****************************
  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 git 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.

* :pep:`644`, require OpenSSL 1.1.1 or newer


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`.)


Better error messages
---------------------

SyntaxErrors
~~~~~~~~~~~~

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:: python

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

but in Python 3.10 a more informative error is emitted:

.. code-block:: python

    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`.)

:exc:`SyntaxError` exceptions raised by the interpreter will now highlight the
full error range of the expression that constitutes the syntax error itself,
instead of just where the problem is detected. In this way, instead of displaying
(before Python 3.10):

.. code-block:: python

    >>> foo(x, z for z in range(10), t, w)
      File "<stdin>", line 1
        foo(x, z for z in range(10), t, w)
               ^
    SyntaxError: Generator expression must be parenthesized

now Python 3.10 will display the exception as:

.. code-block:: python

    >>> foo(x, z for z in range(10), t, w)
      File "<stdin>", line 1
        foo(x, z for z in range(10), t, w)
               ^^^^^^^^^^^^^^^^^^^^
    SyntaxError: Generator expression must be parenthesized

This improvement was contributed by Pablo Galindo in :issue:`43914`.

A considerable amount of new specialized messages for :exc:`SyntaxError` exceptions
have been incorporated. Some of the most notable ones are as follows:

* Missing ``:`` before blocks:

    .. code-block:: python

        >>> if rocket.position > event_horizon
          File "<stdin>", line 1
            if rocket.position > event_horizon
                                              ^
        SyntaxError: expected ':'

    (Contributed by Pablo Galindo in :issue:`42997`)

* Unparenthesised tuples in comprehensions targets:

    .. code-block:: python

        >>> {x,y for x,y in zip('abcd', '1234')}
          File "<stdin>", line 1
            {x,y for x,y in zip('abcd', '1234')}
             ^
        SyntaxError: did you forget parentheses around the comprehension target?

    (Contributed by Pablo Galindo in :issue:`43017`)

* Missing commas in collection literals and between expressions:

    .. code-block:: python

        >>> items = {
        ... x: 1,
        ... y: 2
        ... z: 3,
          File "<stdin>", line 3
            y: 2
               ^
        SyntaxError: invalid syntax. Perhaps you forgot a comma?

    (Contributed by Pablo Galindo in :issue:`43822`)

* Multiple Exception types without parentheses:

    .. code-block:: python

        >>> try:
        ...     build_dyson_sphere()
        ... except NotEnoughScienceError, NotEnoughResourcesError:
          File "<stdin>", line 3
            except NotEnoughScienceError, NotEnoughResourcesError:
                   ^
        SyntaxError: multiple exception types must be parenthesized

    (Contributed by Pablo Galindo in :issue:`43149`)

* Missing ``:`` and values in dictionary literals:

    .. code-block:: python

        >>> values = {
        ... x: 1,
        ... y: 2,
        ... z:
        ... }
          File "<stdin>", line 4
            z:
             ^
        SyntaxError: expression expected after dictionary key and ':'

        >>> values = {x:1, y:2, z w:3}
          File "<stdin>", line 1
            values = {x:1, y:2, z w:3}
                                ^
        SyntaxError: ':' expected after dictionary key

    (Contributed by Pablo Galindo in :issue:`43823`)

* ``try`` blocks without ``except`` or ``finally`` blocks:

    .. code-block:: python

        >>> try:
        ...     x = 2
        ... something = 3
          File "<stdin>", line 3
            something  = 3
            ^^^^^^^^^
        SyntaxError: expected 'except' or 'finally' block

    (Contributed by Pablo Galindo in :issue:`44305`)

* Usage of ``=`` instead of ``==`` in comparisons:

    .. code-block:: python

        >>> if rocket.position = event_horizon:
          File "<stdin>", line 1
            if rocket.position = event_horizon:
                               ^
        SyntaxError: cannot assign to attribute here. Maybe you meant '==' instead of '='?

    (Contributed by Pablo Galindo in :issue:`43797`)

* Usage of ``*`` in f-strings:

    .. code-block:: python

        >>> f"Black holes {*all_black_holes} and revelations"
          File "<stdin>", line 1
            (*all_black_holes)
             ^
        SyntaxError: f-string: cannot use starred expression here

    (Contributed by Pablo Galindo in :issue:`41064`)

IndentationErrors
~~~~~~~~~~~~~~~~~

Many :exc:`IndentationError` exceptions now have more context regarding what kind of block
was expecting an indentation, including the location of the statement:

.. code-block:: python

    >>> def foo():
    ...    if lel:
    ...    x = 2
      File "<stdin>", line 3
        x = 2
        ^
    IndentationError: expected an indented block after 'if' statement in line 2


AttributeErrors
~~~~~~~~~~~~~~~

When printing :exc:`AttributeError`, :c:func:`PyErr_Display` will offer
suggestions of similar attribute names in the object that the exception was
raised from:

.. code-block:: python

    >>> collections.namedtoplo
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?

(Contributed by Pablo Galindo in :issue:`38530`.)

   .. warning::
      Notice this won't work if :c:func:`PyErr_Display` is not called to display the error
      which can happen if some other custom error display function is used. This is a common
      scenario in some REPLs like IPython.

NameErrors
~~~~~~~~~~

When printing :exc:`NameError` raised by the interpreter, :c:func:`PyErr_Display`
will offer suggestions of similar variable names in the function that the exception
was raised from:

.. code-block:: python

    >>> schwarzschild_black_hole = None
    >>> schwarschild_black_hole
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'schwarschild_black_hole' is not defined. Did you mean: schwarzschild_black_hole?

