
****************************
  What's new in Python 3.15
****************************

:Editor: Hugo van Kemenade

.. 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 issue number as a comment:

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

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

This article explains the new features in Python 3.15, compared to 3.14.

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 3.15 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 3.15.
   Brevity is key.


.. PEP-sized items next.

* :pep:`810`: :ref:`Explicit lazy imports for faster startup times
  <whatsnew315-lazy-imports>`
* :pep:`814`: :ref:`Add frozendict built-in type
  <whatsnew315-frozendict>`
* :pep:`799`: :ref:`A dedicated profiling package for organizing Python
  profiling tools <whatsnew315-profiling-package>`
* :pep:`799`: :ref:`Tachyon: High frequency statistical sampling profiler
  <whatsnew315-sampling-profiler>`
* :pep:`798`: :ref:`Unpacking in comprehensions
  <whatsnew315-unpacking-in-comprehensions>`
* :pep:`686`: :ref:`Python now uses UTF-8 as the default encoding
  <whatsnew315-utf8-default>`
* :pep:`728`: ``TypedDict`` with typed extra items
* :pep:`747`: :ref:`Annotating type forms with TypeForm
  <whatsnew315-typeform>`
* :pep:`782`: :ref:`A new PyBytesWriter C API to create a Python bytes object
  <whatsnew315-pybyteswriter>`
* :ref:`The JIT compiler has been significantly upgraded <whatsnew315-jit>`
* :ref:`Improved error messages <whatsnew315-improved-error-messages>`
* :ref:`The official Windows 64-bit binaries now use the tail-calling interpreter
  <whatsnew315-windows-tail-calling-interpreter>`

New features
============

.. _whatsnew315-lazy-imports:

:pep:`810`: Explicit lazy imports
---------------------------------

Large Python applications often suffer from slow startup times. A
significant contributor to this problem is the import system: when a module
is imported, Python must locate the file, read it from disk, compile it to
bytecode, and execute all top-level code. For applications with deep
dependency trees, this process can take seconds, even when most of the
imported code is never actually used during a particular run.

Developers have worked around this by moving imports inside functions, using
:mod:`importlib` to load modules on demand, or restructuring code to avoid
unnecessary dependencies. These approaches work but make code harder to read
and maintain, scatter import statements throughout the codebase, and require
discipline to apply consistently.

Python now provides a cleaner solution through explicit :keyword:`lazy`
imports using the new ``lazy`` soft keyword. When you mark an import as
lazy, Python defers the actual module loading until the imported name is
first used. This gives you the organizational benefits of declaring all
imports at the top of the file while only paying the loading cost for
modules you actually use.

The ``lazy`` keyword works with both ``import`` and ``from ... import``
statements. When you write ``lazy import heavy_module``, Python does not
immediately load the module. Instead, it creates a lightweight proxy object.
The actual module loading happens transparently when you first access the
name:

.. code-block:: python

   lazy import json
   lazy from pathlib import Path

   print("Starting up...")  # json and pathlib not loaded yet

   data = json.loads('{"key": "value"}')  # json loads here
   p = Path(".")  # pathlib loads here

This mechanism is particularly useful for applications that import many
modules at the top level but may only use a subset of them in any given run.
The deferred loading reduces startup latency without requiring code
restructuring or conditional imports scattered throughout the codebase.

In the case where loading a lazily imported module fails (for example, if
the module does not exist), Python raises the exception at the point of
first use rather than at import time. The associated traceback includes both
the location where the name was accessed and the original import statement,
making it straightforward to diagnose & debug the failure.

For cases where you want to enable lazy loading globally without modifying
source code, Python provides the :option:`-X lazy_imports <-X>` command-line
option and the :envvar:`PYTHON_LAZY_IMPORTS` environment variable. Both
accept three values: ``all`` makes all imports lazy by default, ``none``
disables lazy imports entirely (even explicit ``lazy`` statements become
eager), and ``normal`` (the default) respects the ``lazy`` keyword in source
code. The :func:`sys.set_lazy_imports` and :func:`sys.get_lazy_imports`
functions allow changing and querying this mode at runtime.

