns can be arbitrarily nested.  For example, if our data is a short
list of points, it could be matched like this::

    match points:
        case []:
            print("No points in the list.")
        case [Point(0, 0)]:
            print("The origin is the only point in the list.")
        case [Point(x, y)]:
            print(f"A single point {x}, {y} is in the list.")
        case [Point(0, y1), Point(0, y2)]:
            print(f"Two points on the Y axis at {y1}, {y2} are in the list.")
        case _:
            print("Something else is found in the list.")

Complex patterns and the wildcard
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To this point, the examples have used ``_`` alone in the last case statement.
A wildcard can be used in more complex patterns, such as ``('error', code, _)``.
For example::

   match test_variable:
       case ('warning', code, 40):
           print("A warning has been received.")
       case ('error', code, _):
           print(f"An error {code} occured.")

In the above case, ``test_variable`` will match for ('error', code, 100) and
('error', code, 800).

Guard
~~~~~

We can add an ``if`` clause to a pattern, known as a "guard".  If the
guard is false, ``match`` goes on to try the next case block.  Note
that value capture happens before the guard is evaluated::

    match point:
        case Point(x, y) if x == y:
            print(f"The point is located on the diagonal Y=X at {x}.")
        case Point(x, y):
            print(f"Point is not on the diagonal.")

Other Key Features
~~~~~~~~~~~~~~~~~~

Several other key features:

- Like unpacking assignments, tuple and list patterns have exactly the
  same meaning and actually match arbitrary sequences. Technically,
  the subject must be an instance of ``collections.abc.Sequence``.
  Therefore, an important exception is that patterns don't match iterators.
  Also, to prevent a common mistake, sequence patterns don't match strings.

- Sequence patterns support wildcards: ``[x, y, *rest]`` and ``(x, y,
  *rest)`` work similar to wildcards in unpacking assignments.  The
  name after ``*`` may also be ``_``, so ``(x, y, *_)`` matches a sequence
  of at least two items without binding the remaining items.

- Mapping patterns: ``{"bandwidth": b, "latency": l}`` captures the
  ``"bandwidth"`` and ``"latency"`` values from a dict.  Unlike sequence
  patterns, extra keys are ignored.  A wildcard ``**rest`` is also
  supported.  (But ``**_`` would be redundant, so it not allowed.)

- Subpatterns may be captured using the ``as`` keyword::

      case (Point(x1, y1), Point(x2, y2) as p2): ...

  This binds x1, y1, x2, y2 like you would expect without the ``as`` clause,
  and p2 to the entire second item of the subject.

- Most literals are compared by equality. However, the singletons ``True``,
  ``False`` and ``None`` are compared by identity.

- Named constants may be used in patterns.  These named constants must be
  dotted names to prevent the constant from being interpreted as a capture
  variable::

      from enum import Enum
      class Color(Enum):
          RED = 0
          GREEN = 1
          BLUE = 2

      match color:
          case Color.RED:
              print("I see red!")
          case Color.GREEN:
              print("Grass is green")
          case Color.BLUE:
              print("I'm feeling the blues :(")

For the full specification see :pep:`634`.  Motivation and rationale
are in :pep:`635`, and a longer tutorial is in :pep:`636`.

Better error messages in the parser
-----------------------------------

When parsing code that contains unclosed parentheses or brackets the interpreter
now includes the location of the unclosed bracket of parentheses instead of displaying
*SyntaxError: unexpected EOF while parsing* or pointing to some incorrect location.
For instance, consider the following code (notice the unclosed '{'):

.. code-block:: python

    expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
                38: 4, 39: 4, 45: 5, 46: 5, 47: 5, 48: 5, 49: 5, 54: 6,
    some_other_code = foo()

previous versions of the interpreter reported confusing places as the location of
the syntax error:

.. code-block:: text

   File "example.py", line 3
       some_other_code = foo()
                       ^
   SyntaxError: invalid syntax

but in Python3.10 a more informative error is emitted:

.. code-block:: text

    File "example.py", line 1
        expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
                   ^
    SyntaxError: '{' was never closed


In a similar way, errors involving unclosed string literals (single and triple
quoted) now point to the start of the string instead of reporting EOF/EOL.

These improvements are inspired by previous work in the PyPy interpreter.

(Contributed by Pablo Galindo in :issue:`42864` and Batuhan Taskaya in
:issue:`40176`.)