(Contributed by Pablo Galindo in :issue:`38530`.)

   .. warning::
      Notice this won't work if :c:func:`PyErr_Display` is not called to display the error,
      which can happen if some other custom error display function is used. This is a common
      scenario in some REPLs like IPython.


PEP 626: Precise line numbers for debugging and other tools
-----------------------------------------------------------

PEP 626 brings more precise and reliable line numbers for debugging, profiling and coverage tools.
Tracing events, with the correct line number, are generated for all lines of code executed and only for lines of code that are executed.

The ``f_lineno`` attribute of frame objects will always contain the expected line number.

The ``co_lnotab`` attribute of code objects is deprecated and will be removed in 3.12.
Code that needs to convert from offset to line number should use the new ``co_lines()`` method instead.

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 exist, 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.

Patterns 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} occurred.")

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 a 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 is 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`.


.. _whatsnew310-pep597:

Optional ``EncodingWarning`` and ``encoding="locale"`` option
-------------------------------------------------------------

The default encoding of :class:`TextIOWrapper` and :func:`open` is
platform and locale dependent. Since UTF-8 is used on most Unix
platforms, omitting ``encoding`` option when opening UTF-8 files
(e.g. JSON, YAML, TOML, Markdown) is a very common bug. For example::

   # BUG: "rb" mode or encoding="utf-8" should be used.
   with open("data.json") as f:
       data = json.load(f)

To find this type of bug, an optional ``EncodingWarning`` is added.
It is emitted when :data:`sys.flags.warn_default_encoding <sys.flags>`
is true and locale-specific default encoding is used.

``-X warn_default_encoding`` option and :envvar:`PYTHONWARNDEFAULTENCODING`
are added to enable the warning.

See :ref:`io-text-encoding` for more information.


New Features Related to Type Hints
==================================

This section covers major changes affecting :pep:`484` type hints and
the :mod:`typing` module.


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.

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`, :class:`typing.ParamSpecArgs`,
:class:`typing.ParamSpecKwargs`, and :pep:`612` for more details.

(Contributed by Ken Jin in :issue:`41559`, with minor enhancements by Jelle
Zijlstra in :issue:`43783`.  PEP written by Mark Mendoza.)


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

: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 value :data:`TypeAlias`
which lets you 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 647: User-Defined Type Guards
---------------------------------

:data:`TypeGuard` has been added to the :mod:`typing` module to annotate
type guard functions and improve information provided to static type checkers
during type narrowing.  For more information, please see :data:`TypeGuard`\ 's
documentation, and :pep:`647`.

(Contributed by Ken Jin and Guido van Rossum in :issue:`43766`.
PEP written by Eric Traut.)

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

* 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.

* 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`.)

* Two new builtin functions -- :func:`aiter` and :func:`anext` have been added
  to provide asynchronous counterparts to :func:`iter` and :func:`next`,
  respectively.
  (Contributed by Joshua Bronson, Daniel Pope, and Justin Wang in :issue:`31861`.)

* Static methods (:func:`@staticmethod <staticmethod>`) and class methods
  (:func:`@classmethod <classmethod>`) now inherit the method attributes
  (``__module__``, ``__name__``, ``__qualname__``, ``__doc__``,
  ``__annotations__``) and have a new ``__wrapped__`` attribute.
  Moreover, static methods are now callable as regular functions.
  (Contributed by Victor Stinner in :issue:`43682`.)

* Annotations for complex targets (everything beside ``simple name`` targets
  defined by :pep:`526`) no longer cause any runtime effects with ``from __future__ import annotations``.
  (Contributed by Batuhan Taskaya in :issue:`42737`.)

* Class and module objects now lazy-create empty annotations dicts on demand.
  The annotations dicts are stored in the object’s ``__dict__`` for
  backwards compatibility.  This improves the best practices for working
  with ``__annotations__``; for more information, please see
  :ref:`annotations-howto`.
  (Contributed by Larry Hastings in :issue:`43901`.)

* Annotations consist of ``yield``, ``yield from``, ``await`` or named expressions
  are now forbidden under ``from __future__ import annotations`` due to their side
  effects.
  (Contributed by Batuhan Taskaya in :issue:`42725`.)

* Usage of unbound variables, ``super()`` and other expressions that might
  alter the processing of symbol table as annotations are now rendered
  effectless under ``from __future__ import annotations``.
  (Contributed by Batuhan Taskaya in :issue:`42725`.)

* Hashes of NaN values of both :class:`float` type and
  :class:`decimal.Decimal` type now depend on object identity. Formerly, they
  always hashed to ``0`` even though NaN values are not equal to one another.
  This caused potentially quadratic runtime behavior due to excessive hash
  collisions when creating dictionaries and sets containing multiple NaNs.
  (Contributed by Raymond Hettinger in :issue:`43475`.)