For more selective control, :func:`sys.set_lazy_imports_filter` accepts a
callable that determines whether a specific module should be loaded lazily.
The filter receives three arguments: the importing module's name (or
``None``), the imported module's name, and the fromlist (or ``None`` for
regular imports). It should return ``True`` to allow the import to be lazy,
or ``False`` to force eager loading. This allows patterns like making only
your own application's modules lazy while keeping third-party dependencies
eager:

.. code-block:: python

   import sys

   def myapp_filter(importing, imported, fromlist):
       return imported.startswith("myapp.")
   sys.set_lazy_imports_filter(myapp_filter)
   sys.set_lazy_imports("all")

   import myapp.slow_module  # lazy (matches filter)
   import json               # eager (does not match filter)

The proxy type itself is available as :data:`types.LazyImportType` for code
that needs to detect lazy imports programmatically.

There are some restrictions on where the ``lazy`` keyword can be used. Lazy
imports are only permitted at module scope; using ``lazy`` inside a
function, class body, or ``try``/``except``/``finally`` block raises a
:exc:`SyntaxError`. Neither star imports nor future imports can be lazy
(``lazy from module import *`` and ``lazy from __future__ import ...`` both
raise :exc:`SyntaxError`).

.. seealso:: :pep:`810` for the full specification and rationale.

(Contributed by Pablo Galindo Salgado and Dino Viehland in :gh:`142349`.)


.. _whatsnew315-frozendict:

:pep:`814`: Add frozendict built-in type
----------------------------------------

A new :term:`immutable` type, :class:`frozendict`, is added to the :mod:`builtins` module.
It does not allow modification after creation. A :class:`!frozendict` is not a subclass of ``dict``;
it inherits directly from ``object``. A :class:`!frozendict` is :term:`hashable`
as long as all of its keys and values are hashable. A :class:`!frozendict` preserves
insertion order, but comparison does not take order into account.

For example::

    >>> a = frozendict(x=1, y=2)
    >>> a
    frozendict({'x': 1, 'y': 2})
    >>> a['z'] = 3
    Traceback (most recent call last):
      File "<python-input-2>", line 1, in <module>
        a['z'] = 3
        ~^^^^^
    TypeError: 'frozendict' object does not support item assignment
    >>> b = frozendict(y=2, x=1)
    >>> hash(a) == hash(b)
    True
    >>> a == b
    True

The following standard library modules have been updated to accept
:class:`!frozendict`: :mod:`copy`, :mod:`decimal`, :mod:`json`, :mod:`marshal`,
:mod:`plistlib` (only for serialization), :mod:`pickle`, :mod:`pprint` and
:mod:`xml.etree.ElementTree`.

:func:`eval` and :func:`exec` accept :class:`!frozendict` for *globals*, and
:func:`type` and :meth:`str.maketrans` accept :class:`!frozendict` for *dict*.

Code checking for :class:`dict` type using ``isinstance(arg, dict)`` can be
updated to ``isinstance(arg, (dict, frozendict))`` to accept also the
:class:`!frozendict` type, or to ``isinstance(arg, collections.abc.Mapping)``
to accept also other mapping types such as :class:`~types.MappingProxyType`.

.. seealso:: :pep:`814` for the full specification and rationale.

(Contributed by Victor Stinner and Donghee Na in :gh:`141510`.)


.. _whatsnew315-profiling-package:

:pep:`799`: A dedicated profiling package
-----------------------------------------

A new :mod:`profiling` module has been added to organize Python's built-in
profiling tools under a single, coherent namespace. This module contains:

* :mod:`profiling.tracing`: deterministic function-call tracing (relocated from
  ``cProfile``).
* :mod:`profiling.sampling`: a new statistical sampling profiler (named Tachyon).

The ``cProfile`` module remains as an alias for backwards compatibility.
The :mod:`profile` module is deprecated and will be removed in Python 3.17.

.. seealso:: :pep:`799` for further details.

(Contributed by Pablo Galindo and László Kiss Kollár in :gh:`138122`.)