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`.)
* If :func:`object.__ipow__` returns :const:`NotImplemented`, the operator will
  correctly fall back to :func:`object.__pow__` and :func:`object.__rpow__` as expected.
  (Contributed by Alex Shkop in :issue:`38302`.)

* Assignment expressions can now be used unparenthesized within set literals
  and set comprehensions, as well as in sequence indexes (but not slices).

* Functions have a new ``__builtins__`` attribute which is used to look for
  builtin symbols when a function is executed, instead of looking into
  ``__globals__['__builtins__']``. The attribute is initialized from
  ``__globals__["__builtins__"]`` if it exists, else from the current builtins.
  (Contributed by Mark Shannon in :issue:`42990`.)


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

* None yet.


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

argparse
--------

Misleading phrase "optional arguments" was replaced with "options" in argparse help. Some tests might require adaptation if they rely on exact output match.
(Contributed by Raymond Hettinger in :issue:`9694`.)

base64
------

Add :func:`base64.b32hexencode` and :func:`base64.b32hexdecode` to support the
Base32 Encoding with Extended Hex Alphabet.

codecs
------

Add a :func:`codecs.unregister` function to unregister a codec search function.
(Contributed by Hai Shi in :issue:`41842`.)

collections.abc
---------------

The ``__args__`` of the :ref:`parameterized generic <types-genericalias>` for
:class:`collections.abc.Callable` are now consistent with :data:`typing.Callable`.
:class:`collections.abc.Callable` generic now flattens type parameters, similar
to what :data:`typing.Callable` currently does.  This means that
``collections.abc.Callable[[int, str], str]`` will have ``__args__`` of
``(int, str, str)``; previously this was ``([int, str], str)``.  To allow this
change, :class:`types.GenericAlias` can now be subclassed, and a subclass will
be returned when subscripting the :class:`collections.abc.Callable` type.  Note
that a :exc:`TypeError` may be raised for invalid forms of parameterizing
:class:`collections.abc.Callable` which may have passed silently in Python 3.9.
(Contributed by Ken Jin in :issue:`42195`.)

contextlib
----------

Add a :func:`contextlib.aclosing` context manager to safely close async generators
and objects representing asynchronously released resources.
(Contributed by Joongi Kim and John Belmonte in :issue:`41229`.)

Add asynchronous context manager support to :func:`contextlib.nullcontext`.
(Contributed by Tom Gringauz in :issue:`41543`.)

curses
------

The extended color functions added in ncurses 6.1 will be used transparently
by :func:`curses.color_content`, :func:`curses.init_color`,
:func:`curses.init_pair`, and :func:`curses.pair_content`. A new function,
:func:`curses.has_extended_color_support`, indicates whether extended color
support is provided by the underlying ncurses library.
(Contributed by Jeffrey Kintscher and Hans Petter Jansson in :issue:`36982`.)

The ``BUTTON5_*`` constants are now exposed in the :mod:`curses` module if
they are provided by the underlying curses library.
(Contributed by Zackery Spytz in :issue:`39273`.)

.. _distutils-deprecated:

distutils
---------

The entire ``distutils`` package is deprecated, to be removed in Python
3.12. Its functionality for specifying package builds has already been
completely replaced by third-party packages ``setuptools`` and
``packaging``, and most other commonly used APIs are available elsewhere
in the standard library (such as :mod:`platform`, :mod:`shutil`,
:mod:`subprocess` or :mod:`sysconfig`). There are no plans to migrate
any other functionality from ``distutils``, and applications that are
using other functions should plan to make private copies of the code.
Refer to :pep:`632` for discussion.

The ``bdist_wininst`` command deprecated in Python 3.8 has been removed.
The ``bdist_wheel`` command is now recommended to distribute binary packages
on Windows.
(Contributed by Victor Stinner in :issue:`42802`.)

doctest
-------

When a module does not define ``__loader__``, fall back to ``__spec__.loader``.
(Contributed by Brett Cannon in :issue:`42133`.)

encodings
---------
:func:`encodings.normalize_encoding` now ignores non-ASCII characters.
(Contributed by Hai Shi in :issue:`39337`.)

glob
----

Added the *root_dir* and *dir_fd* parameters in :func:`~glob.glob` and
:func:`~glob.iglob` which allow to specify the root directory for searching.
(Contributed by Serhiy Storchaka in :issue:`38144`.)

inspect
-------

When a module does not define ``__loader__``, fall back to ``__spec__.loader``.
(Contributed by Brett Cannon in :issue:`42133`.)