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

* None yet.


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

asyncio
-------

Add missing :meth:`~asyncio.events.AbstractEventLoop.connect_accepted_socket`
method.
(Contributed by Alex Grönholm in :issue:`41332`.)

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`.)

array
-----

The :meth:`~array.array.index` method of :class:`array.array` now has
optional *start* and *stop* parameters.
(Contributed by Anders Lorentsen and Zackery Spytz in :issue:`31956`.)

asynchat, asyncore, smtpd
-------------------------
These modules have been marked as deprecated in their module documentation
since Python 3.6.  An import-time :class:`DeprecationWarning` has now been
added to all three of these modules.

base64
------

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

bdb
---

Add :meth:`~bdb.Breakpoint.clearBreakpoints` to reset all set breakpoints.
(Contributed by Irit Katriel in :issue:`24160`.)

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`.)

Add :class:`AsyncContextDecorator`, for supporting usage of async context managers
as decorators.

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`.)

dataclasses
-----------

Add ``slots`` parameter in :func:`dataclasses.dataclass` decorator.
(Contributed by Yurii Karabas in :issue:`42269`)

.. _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`.)

enum
----

:class:`Enum` :func:`__repr__` now returns ``enum_name.member_name`` and
:func:`__str__` now returns ``member_name``.  Stdlib enums available as
module constants have a :func:`repr` of ``module_name.member_name``.
(Contributed by Ethan Furman in :issue:`40066`.)

Add :class:`enum.StrEnum` for enums where all members are strings.
(Contributed by Ethan Furman in :issue:`41816`.)

fileinput
---------

Add *encoding* and *errors* parameters in :func:`fileinput.input` and
:class:`fileinput.FileInput`.
(Contributed by Inada Naoki in :issue:`43712`.)

:func:`fileinput.hook_compressed` now returns :class:`TextIOWrapper` object
when *mode* is "r" and file is compressed, like uncompressed files.
(Contributed by Inada Naoki in :issue:`5758`.)

faulthandler
------------

The :mod:`faulthandler` module now detects if a fatal error occurs during a
garbage collector collection.
(Contributed by Victor Stinner in :issue:`44466`.)

gc
--

Add audit hooks for :func:`gc.get_objects`, :func:`gc.get_referrers` and
:func:`gc.get_referents`. (Contributed by Pablo Galindo in :issue:`43439`.)

glob
----

Add 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`.)

hashlib
-------

The hashlib module requires OpenSSL 1.1.1 or newer.
(Contributed by Christian Heimes in :pep:`644` and :issue:`43669`.)

The hashlib module has preliminary support for OpenSSL 3.0.0.
(Contributed by Christian Heimes in :issue:`38820` and other issues.)

The pure-Python fallback of :func:`~hashlib.pbkdf2_hmac` is deprecated. In
the future PBKDF2-HMAC will only be available when Python has been built with
OpenSSL support.
(Contributed by Christian Heimes in :issue:`43880`.)

hmac
----

The hmac module now uses OpenSSL's HMAC implementation internally.
(Contributed by Christian Heimes in :issue:`40645`.)

IDLE and idlelib
----------------

Make IDLE invoke :func:`sys.excepthook` (when started without '-n').
User hooks were previously ignored.  (Patch by Ken Hilton in
:issue:`43008`.)

This change was backported to a 3.9 maintenance release.

Add a Shell sidebar.  Move the primary prompt ('>>>') to the sidebar.
Add secondary prompts ('...') to the sidebar.  Left click and optional
drag selects one or more lines of text, as with the editor
line number sidebar.  Right click after selecting text lines displays
a context menu with 'copy with prompts'.  This zips together prompts
from the sidebar with lines from the selected text.  This option also
appears on the context menu for the text.  (Contributed by Tal Einat
in :issue:`37903`.)

Use spaces instead of tabs to indent interactive code.  This makes
interactive code entries 'look right'.  Making this feasible was a
major motivation for adding the shell sidebar.  Contributed by
Terry Jan Reedy in :issue:`37892`.)

We expect to backport these shell changes to a future 3.9 maintenance
release.

Highlight the new :ref:`soft keywords <soft-keywords>` :keyword:`match`,
:keyword:`case <match>`, and :keyword:`_ <wildcard-patterns>` in
pattern-matching statements. However, this highlighting is not perfect
and will be incorrect in some rare cases, including some ``_``-s in
``case`` patterns.  (Contributed by Tal Einat in bpo-44010.)

importlib.metadata
------------------

Feature parity with ``importlib_metadata`` 4.6
(`history <https://importlib-metadata.readthedocs.io/en/latest/history.html>`_).

:ref:`importlib.metadata entry points <entry-points>`
now provides a nicer experience
for selecting entry points by group and name through a new
:class:`importlib.metadata.EntryPoints` class. See the Compatibility
Note in the docs for more info on the deprecation and usage.