.. _whatsnew315-sampling-profiler:

Tachyon: High frequency statistical sampling profiler
-----------------------------------------------------

.. image:: ../library/tachyon-logo.png
   :alt: Tachyon profiler logo
   :align: center
   :width: 200px

A new statistical sampling profiler (Tachyon) has been added as
:mod:`profiling.sampling`. This profiler enables low-overhead performance analysis of
running Python processes without requiring code modification or process restart.

Unlike deterministic profilers (such as :mod:`profiling.tracing`) that instrument
every function call, the sampling profiler periodically captures stack traces from
running processes.  This approach provides virtually zero overhead while achieving
sampling rates of **up to 1,000,000 Hz**, making it the fastest sampling profiler
available for Python (at the time of its contribution) and ideal for debugging
performance issues in production environments. This capability is particularly
valuable for debugging performance issues in production systems where traditional
profiling approaches would be too intrusive.

Key features include:

* **Zero-overhead profiling**: Attach to any running Python process without
  affecting its performance. Ideal for production debugging where you can't afford
  to restart or slow down your application.

* **No code modification required**: Profile existing applications without restart.
  Simply point the profiler at a running process by PID and start collecting data.

* **Flexible target modes**:

  * Profile running processes by PID (``attach``) - attach to already-running applications
  * Run and profile scripts directly (``run``) - profile from the very start of execution
  * Execute and profile modules (``run -m``) - profile packages run as ``python -m module``

* **Multiple profiling modes**: Choose what to measure based on your performance investigation:

  * **Wall-clock time** (``--mode wall``, default): Measures real elapsed time including I/O,
    network waits, and blocking operations. Use this to understand where your program spends
    calendar time, including when waiting for external resources.
  * **CPU time** (``--mode cpu``): Measures only active CPU execution time, excluding I/O waits
    and blocking. Use this to identify CPU-bound bottlenecks and optimize computational work.
  * **GIL-holding time** (``--mode gil``): Measures time spent holding Python's Global Interpreter
    Lock. Use this to identify which threads dominate GIL usage in multi-threaded applications.
  * **Exception handling time** (``--mode exception``): Captures samples only from threads with
    an active exception. Use this to analyze exception handling overhead.

* **Thread-aware profiling**: Option to profile all threads (``-a``) or just the main thread,
  essential for understanding multi-threaded application behavior.

* **Multiple output formats**: Choose the visualization that best fits your workflow:

  * ``--pstats``: Detailed tabular statistics compatible with :mod:`pstats`. Shows function-level
    timing with direct and cumulative samples. Best for detailed analysis and integration with
    existing Python profiling tools.
  * ``--collapsed``: Generates collapsed stack traces (one line per stack). This format is
    specifically designed for creating flame graphs with external tools like Brendan Gregg's
    FlameGraph scripts or speedscope.
  * ``--flamegraph``: Generates a self-contained interactive HTML flame graph using D3.js.
    Opens directly in your browser for immediate visual analysis. Flame graphs show the call
    hierarchy where width represents time spent, making it easy to spot bottlenecks at a glance.
  * ``--gecko``: Generates Gecko Profiler format compatible with `Firefox Profiler
    <https://profiler.firefox.com>`__. Upload the output to Firefox Profiler for advanced
    timeline-based analysis with features like stack charts, markers, and network activity.
  * ``--heatmap``: Generates an interactive HTML heatmap visualization with line-level sample
    counts. Creates a directory with per-file heatmaps showing exactly where time is spent
    at the source code level.

* **Live interactive mode**: Real-time TUI profiler with a top-like interface (``--live``).
  Monitor performance as your application runs with interactive sorting and filtering.

* **Async-aware profiling**: Profile async/await code with task-based stack reconstruction
  (``--async-aware``). See which coroutines are consuming time, with options to show only
  running tasks or all tasks including those waiting.

* **Opcode-level profiling**: Gather bytecode opcode information for instruction-level
  profiling (``--opcodes``). Shows which bytecode instructions are executing, including
  specializations from the adaptive interpreter.

See :mod:`profiling.sampling` for the complete documentation, including all
available output formats, profiling modes, and configuration options.

(Contributed by Pablo Galindo and László Kiss Kollár in :gh:`135953` and :gh:`138122`.)