Added *globalns* and *localns* parameters in :func:`~inspect.signature` and
:meth:`inspect.Signature.from_callable` to retrieve the annotations in given
local and global namespaces.
(Contributed by Batuhan Taskaya in :issue:`41960`.)

linecache
---------

When a module does not define ``__loader__``, fall back to ``__spec__.loader``.
(Contributed by Brett Cannon in :issue:`42133`.)

os
--

Added :func:`os.cpu_count()` support for VxWorks RTOS.
(Contributed by Peixing Xin in :issue:`41440`.)

Added a new function :func:`os.eventfd` and related helpers to wrap the
``eventfd2`` syscall on Linux.
(Contributed by Christian Heimes in :issue:`41001`.)

Added :func:`os.splice()` that allows to move data between two file
descriptors without copying between kernel address space and user
address space, where one of the file descriptors must refer to a
pipe. (Contributed by Pablo Galindo in :issue:`41625`.)

Added :data:`~os.O_EVTONLY`, :data:`~os.O_FSYNC`, :data:`~os.O_SYMLINK`
and :data:`~os.O_NOFOLLOW_ANY` for macOS.
(Contributed by Dong-hee Na in :issue:`43106`.)

pathlib
-------

Added slice support to :attr:`PurePath.parents <pathlib.PurePath.parents>`.
(Contributed by Joshua Cannon in :issue:`35498`)

Added negative indexing support to :attr:`PurePath.parents
<pathlib.PurePath.parents>`.
(Contributed by Yaroslav Pankovych in :issue:`21041`)

platform
--------

Added :func:`platform.freedesktop_os_release()` to retrieve operation system
identification from `freedesktop.org os-release
<https://www.freedesktop.org/software/systemd/man/os-release.html>`_ standard file.
(Contributed by Christian Heimes in :issue:`28468`)

py_compile
----------

Added ``--quiet`` option to command-line interface of :mod:`py_compile`.
(Contributed by Gregory Schevchenko in :issue:`38731`.)

pyclbr
------

Added an ``end_lineno`` attribute to the ``Function`` and ``Class``
objects in the tree returned by :func:`pyclbr.readline` and
:func:`pyclbr.readline_ex`.  It matches the existing (start) ``lineno``.
(Contributed by Aviral Srivastava in :issue:`38307`.)

shelve
------

The :mod:`shelve` module now uses :data:`pickle.DEFAULT_PROTOCOL` by default
instead of :mod:`pickle` protocol ``3`` when creating shelves.
(Contributed by Zackery Spytz in :issue:`34204`.)

site
----

When a module does not define ``__loader__``, fall back to ``__spec__.loader``.
(Contributed by Brett Cannon in :issue:`42133`.)

socket
------

The exception :exc:`socket.timeout` is now an alias of :exc:`TimeoutError`.
(Contributed by Christian Heimes in :issue:`42413`.)

sys
---

Add :data:`sys.orig_argv` attribute: the list of the original command line
arguments passed to the Python executable.
(Contributed by Victor Stinner in :issue:`23427`.)

Add :data:`sys.stdlib_module_names`, containing the list of the standard library
module names.
(Contributed by Victor Stinner in :issue:`42955`.)

threading
---------

Added :func:`threading.gettrace` and :func:`threading.getprofile` to
retrieve the functions set by :func:`threading.settrace` and
:func:`threading.setprofile` respectively.
(Contributed by Mario Corchero in :issue:`42251`.)

Add :data:`threading.__excepthook__` to allow retrieving the original value
of :func:`threading.excepthook` in case it is set to a broken or a different
value.
(Contributed by Mario Corchero in :issue:`42308`.)

traceback
---------

The :func:`~traceback.format_exception`,
:func:`~traceback.format_exception_only`, and
:func:`~traceback.print_exception` functions can now take an exception object
as a positional-only argument.
(Contributed by Zackery Spytz and Matthias Bussonnier in :issue:`26389`.)

types
-----

Reintroduced the :data:`types.EllipsisType`, :data:`types.NoneType`
and :data:`types.NotImplementedType` classes, providing a new set
of types readily interpretable by type checkers.
(Contributed by Bas van Beek in :issue:`41810`.)

typing
------

The behavior of :class:`typing.Literal` was changed to conform with :pep:`586`
and to match the behavior of static type checkers specified in the PEP.