Added :func:`importlib.metadata.packages_distributions` for resolving
top-level Python modules and packages to their
:class:`importlib.metadata.Distribution`.

inspect
-------

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

Add :func:`inspect.get_annotations`, which safely computes the annotations
defined on an object. It works around the quirks of accessing the annotations
on various types of objects, and makes very few assumptions about the object
it examines. :func:`inspect.get_annotations` can also correctly un-stringize
stringized annotations.  :func:`inspect.get_annotations` is now considered
best practice for accessing the annotations dict defined on any Python object;
for more information on best practices for working with annotations, please see
:ref:`annotations-howto`.
Relatedly, :func:`inspect.signature`,
:func:`inspect.Signature.from_callable`, and :func:`inspect.Signature.from_function`
now call :func:`inspect.get_annotations` to retrieve annotations. This means
:func:`inspect.signature` and :func:`inspect.Signature.from_callable` can
also now un-stringize stringized annotations.
(Contributed by Larry Hastings in :issue:`43817`.)

linecache
---------

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

os
--

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

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

Add :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`.)

Add :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`.)

os.path
-------

:func:`os.path.realpath` now accepts a *strict* keyword-only argument. When set
to ``True``, :exc:`OSError` is raised if a path doesn't exist or a symlink loop
is encountered.
(Contributed by Barney Gale in :issue:`43757`.)

pathlib
-------

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

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

Add :meth:`Path.hardlink_to <pathlib.Path.hardlink_to>` method that
supersedes :meth:`~pathlib.Path.link_to`. The new method has the same argument
order as :meth:`~pathlib.Path.symlink_to`.
(Contributed by Barney Gale in :issue:`39950`.)

:meth:`pathlib.Path.stat` and :meth:`~pathlib.Path.chmod` now accept a
*follow_symlinks* keyword-only argument for consistency with corresponding
functions in the :mod:`os` module.
(Contributed by Barney Gale in :issue:`39906`.)

platform
--------

Add :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`)

pprint
------

:func:`pprint.pprint` now accepts a new ``underscore_numbers`` keyword argument.
(Contributed by sblondon in :issue:`42914`.)

:mod:`pprint` can now pretty-print :class:`dataclasses.dataclass` instances.
(Contributed by Lewis Gaul in :issue:`43080`.)

py_compile
----------

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

pyclbr
------

Add 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`.)

statistics
----------

Add :func:`~statistics.covariance`, Pearson's
:func:`~statistics.correlation`, and simple
:func:`~statistics.linear_regression` functions.
(Contributed by Tymoteusz Wołodźko in :issue:`38490`.)

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`.)

Add option to create MPTCP sockets with ``IPPROTO_MPTCP``
(Contributed by Rui Cunha in :issue:`43571`.)

Add ``IP_RECVTOS`` option to receive the type of service (ToS) or DSCP/ECN fields
(Contributed by Georg Sauthoff in :issue:`44077`.)

ssl
---

The ssl module requires OpenSSL 1.1.1 or newer.
(Contributed by Christian Heimes in :pep:`644` and :issue:`43669`.)

The ssl module has preliminary support for OpenSSL 3.0.0 and new option
:data:`~ssl.OP_IGNORE_UNEXPECTED_EOF`.
(Contributed by Christian Heimes in :issue:`38820`, :issue:`43794`,
:issue:`43788`, :issue:`43791`, :issue:`43799`, :issue:`43920`,
:issue:`43789`, and :issue:`43811`.)

Deprecated function and use of deprecated constants now result in
a :exc:`DeprecationWarning`. :attr:`ssl.SSLContext.options` has
:data:`~ssl.OP_NO_SSLv2` and :data:`~ssl.OP_NO_SSLv3` set by default and
therefore cannot warn about setting the flag again. The
:ref:`deprecation section <whatsnew310-deprecated>` has a list of deprecated
features.
(Contributed by Christian Heimes in :issue:`43880`.)

The ssl module now has more secure default settings. Ciphers without forward
secrecy or SHA-1 MAC are disabled by default. Security level 2 prohibits
weak RSA, DH, and ECC keys with less than 112 bits of security.
:class:`~ssl.SSLContext` defaults to minimum protocol version TLS 1.2.
Settings are based on Hynek Schlawack's research.
(Contributed by Christian Heimes in :issue:`43998`.)

The deprecated protocols SSL 3.0, TLS 1.0, and TLS 1.1 are no longer
officially supported. Python does not block them actively. However
OpenSSL build options, distro configurations, vendor patches, and cipher
suites may prevent a successful handshake.

Add a *timeout* parameter to the :func:`ssl.get_server_certificate` function.
(Contributed by Zackery Spytz in :issue:`31870`.)

The ssl module uses heap-types and multi-phase initialization.
(Contributed by Christian Heimes in :issue:`42333`.)

A new verify flag :data:`~ssl.VERIFY_X509_PARTIAL_CHAIN` has been added.
(Contributed by l0x in :issue:`40849`.)

sqlite3
-------

Add audit events for :func:`~sqlite3.connect/handle`,
:meth:`~sqlite3.Connection.enable_load_extension`, and
:meth:`~sqlite3.Connection.load_extension`.
(Contributed by Erlend E. Aasland in :issue:`43762`.)

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`.)