.. _whatsnew315-unpacking-in-comprehensions:

:pep:`798`: Unpacking in Comprehensions
---------------------------------------

List, set, and dictionary comprehensions, as well as generator expressions, now
support unpacking with ``*`` and ``**``.  This extends the unpacking syntax
from :pep:`448` to comprehensions, providing a new syntax for combining an
arbitrary number of iterables or dictionaries into a single flat structure.
This new syntax is a direct alternative to nested comprehensions,
:func:`itertools.chain`, and :meth:`itertools.chain.from_iterable`.  For
example::

    >>> lists = [[1, 2], [3, 4], [5]]
    >>> [*L for L in lists]  # equivalent to [x for L in lists for x in L]
    [1, 2, 3, 4, 5]

    >>> sets = [{1, 2}, {2, 3}, {3, 4}]
    >>> {*s for s in sets}  # equivalent to {x for s in sets for x in s}
    {1, 2, 3, 4}

    >>> dicts = [{'a': 1}, {'b': 2}, {'a': 3}]
    >>> {**d for d in dicts}  # equivalent to {k: v for d in dicts for k,v in d.items()}
    {'a': 3, 'b': 2}

Generator expressions can similarly use unpacking to yield values from multiple
iterables::

    >>> gen = (*L for L in lists)  # equivalent to (x for L in lists for x in L)
    >>> list(gen)
    [1, 2, 3, 4, 5]

This change also extends to asynchronous generator expressions, such that, for
example, ``(*a async for a in agen())`` is equivalent to ``(x async for a in
agen() for x in a)``.

.. seealso:: :pep:`798` for further details.

(Contributed by Adam Hartz in :gh:`143055`.)


.. _whatsnew315-improved-error-messages:

Improved error messages
-----------------------

* The interpreter now provides more helpful suggestions in :exc:`AttributeError`
  exceptions when accessing an attribute on an object that does not exist, but
  a similar attribute is available through one of its members.

  For example, if the object has an attribute that itself exposes the requested
  name, the error message will suggest accessing it via that inner attribute:

  .. code-block:: python

     @dataclass
     class Circle:
        radius: float

        @property
        def area(self) -> float:
           return pi * self.radius**2

     class Container:
        def __init__(self, inner: Circle) -> None:
           self.inner = inner

     circle = Circle(radius=4.0)
     container = Container(circle)
     print(container.area)

  Running this code now produces a clearer suggestion:

  .. code-block:: pycon

     Traceback (most recent call last):
     File "/home/pablogsal/github/python/main/lel.py", line 42, in <module>
        print(container.area)
              ^^^^^^^^^^^^^^
     AttributeError: 'Container' object has no attribute 'area'. Did you mean '.inner.area' instead of '.area'?


Other language changes
======================

.. _whatsnew315-utf8-default:

* Python now uses UTF-8_ as the default encoding, independent of the system's
  environment. This means that I/O operations without an explicit encoding,
  for example, ``open('flying-circus.txt')``, will use UTF-8.
  UTF-8 is a widely-supported Unicode_ character encoding that has become a
  *de facto* standard for representing text, including nearly every webpage
  on the internet, many common file formats, programming languages, and more.

  This only applies when no ``encoding`` argument is given. For best
  compatibility between versions of Python, ensure that an explicit ``encoding``
  argument is always provided. The :ref:`opt-in encoding warning <io-encoding-warning>`
  can be used to identify code that may be affected by this change.
  The special ``encoding='locale'`` argument uses the current locale
  encoding, and has been supported since Python 3.10.

  To retain the previous behaviour, Python's UTF-8 mode may be disabled with
  the :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` environment variable or the
  :option:`-X utf8=0 <-X>` command-line option.

  .. seealso:: :pep:`686` for further details.

  .. _UTF-8: https://en.wikipedia.org/wiki/UTF-8
  .. _Unicode: https://home.unicode.org/

  (Contributed by Adam Turner in :gh:`133711`; PEP 686 written by Inada Naoki.)

* Several error messages incorrectly using the term "argument" have been corrected.
  (Contributed by Stan Ulbrych in :gh:`133382`.)