1. ``Literal`` now de-duplicates parameters.
2. Equality comparisons between ``Literal`` objects are now order independent.
3. ``Literal`` comparisons now respects types.  For example,
   ``Literal[0] == Literal[False]`` previously evaluated to ``True``.  It is
   now ``False``.  To support this change, the internally used type cache now
   supports differentiating types.
4. ``Literal`` objects will now raise a :exc:`TypeError` exception during
   equality comparisons if one of their parameters are not :term:`immutable`.
   Note that declaring ``Literal`` with mutable parameters will not throw
   an error::

      >>> from typing import Literal
      >>> Literal[{0}]
      >>> Literal[{0}] == Literal[{False}]
      Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
      TypeError: unhashable type: 'set'

(Contributed by Yurii Karabas in :issue:`42345`.)

unittest
--------

Add new method :meth:`~unittest.TestCase.assertNoLogs` to complement the
existing :meth:`~unittest.TestCase.assertLogs`. (Contributed by Kit Yan Choi
in :issue:`39385`.)

urllib.parse
------------

Python versions earlier than Python 3.10 allowed using both ``;`` and ``&`` as
query parameter separators in :func:`urllib.parse.parse_qs` and
:func:`urllib.parse.parse_qsl`.  Due to security concerns, and to conform with
newer W3C recommendations, this has been changed to allow only a single
separator key, with ``&`` as the default.  This change also affects
:func:`cgi.parse` and :func:`cgi.parse_multipart` as they use the affected
functions internally.  For more details, please see their respective
documentation.
(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)

xml
---

Add a :class:`~xml.sax.handler.LexicalHandler` class to the
:mod:`xml.sax.handler` module.
(Contributed by Jonathan Gossage and Zackery Spytz in :issue:`35018`.)

zipimport
---------
Add methods related to :pep:`451`: :meth:`~zipimport.zipimporter.find_spec`,
:meth:`zipimport.zipimporter.create_module`, and
:meth:`zipimport.zipimporter.exec_module`.
(Contributed by Brett Cannon in :issue:`42131`.


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

* Constructors :func:`str`, :func:`bytes` and :func:`bytearray` are now faster
  (around 30--40% for small objects).
  (Contributed by Serhiy Storchaka in :issue:`41334`.)

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

* The ``LOAD_ATTR`` instruction now uses new "per opcode cache" mechanism.  It
  is about 36% faster now for regular attributes and 44% faster for slots.
  (Contributed by Pablo Galindo and Yury Selivanov in :issue:`42093` and Guido
  van Rossum in :issue:`42927`, based on ideas implemented originally in PyPy
  and MicroPython.)

* When building Python with ``--enable-optimizations`` now
  ``-fno-semantic-interposition`` is added to both the compile and link line.
  This speeds builds of the Python interpreter created with ``--enable-shared``
  with ``gcc`` by up to 30%. See `this article
  <https://developers.redhat.com/blog/2020/06/25/red-hat-enterprise-linux-8-2-brings-faster-python-3-8-run-speeds/>`_
  for more details. (Contributed by Victor Stinner and Pablo Galindo in
  :issue:`38980`.)


* Function parameters and their annotations are no longer computed at runtime,
  but rather at compilation time.  They are stored as a tuple of strings at the
  bytecode level.  It is now around 100% faster to create a function with parameter
  annotations.  (Contributed by Yurii Karabas and Inada Naoki in :issue:`42202`)

* Substring search functions such as ``str1 in str2`` and ``str2.find(str1)``
  now sometimes use Crochemore & Perrin's "Two-Way" string searching
  algorithm to avoid quadratic behavior on long strings.  (Contributed
  by Dennis Sweeney in :issue:`41972`)

Deprecated
==========

* Starting in this release, there will be a concerted effort to begin
  cleaning up old import semantics that were kept for Python 2.7
  compatibility. Specifically,
  :meth:`~importlib.abc.PathEntryFinder.find_loader`/:meth:`~importlib.abc.Finder.find_module`
  (superseded by :meth:`~importlib.abc.Finder.find_spec`),
  :meth:`~importlib.abc.Loader.load_module`
  (superseded by :meth:`~importlib.abc.Loader.exec_module`),
  :meth:`~importlib.abc.Loader.module_repr` (which the import system
  takes care of for you), the ``__package__`` attribute
  (superseded by ``__spec__.parent``), the ``__loader__`` attribute
  (superseded by ``__spec__.loader``), and the ``__cached__`` attribute
  (superseded by ``__spec__.cached``) will slowly be removed (as well
  as other classes and methods in :mod:`importlib`).
  :exc:`ImportWarning` and/or :exc:`DeprecationWarning` will be raised
  as appropriate to help identify code which needs updating during
  this transition.