_thread
-------

:func:`_thread.interrupt_main` now takes an optional signal number to
simulate (the default is still :data:`signal.SIGINT`).
(Contributed by Antoine Pitrou in :issue:`43356`.)

threading
---------

Add :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
-----

Reintroduce 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
------

For major changes, see `New Features Related to Type Hints`_.

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 any of their parameters are not :term:`hashable`.
   Note that declaring ``Literal`` with unhashable 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`.)

Add new function :func:`typing.is_typeddict` to introspect if an annotation
is a :class:`typing.TypedDict`.
(Contributed by Patrick Reader in :issue:`41792`)

Subclasses of ``typing.Protocol`` which only have data variables declared
will now raise a ``TypeError`` when checked with ``isinstance`` unless they
are decorated with :func:`runtime_checkable`.  Previously, these checks
passed silently.  Users should decorate their
subclasses with the :func:`runtime_checkable` decorator
if they want runtime protocols.
(Contributed by Yurii Karabas in :issue:`38908`)

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`.)

Add :meth:`~zipimport.zipimporter.invalidate_caches` method.
(Contributed by Desmond Cheong in :issue:`14678`.)


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 1.4x faster in
  average. On Linux, ``python3 -I -m module-name`` imports 69 modules on Python
  3.9, whereas it only imports 51 modules (-18) on Python 3.10.
  (Contributed by Victor Stinner in :issue:`41006` and :issue:`41718`.)

* 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 :option:`--enable-optimizations` now
  ``-fno-semantic-interposition`` is added to both the compile and link line.
  This speeds builds of the Python interpreter created with :option:`--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`.)

* Use a new output buffer management code for :mod:`bz2` / :mod:`lzma` /
  :mod:`zlib` modules, and add ``.readall()`` function to
  ``_compression.DecompressReader`` class. bz2 decompression is now 1.09x ~ 1.17x
  faster, lzma decompression 1.20x ~ 1.32x faster, ``GzipFile.read(-1)`` 1.11x
  ~ 1.18x faster. (Contributed by Ma Lin, reviewed by Gregory P. Smith, in :issue:`41486`)

* When using stringized annotations, annotations dicts for functions are no longer
  created when the function is created.  Instead, they are stored as a tuple of
  strings, and the function object lazily converts this into the annotations dict
  on demand.  This optimization cuts the CPU time needed to define an annotated
  function by half.
  (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`)

* Add micro-optimizations to ``_PyType_Lookup()`` to improve type attribute cache lookup
  performance in the common case of cache hits. This makes the interpreter 1.04 times faster
  on average. (Contributed by Dino Viehland in :issue:`43452`)

* The following built-in functions now support the faster :pep:`590` vectorcall calling convention:
  :func:`map`, :func:`filter`, :func:`reversed`, :func:`bool` and :func:`float`.
  (Contributed by Dong-hee Na and Jeroen Demeyer in :issue:`43575`, :issue:`43287`, :issue:`41922`, :issue:`41873` and :issue:`41870`)

* :class:`BZ2File` performance is improved by removing internal ``RLock``.
  This makes :class:`BZ2File` thread unsafe in the face of multiple simultaneous
  readers or writers, just like its equivalent classes in :mod:`gzip` and
  :mod:`lzma` have always been.  (Contributed by Inada Naoki in :issue:`43785`).

.. _whatsnew310-deprecated:

Deprecated
==========

* Currently Python accepts numeric literals immediately followed by keywords,
  for example ``0in x``, ``1or x``, ``0if 1else 2``.  It allows confusing
  and ambigious expressions like ``[0x1for x in y]`` (which can be
  interpreted as ``[0x1 for x in y]`` or ``[0x1f or x in y]``).  Starting in
  this release, a deprecation warning is raised if the numeric literal is
  immediately followed by one of keywords :keyword:`and`, :keyword:`else`,
  :keyword:`for`, :keyword:`if`, :keyword:`in`, :keyword:`is` and :keyword:`or`.
  In future releases it will be changed to syntax warning, and finally to
  syntax error.
  (Contributed by Serhiy Storchaka in :issue:`43833`).

* 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`.)

* The use of :meth:`importlib.abc.MetaPathFinder.find_module` and
  :meth:`importlib.abc.PathEntryFinder.find_module` by the import system now
  trigger an :exc:`ImportWarning` as
  :meth:`importlib.abc.MetaPathFinder.find_spec` and
  :meth:`importlib.abc.PathEntryFinder.find_spec`
  are preferred, respectively. You can use
  :func:`importlib.util.spec_from_loader` to help in porting.
  (Contributed by Brett Cannon in :issue:`42134`.)

* The use of :meth:`importlib.abc.PathEntryFinder.find_loader` by the import
  system now triggers an :exc:`ImportWarning` as
  :meth:`importlib.abc.PathEntryFinder.find_spec` is preferred. You can use
  :func:`importlib.util.spec_from_loader` to help in porting.
  (Contributed by Brett Cannon in :issue:`43672`.)