* The entire ``distutils`` namespace is deprecated, to be removed in
  Python 3.12. Refer to the :ref:`module changes <distutils-deprecated>`
  section for more information.

* Non-integer arguments to :func:`random.randrange` are deprecated.
  The :exc:`ValueError` is deprecated in favor of a :exc:`TypeError`.
  (Contributed by Serhiy Storchaka and Raymond Hettinger in :issue:`37319`.)

* The various ``load_module()`` methods of :mod:`importlib` have been
  documented as deprecated since Python 3.6, but will now also trigger
  a :exc:`DeprecationWarning`. Use
  :meth:`~importlib.abc.Loader.exec_module` instead.
  (Contributed by Brett Cannon in :issue:`26131`.)

* :meth:`zimport.zipimporter.load_module` has been deprecated in
  preference for :meth:`~zipimport.zipimporter.exec_module`.
  (Contributed by Brett Cannon in :issue:`26131`.)

* The use of :meth:`~importlib.abc.Loader.load_module` by the import
  system now triggers an :exc:`ImportWarning` as
  :meth:`~importlib.abc.Loader.exec_module` is preferred.
  (Contributed by Brett Cannon in :issue:`26131`.)

* ``sqlite3.OptimizedUnicode`` has been undocumented and obsolete since Python
  3.3, when it was made an alias to :class:`str`.  It is now deprecated,
  scheduled for removal in Python 3.12.
  (Contributed by Erlend E. Aasland in :issue:`42264`.)

* The undocumented built-in function ``sqlite3.enable_shared_cache`` is now
  deprecated, scheduled for removal in Python 3.12.  Its use is strongly
  discouraged by the SQLite3 documentation.  See `the SQLite3 docs
  <https://sqlite.org/c3ref/enable_shared_cache.html>`_ for more details.
  If shared cache must be used, open the database in URI mode using the
  ``cache=shared`` query parameter.
  (Contributed by Erlend E. Aasland in :issue:`24464`.)


Removed
=======

* Removed special methods ``__int__``, ``__float__``, ``__floordiv__``,
  ``__mod__``, ``__divmod__``, ``__rfloordiv__``, ``__rmod__`` and
  ``__rdivmod__`` of the :class:`complex` class.  They always raised
  a :exc:`TypeError`.
  (Contributed by Serhiy Storchaka in :issue:`41974`.)

* The ``ParserBase.error()`` method from the private and undocumented ``_markupbase``
  module has been removed.  :class:`html.parser.HTMLParser` is the only subclass of
  ``ParserBase`` and its ``error()`` implementation has already been removed in
  Python 3.5.
  (Contributed by Berker Peksag in :issue:`31844`.)

* Removed the ``unicodedata.ucnhash_CAPI`` attribute which was an internal
  PyCapsule object. The related private ``_PyUnicode_Name_CAPI`` structure was
  moved to the internal C API.
  (Contributed by Victor Stinner in :issue:`42157`.)

* Removed the ``parser`` module, which was deprecated in 3.9 due to the
  switch to the new PEG parser, as well as all the C source and header files
  that were only being used by the old parser, including ``node.h``, ``parser.h``,
  ``graminit.h`` and ``grammar.h``.

* Removed the Public C API functions :c:func:`PyParser_SimpleParseStringFlags`,
  :c:func:`PyParser_SimpleParseStringFlagsFilename`,
  :c:func:`PyParser_SimpleParseFileFlags` and :c:func:`PyNode_Compile`
  that were deprecated in 3.9 due to the switch to the new PEG parser.

* Removed the ``formatter`` module, which was deprecated in Python 3.4.
  It is somewhat obsolete, little used, and not tested. It was originally
  scheduled to be removed in Python 3.6, but such removals were delayed until
  after Python 2.7 EOL. Existing users should copy whatever classes they use
  into their code.
  (Contributed by Dong-hee Na and Terry J. Reedy in :issue:`42299`.)

* Removed the :c:func:`PyModule_GetWarningsModule` function that was useless
  now due to the _warnings module was converted to a builtin module in 2.6.
  (Contributed by Hai Shi in :issue:`42599`.)

* Remove deprecated aliases to :ref:`collections-abstract-base-classes` from
  the :mod:`collections` module.
  (Contributed by Victor Stinner in :issue:`37324`.)