* The various implementations of
  :meth:`importlib.abc.MetaPathFinder.find_module` (
  :meth:`importlib.machinery.BuiltinImporter.find_module`,
  :meth:`importlib.machinery.FrozenImporter.find_module`,
  :meth:`importlib.machinery.WindowsRegistryFinder.find_module`,
  :meth:`importlib.machinery.PathFinder.find_module`,
  :meth:`importlib.abc.MetaPathFinder.find_module`),
  :meth:`importlib.abc.PathEntryFinder.find_module` (
  :meth:`importlib.machinery.FileFinder.find_module`,
  ), and
  :meth:`importlib.abc.PathEntryFinder.find_loader` (
  :meth:`importlib.machinery.FileFinder.find_loader`
  ) now raise :exc:`DeprecationWarning` and are slated for removal in
  Python 3.12 (previously they were documented as deprecated in Python 3.4).
  (Contributed by Brett Cannon in :issue:`42135`.)

* :class:`importlib.abc.Finder` is deprecated (including its sole method,
  :meth:`~importlib.abc.Finder.find_module`). Both
  :class:`importlib.abc.MetaPathFinder` and :class:`importlib.abc.PathEntryFinder`
  no longer inherit from the class. Users should inherit from one of these two
  classes as appropriate instead.
  (Contributed by Brett Cannon in :issue:`42135`.)

* The deprecations of :mod:`imp`, :func:`importlib.find_loader`,
  :func:`importlib.util.set_package_wrapper`,
  :func:`importlib.util.set_loader_wrapper`,
  :func:`importlib.util.module_for_loader`,
  :class:`pkgutil.ImpImporter`, and
  :class:`pkgutil.ImpLoader` have all been updated to list Python 3.12 as the
  slated version of removal (they began raising :exc:`DeprecationWarning` in
  previous versions of Python).
  (Contributed by Brett Cannon in :issue:`43720`.)

* The import system now uses the ``__spec__`` attribute on modules before
  falling back on :meth:`~importlib.abc.Loader.module_repr` for a module's
  ``__repr__()`` method. Removal of the use of ``module_repr()`` is scheduled
  for Python 3.12.
  (Contributed by Brett Cannon in :issue:`42137`.)

* :meth:`importlib.abc.Loader.module_repr`,
  :meth:`importlib.machinery.FrozenLoader.module_repr`, and
  :meth:`importlib.machinery.BuiltinLoader.module_repr` are deprecated and
  slated for removal in Python 3.12.
  (Contributed by Brett Cannon in :issue:`42136`.)

* ``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`.)

* :func:`asyncio.get_event_loop` now emits a deprecation warning if there is
  no running event loop. In the future it will be an alias of
  :func:`~asyncio.get_running_loop`.
  :mod:`asyncio` functions which implicitly create a :class:`~asyncio.Future`
  or :class:`~asyncio.Task` objects now emit
  a deprecation warning if there is no running event loop and no explicit
  *loop* argument is passed: :func:`~asyncio.ensure_future`,
  :func:`~asyncio.wrap_future`, :func:`~asyncio.gather`,
  :func:`~asyncio.shield`, :func:`~asyncio.as_completed` and constructors of
  :class:`~asyncio.Future`, :class:`~asyncio.Task`,
  :class:`~asyncio.StreamReader`, :class:`~asyncio.StreamReaderProtocol`.
  (Contributed by Serhiy Storchaka in :issue:`39529`.)

* 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 a 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`.)

* The following ``threading`` methods are now deprecated:

  * ``threading.currentThread`` => :func:`threading.current_thread`

  * ``threading.activeCount`` => :func:`threading.active_count`

  * ``threading.Condition.notifyAll`` =>
    :meth:`threading.Condition.notify_all`

  * ``threading.Event.isSet`` => :meth:`threading.Event.is_set`

  * ``threading.Thread.setName`` => :attr:`threading.Thread.name`

  * ``threading.thread.getName`` => :attr:`threading.Thread.name`

  * ``threading.Thread.isDaemon`` => :attr:`threading.Thread.daemon`

  * ``threading.Thread.setDaemon`` => :attr:`threading.Thread.daemon`

  (Contributed by Jelle Zijlstra in :issue:`21574`.)

* :meth:`pathlib.Path.link_to` is deprecated and slated for removal in
  Python 3.12. Use :meth:`pathlib.Path.hardlink_to` instead.
  (Contributed by Barney Gale in :issue:`39950`.)

* ``cgi.log()`` is deprecated and slated for removal in Python 3.12.
  (Contributed by Inada Naoki in :issue:`41139`.)

* The following :mod:`ssl` features have been deprecated since Python 3.6,
  Python 3.7, or OpenSSL 1.1.0 and will be removed in 3.11:

  * :data:`~ssl.OP_NO_SSLv2`, :data:`~ssl.OP_NO_SSLv3`, :data:`~ssl.OP_NO_TLSv1`,
    :data:`~ssl.OP_NO_TLSv1_1`, :data:`~ssl.OP_NO_TLSv1_2`, and
    :data:`~ssl.OP_NO_TLSv1_3` are replaced by
    :attr:`sslSSLContext.minimum_version` and
    :attr:`sslSSLContext.maximum_version`.

  * :data:`~ssl.PROTOCOL_SSLv2`, :data:`~ssl.PROTOCOL_SSLv3`,
    :data:`~ssl.PROTOCOL_SSLv23`, :data:`~ssl.PROTOCOL_TLSv1`,
    :data:`~ssl.PROTOCOL_TLSv1_1`, :data:`~ssl.PROTOCOL_TLSv1_2`, and
    :data:`~ssl.PROTOCOL_TLS` are deprecated in favor of
    :data:`~ssl.PROTOCOL_TLS_CLIENT` and :data:`~ssl.PROTOCOL_TLS_SERVER`

  * :func:`~ssl.wrap_socket` is replaced by :meth:`ssl.SSLContext.wrap_socket`

  * :func:`~ssl.match_hostname`

  * :func:`~ssl.RAND_pseudo_bytes`, :func:`~ssl.RAND_egd`

  * NPN features like :meth:`ssl.SSLSocket.selected_npn_protocol` and
    :meth:`ssl.SSLContext.set_npn_protocols` are replaced by ALPN.

.. _whatsnew310-removed:

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 was already 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 ``PyParser_SimpleParseStringFlags``,
  ``PyParser_SimpleParseStringFlagsFilename``,
  ``PyParser_SimpleParseFileFlags`` and ``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 syntax
----------------------------

* Deprecation warning is now emitted when compiling previously valid syntax
  if the numeric literal is immediately followed by a keyword (like in ``0in x``).
  If future releases it will be changed to syntax warning, and finally to a
  syntax error.  To get rid of the warning and make the code compatible with
  future releases just add a space between the numeric literal and the
  following keyword.
  (Contributed by Serhiy Storchaka in :issue:`43833`).


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 looks 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`.)

Changes in the C API
--------------------

* The C API functions ``PyParser_SimpleParseStringFlags``,
  ``PyParser_SimpleParseStringFlagsFilename``,
  ``PyParser_SimpleParseFileFlags``, ``PyNode_Compile`` and the type
  used by these functions, ``struct _node``, were removed due to the switch
  to the new PEG parser.

  Source should be now be compiled directly to a code object using, for
  example, :c:func:`Py_CompileString`. The resulting code object can then be
  evaluated using, for example, :c:func:`PyEval_EvalCode`.

  Specifically:

  * A call to ``PyParser_SimpleParseStringFlags`` followed by
    ``PyNode_Compile`` can be replaced by calling :c:func:`Py_CompileString`.

  * There is no direct replacement for ``PyParser_SimpleParseFileFlags``.
    To compile code from a ``FILE *`` argument, you will need to read
    the file in C and pass the resulting buffer to :c:func:`Py_CompileString`.

  * To compile a file given a ``char *`` filename, explicitly open the file, read
    it and compile the result. One way to do this is using the :py:mod:`io`
    module with :c:func:`PyImport_ImportModule`, :c:func:`PyObject_CallMethod`,
    :c:func:`PyBytes_AsString` and :c:func:`Py_CompileString`,
    as sketched below. (Declarations and error handling are omitted.) ::

       io_module = Import_ImportModule("io");
       fileobject = PyObject_CallMethod(io_module, "open", "ss", filename, "rb");
       source_bytes_object = PyObject_CallMethod(fileobject, "read", "");
       result = PyObject_CallMethod(fileobject, "close", "");
       source_buf = PyBytes_AsString(source_bytes_object);
       code = Py_CompileString(source_buf, filename, Py_file_input);

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

* The ``MAKE_FUNCTION`` instruction now accepts either a dict or a tuple of
  strings as the function's annotations.
  (Contributed by Yurii Karabas and Inada Naoki in :issue:`42202`)

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

* :pep:`644`: Python now requires OpenSSL 1.1.1 or newer. OpenSSL 1.0.2 is no
  longer supported.
  (Contributed by Christian Heimes in :issue:`43669`.)

* 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`.)

* Add :option:`--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 :option:`--with-wheel-pkg-dir=PATH option <--with-wheel-pkg-dir>`
  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 :option:`configure --without-static-libpython option
  <--without-static-libpython>` 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 :option:`--with-tcltk-includes`
  and :option:`--with-tcltk-libs` configuration options.
  (Contributed by Manolis Stamatogiannakis in :issue:`42603`.)

* Add :option:`--with-openssl-rpath` option to ``configure`` script. The option
  simplifies building Python with a custom OpenSSL installation, e.g.
  ``./configure --with-openssl=/path/to/openssl --with-openssl-rpath=auto``.
  (Contributed by Christian Heimes in :issue:`43466`.)


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

PEP 652: Maintaining the Stable ABI
-----------------------------------

The Stable ABI (Application Binary Interface) for extension modules or
embedding Python is now explicitly defined.
:ref:`stable` describes C API and ABI stability guarantees along with best
practices for using the Stable ABI.

(Contributed by Petr Viktorin in :pep:`652` and :issue:`43795`.)

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`.)

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

* Add :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`.)

* Add :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
  :ref:`static types <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`.)

* Add :c:func:`PyErr_SetInterruptEx` which allows passing a signal number
  to simulate.
  (Contributed by Antoine Pitrou in :issue:`43356`.)