* The ``loop`` parameter has been removed from most of :mod:`asyncio`\ 's
  :doc:`high-level API <../library/asyncio-api-index>` following deprecation
  in Python 3.8.  The motivation behind this change is multifold:

  1. This simplifies the high-level API.
  2. The functions in the high-level API have been implicitly getting the
     current thread's running event loop since Python 3.7.  There isn't a need to
     pass the event loop to the API in most normal use cases.
  3. Event loop passing is error-prone especially when dealing with loops
     running in different threads.

  Note that the low-level API will still accept ``loop``.
  See `Changes in the Python API`_ for examples of how to replace existing code.

  (Contributed by Yurii Karabas, Andrew Svetlov, Yury Selivanov and Kyle Stanley
  in :issue:`42392`.)


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

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


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

* The *etype* parameters of the :func:`~traceback.format_exception`,
  :func:`~traceback.format_exception_only`, and
  :func:`~traceback.print_exception` functions in the :mod:`traceback` module
  have been renamed to *exc*.
  (Contributed by Zackery Spytz and Matthias Bussonnier in :issue:`26389`.)

* :mod:`atexit`: At Python exit, if a callback registered with
  :func:`atexit.register` fails, its exception is now logged. Previously, only
  some exceptions were logged, and the last exception was always silently
  ignored.
  (Contributed by Victor Stinner in :issue:`42639`.)

* :class:`collections.abc.Callable` generic now flattens type parameters, similar
  to what :data:`typing.Callable` currently does.  This means that
  ``collections.abc.Callable[[int, str], str]`` will have ``__args__`` of
  ``(int, str, str)``; previously this was ``([int, str], str)``.  Code which
  accesses the arguments via :func:`typing.get_args` or ``__args__`` need to account
  for this change.  Furthermore, :exc:`TypeError` may be raised for invalid forms
  of parameterizing :class:`collections.abc.Callable` which may have passed
  silently in Python 3.9.
  (Contributed by Ken Jin in :issue:`42195`.)

* :meth:`socket.htons` and :meth:`socket.ntohs` now raise :exc:`OverflowError`
  instead of :exc:`DeprecationWarning` if the given parameter will not fit in
  a 16-bit unsigned integer.
  (Contributed by Erlend E. Aasland in :issue:`42393`.)

* The ``loop`` parameter has been removed from most of :mod:`asyncio`\ 's
  :doc:`high-level API <../library/asyncio-api-index>` following deprecation
  in Python 3.8.

  A coroutine that currently look like this::

     async def foo(loop):
         await asyncio.sleep(1, loop=loop)

  Should be replaced with this::

     async def foo():
         await asyncio.sleep(1)

  If ``foo()`` was specifically designed *not* to run in the current thread's
  running event loop (e.g. running in another thread's event loop), consider
  using :func:`asyncio.run_coroutine_threadsafe` instead.

  (Contributed by Yurii Karabas, Andrew Svetlov, Yury Selivanov and Kyle Stanley
  in :issue:`42392`.)

* The :data:`types.FunctionType` constructor now inherits the current builtins
  if the *globals* dictionary has no ``"__builtins__"`` key, rather than using
  ``{"None": None}`` as builtins: same behavior as :func:`eval` and
  :func:`exec` functions.  Defining a function with ``def function(...): ...``
  in Python is not affected, globals cannot be overriden with this syntax: it
  also inherits the current builtins.
  (Contributed by Victor Stinner in :issue:`42990`.)

CPython bytecode changes
========================

* The ``MAKE_FUNCTION`` instruction accepts tuple of strings as annotations
  instead of dictionary.
  (Contributed by Yurii Karabas and Inada Naoki in :issue:`42202`)

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`.)

* :mod:`sqlite3` requires SQLite 3.7.15 or higher. (Contributed by Sergey Fedoseev
  and Erlend E. Aasland :issue:`40744` and :issue:`40810`.)

* The :mod:`atexit` module must now always be built as a built-in module.
  (Contributed by Victor Stinner in :issue:`42639`.)

* Added ``--disable-test-modules`` option to the ``configure`` script:
  don't build nor install test modules.
  (Contributed by Xavier de Gaye, Thomas Petazzoni and Peixing Xin in :issue:`27640`.)

* Add ``--with-wheel-pkg-dir=PATH`` option to the ``./configure`` script. If
  specified, the :mod:`ensurepip` module looks for ``setuptools`` and ``pip``
  wheel packages in this directory: if both are present, these wheel packages
  are used instead of ensurepip bundled wheel packages.