* The limited C API is now supported if :ref:`Python is built in debug mode
  <debug-build>` (if the ``Py_DEBUG`` macro is defined). In the limited C API,
  the :c:func:`Py_INCREF` and :c:func:`Py_DECREF` functions are now implemented
  as opaque function
  calls, rather than accessing directly the :c:member:`PyObject.ob_refcnt`
  member, if Python is built in debug mode and the ``Py_LIMITED_API`` macro
  targets Python 3.10 or newer. It became possible to support the limited C API
  in debug mode because the :c:type:`PyObject` structure is the same in release
  and debug mode since Python 3.8 (see :issue:`36465`).

  The limited C API is still not supported in the :option:`--with-trace-refs`
  special build (``Py_TRACE_REFS`` macro).
  (Contributed by Victor Stinner in :issue:`43688`.)

* Add the :c:func:`Py_Is(x, y) <Py_Is>` function to test if the *x* object is
  the *y* object, the same as ``x is y`` in Python. Add also the
  :c:func:`Py_IsNone`, :c:func:`Py_IsTrue`, :c:func:`Py_IsFalse` functions to
  test if an object is, respectively, the ``None`` singleton, the ``True``
  singleton or the ``False`` singleton.
  (Contributed by Victor Stinner in :issue:`43753`.)

* Add new functions to control the garbage collector from C code:
  :c:func:`PyGC_Enable()`,
  :c:func:`PyGC_Disable()`,
  :c:func:`PyGC_IsEnabled()`.
  These functions allow to activate, deactivate and query the state of the garbage collector from C code without
  having to import the :mod:`gc` module.

* Add a new :c:data:`Py_TPFLAGS_DISALLOW_INSTANTIATION` type flag to disallow
  creating type instances.
  (Contributed by Victor Stinner in :issue:`43916`.)

* Add a new :c:data:`Py_TPFLAGS_IMMUTABLETYPE` type flag for creating immutable
  type objects: type attributes cannot be set nor deleted.
  (Contributed by Victor Stinner and Erlend E. Aasland in :issue:`43908`.)

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`)

* Use the :c:data:`Py_TPFLAGS_IMMUTABLETYPE` type flag to create immutable type
  objects. Do not rely on :c:data:`Py_TPFLAGS_HEAPTYPE` to decide if a type
  object is mutable or not; check if :c:data:`Py_TPFLAGS_IMMUTABLETYPE` is set
  instead.
  (Contributed by Victor Stinner and Erlend E. Aasland in :issue:`43908`.)

* The undocumented function ``Py_FrozenMain`` has been removed from the
  limited API. The function is mainly useful for custom builds of Python.
  (Contributed by Petr Viktorin in :issue:`26241`)

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`.)

* Remove the ``PyAST_Validate()`` function. It is no longer possible to build a
  AST object (``mod_ty`` type) with the public C API. The function was already
  excluded from the limited C API (:pep:`384`).
  (Contributed by Victor Stinner in :issue:`43244`.)

* Remove the ``symtable.h`` header file and the undocumented functions:

  * ``PyST_GetScope()``
  * ``PySymtable_Build()``
  * ``PySymtable_BuildObject()``
  * ``PySymtable_Free()``
  * ``Py_SymtableString()``
  * ``Py_SymtableStringObject()``

  The ``Py_SymtableString()`` function was part the stable ABI by mistake but
  it could not be used, because the ``symtable.h`` header file was excluded
  from the limited C API.

  Use Python :mod:`symtable` module instead.
  (Contributed by Victor Stinner in :issue:`43244`.)

* Remove :c:func:`PyOS_ReadlineFunctionPointer` from the limited C API headers
  and from ``python3.dll``, the library that provides the stable ABI on
  Windows. Since the function takes a ``FILE*`` argument, its ABI stability
  cannot be guaranteed.
  (Contributed by Petr Viktorin in :issue:`43868`.)

* Remove ``ast.h``, ``asdl.h``, and ``Python-ast.h`` header files.
  These functions were undocumented and excluded from the limited C API.
  Most names defined by these header files were not prefixed by ``Py`` and so
  could create names conflicts. For example, ``Python-ast.h`` defined a
  ``Yield`` macro which was conflict with the ``Yield`` name used by the
  Windows ``<winbase.h>`` header. Use the Python :mod:`ast` module instead.
  (Contributed by Victor Stinner in :issue:`43244`.)

* Remove the compiler and parser functions using ``struct _mod`` type, because
  the public AST C API was removed:

  * ``PyAST_Compile()``
  * ``PyAST_CompileEx()``
  * ``PyAST_CompileObject()``
  * ``PyFuture_FromAST()``
  * ``PyFuture_FromASTObject()``
  * ``PyParser_ASTFromFile()``
  * ``PyParser_ASTFromFileObject()``
  * ``PyParser_ASTFromFilename()``
  * ``PyParser_ASTFromString()``
  * ``PyParser_ASTFromStringObject()``

  These functions were undocumented and excluded from the limited C API.
  (Contributed by Victor Stinner in :issue:`43244`.)

* Remove the ``pyarena.h`` header file with functions:

  * ``PyArena_New()``
  * ``PyArena_Free()``
  * ``PyArena_Malloc()``
  * ``PyArena_AddPyObject()``

  These functions were undocumented, excluded from the limited C API, and were
  only used internally by the compiler.
  (Contributed by Victor Stinner in :issue:`43244`.)