  Some Linux distribution packaging policies recommend against bundling
  dependencies. For example, Fedora installs wheel packages in the
  ``/usr/share/python-wheels/`` directory and don't install the
  ``ensurepip._bundled`` package.

  (Contributed by Victor Stinner in :issue:`42856`.)

* Add a new configure ``--without-static-libpython`` option to not build the
  ``libpythonMAJOR.MINOR.a`` static library and not install the ``python.o``
  object file.

  (Contributed by Victor Stinner in :issue:`43103`.)

* The ``configure`` script now uses the ``pkg-config`` utility, if available,
  to detect the location of Tcl/Tk headers and libraries.  As before, those
  locations can be explicitly specified with the ``--with-tcltk-includes``
  and ``--with-tcltk-libs`` configuration options.
  (Contributed by Manolis Stamatogiannakis in :issue:`42603`.)


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`.)

* Add a new :c:member:`~PyConfig.orig_argv` member to the :c:type:`PyConfig`
  structure: the list of the original command line arguments passed to the
  Python executable.
  (Contributed by Victor Stinner in :issue:`23427`.)

* The :c:func:`PyDateTime_DATE_GET_TZINFO` and
  :c:func:`PyDateTime_TIME_GET_TZINFO` macros have been added for accessing
  the ``tzinfo`` attributes of :class:`datetime.datetime` and
  :class:`datetime.time` objects.
  (Contributed by Zackery Spytz in :issue:`30155`.)

* Add a :c:func:`PyCodec_Unregister` function to unregister a codec
  search function.
  (Contributed by Hai Shi in :issue:`41842`.)

* The :c:func:`PyIter_Send` function was added to allow
  sending value into iterator without raising ``StopIteration`` exception.
  (Contributed by Vladimir Matveev in :issue:`41756`.)

* Added :c:func:`PyUnicode_AsUTF8AndSize` to the limited C API.
  (Contributed by Alex Gaynor in :issue:`41784`.)

* Added :c:func:`PyModule_AddObjectRef` function: similar to
  :c:func:`PyModule_AddObject` but don't steal a reference to the value on
  success.
  (Contributed by Victor Stinner in :issue:`1635741`.)

* Added :c:func:`Py_NewRef` and :c:func:`Py_XNewRef` functions to increment the
  reference count of an object and return the object.
  (Contributed by Victor Stinner in :issue:`42262`.)

* The :c:func:`PyType_FromSpecWithBases` and :c:func:`PyType_FromModuleAndSpec`
  functions now accept a single class as the *bases* argument.
  (Contributed by Serhiy Storchaka in :issue:`42423`.)

* The :c:func:`PyType_FromModuleAndSpec` function now accepts NULL ``tp_doc``
  slot.
  (Contributed by Hai Shi in :issue:`41832`.)

* The :c:func:`PyType_GetSlot` function can accept static types.
  (Contributed by Hai Shi and Petr Viktorin in :issue:`41073`.)

* Add a new :c:func:`PySet_CheckExact` function to the C-API to check if an
  object is an instance of :class:`set` but not an instance of a subtype.
  (Contributed by Pablo Galindo in :issue:`43277`.)

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

* The ``PY_SSIZE_T_CLEAN`` macro must now be defined to use
  :c:func:`PyArg_ParseTuple` and :c:func:`Py_BuildValue` formats which use
  ``#``: ``es#``, ``et#``, ``s#``, ``u#``, ``y#``, ``z#``, ``U#`` and ``Z#``.
  See :ref:`Parsing arguments and building values
  <arg-parsing>` and the :pep:`353`.
  (Contributed by Victor Stinner in :issue:`40943`.)

* 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`.)

* 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`.)

* ``PyUnicode_FromUnicode(NULL, size)`` and ``PyUnicode_FromStringAndSize(NULL, size)``
  raise ``DeprecationWarning`` now.  Use :c:func:`PyUnicode_New` to allocate
  Unicode object without initial data.
  (Contributed by Inada Naoki in :issue:`36346`.)

* The private ``_PyUnicode_Name_CAPI`` structure of the PyCapsule API
  ``unicodedata.ucnhash_CAPI`` has been moved to the internal C API.
  (Contributed by Victor Stinner in :issue:`42157`.)

* :c:func:`Py_GetPath`, :c:func:`Py_GetPrefix`, :c:func:`Py_GetExecPrefix`,
  :c:func:`Py_GetProgramFullPath`, :c:func:`Py_GetPythonHome` and
  :c:func:`Py_GetProgramName` functions now return ``NULL`` if called before
  :c:func:`Py_Initialize` (before Python is initialized). Use the new
  :ref:`Python Initialization Configuration API <init-config>` to get the
  :ref:`Python Path Configuration.  <init-path-config>`.
  (Contributed by Victor Stinner in :issue:`42260`.)

* :c:func:`PyList_SET_ITEM`, :c:func:`PyTuple_SET_ITEM` and
  :c:func:`PyCell_SET` macros can no longer be used as l-value or r-value.
  For example, ``x = PyList_SET_ITEM(a, b, c)`` and
  ``PyList_SET_ITEM(a, b, c) = x`` now fail with a compiler error. It prevents
  bugs like ``if (PyList_SET_ITEM (a, b, c) < 0) ...`` test.
  (Contributed by Zackery Spytz and Victor Stinner in :issue:`30459`.)

* The non-limited API files ``odictobject.h``, ``parser_interface.h``,
  ``picklebufobject.h``, ``pyarena.h``, ``pyctype.h``, ``pydebug.h``,
  ``pyfpe.h``, and ``pytime.h`` have been moved to the ``Include/cpython``
  directory. These files must not be included directly, as they are already
  included in ``Python.h``: :ref:`Include Files <api-includes>`. If they have
  been included directly, consider including ``Python.h`` instead.
  (Contributed by Nicholas Sim in :issue:`35134`)

Deprecated
----------

* The ``PyUnicode_InternImmortal()`` function is now deprecated
  and will be removed in Python 3.12: use :c:func:`PyUnicode_InternInPlace`
  instead.
  (Contributed by Victor Stinner in :issue:`41692`.)

Removed
-------

* ``PyObject_AsCharBuffer()``, ``PyObject_AsReadBuffer()``, ``PyObject_CheckReadBuffer()``,
  and ``PyObject_AsWriteBuffer()`` are removed. Please migrate to new buffer protocol;
  :c:func:`PyObject_GetBuffer` and :c:func:`PyBuffer_Release`.
  (Contributed by Inada Naoki in :issue:`41103`.)

* Removed ``Py_UNICODE_str*`` functions manipulating ``Py_UNICODE*`` strings.
  (Contributed by Inada Naoki in :issue:`41123`.)

   * ``Py_UNICODE_strlen``: use :c:func:`PyUnicode_GetLength` or
     :c:macro:`PyUnicode_GET_LENGTH`
   * ``Py_UNICODE_strcat``: use :c:func:`PyUnicode_CopyCharacters` or
     :c:func:`PyUnicode_FromFormat`
   * ``Py_UNICODE_strcpy``, ``Py_UNICODE_strncpy``: use
     :c:func:`PyUnicode_CopyCharacters` or :c:func:`PyUnicode_Substring`
   * ``Py_UNICODE_strcmp``: use :c:func:`PyUnicode_Compare`
   * ``Py_UNICODE_strncmp``: use :c:func:`PyUnicode_Tailmatch`
   * ``Py_UNICODE_strchr``, ``Py_UNICODE_strrchr``: use
     :c:func:`PyUnicode_FindChar`

* Removed ``PyUnicode_GetMax()``. Please migrate to new (:pep:`393`) APIs.
  (Contributed by Inada Naoki in :issue:`41103`.)

* Removed ``PyLong_FromUnicode()``. Please migrate to :c:func:`PyLong_FromUnicodeObject`.
  (Contributed by Inada Naoki in :issue:`41103`.)

* Removed ``PyUnicode_AsUnicodeCopy()``. Please use :c:func:`PyUnicode_AsUCS4Copy` or
  :c:func:`PyUnicode_AsWideCharString`
  (Contributed by Inada Naoki in :issue:`41103`.)

* Removed ``_Py_CheckRecursionLimit`` variable: it has been replaced by
  ``ceval.recursion_limit`` of the :c:type:`PyInterpreterState` structure.
  (Contributed by Victor Stinner in :issue:`41834`.)

* Removed undocumented macros ``Py_ALLOW_RECURSION`` and
  ``Py_END_ALLOW_RECURSION`` and the ``recursion_critical`` field of the
  :c:type:`PyInterpreterState` structure.
  (Contributed by Serhiy Storchaka in :issue:`41936`.)

* Removed the undocumented ``PyOS_InitInterrupts()`` function. Initializing
  Python already implicitly installs signal handlers: see
  :c:member:`PyConfig.install_signal_handlers`.
  (Contributed by Victor Stinner in :issue:`41713`.)