* The interpreter now tries to provide a suggestion when
  :func:`delattr` fails due to a missing attribute.
  When an attribute name that closely resembles an existing attribute is used,
  the interpreter will suggest the correct attribute name in the error message.
  For example:

  .. doctest::

     >>> class A:
     ...     pass
     >>> a = A()
     >>> a.abcde = 1
     >>> del a.abcdf  # doctest: +ELLIPSIS
     Traceback (most recent call last):
     ...
     AttributeError: 'A' object has no attribute 'abcdf'. Did you mean: 'abcde'?

  (Contributed by Nikita Sobolev and Pranjal Prajapati in :gh:`136588`.)

* Unraisable exceptions are now highlighted with color by default. This can be
  controlled by :ref:`environment variables <using-on-controlling-color>`.
  (Contributed by Peter Bierma in :gh:`134170`.)

* The :meth:`~object.__repr__` of :class:`ImportError` and :class:`ModuleNotFoundError`
  now shows "name" and "path" as ``name=<name>`` and ``path=<path>`` if they were given
  as keyword arguments at construction time.
  (Contributed by Serhiy Storchaka, Oleg Iarygin, and Yoav Nir in :gh:`74185`.)

* The :attr:`~object.__dict__` and :attr:`!__weakref__` descriptors now use a
  single descriptor instance per interpreter, shared across all types that
  need them.
  This speeds up class creation, and helps avoid reference cycles.
  (Contributed by Petr Viktorin in :gh:`135228`.)

* The :option:`-W` option and the :envvar:`PYTHONWARNINGS` environment variable
  can now specify regular expressions instead of literal strings to match
  the warning message and the module name, if the corresponding field starts
  and ends with a forward slash (``/``).
  (Contributed by Serhiy Storchaka in :gh:`134716`.)

* Functions that take timestamp or timeout arguments now accept any real
  numbers (such as :class:`~decimal.Decimal` and :class:`~fractions.Fraction`),
  not only integers or floats, although this does not improve precision.
  (Contributed by Serhiy Storchaka in :gh:`67795`.)

.. _whatsnew315-bytearray-take-bytes:

* Added :meth:`bytearray.take_bytes(n=None, /) <bytearray.take_bytes>` to take
  bytes out of a :class:`bytearray` without copying. This enables optimizing code
  which must return :class:`bytes` after working with a mutable buffer of bytes
  such as data buffering, network protocol parsing, encoding, decoding,
  and compression. Common code patterns which can be optimized with
  :func:`~bytearray.take_bytes` are listed below.

  .. list-table:: Suggested optimizing refactors
      :header-rows: 1

      * - Description
        - Old
        - New

      * - Return :class:`bytes` after working with :class:`bytearray`
        - .. code:: python

              def read() -> bytes:
                  buffer = bytearray(1024)
                  ...
                  return bytes(buffer)

        - .. code:: python

              def read() -> bytes:
                  buffer = bytearray(1024)
                  ...
                  return buffer.take_bytes()

      * - Empty a buffer getting the bytes
        - .. code:: python

              buffer = bytearray(1024)
              ...
              data = bytes(buffer)
              buffer.clear()

        - .. code:: python

              buffer = bytearray(1024)
              ...
              data = buffer.take_bytes()

      * - Split a buffer at a specific separator
        - .. code:: python

              buffer = bytearray(b'abc\ndef')
              n = buffer.find(b'\n')
              data = bytes(buffer[:n + 1])
              del buffer[:n + 1]
              assert data == b'abc'
              assert buffer == bytearray(b'def')

        - .. code:: python

              buffer = bytearray(b'abc\ndef')
              n = buffer.find(b'\n')
              data = buffer.take_bytes(n + 1)

      * - Split a buffer at a specific separator; discard after the separator
        - .. code:: python

              buffer = bytearray(b'abc\ndef')
              n = buffer.find(b'\n')
              data = bytes(buffer[:n])
              buffer.clear()
              assert data == b'abc'
              assert len(buffer) == 0

        - .. code:: python

              buffer = bytearray(b'abc\ndef')
              n = buffer.find(b'\n')
              buffer.resize(n)
              data = buffer.take_bytes()

  (Contributed by Cody Maloney in :gh:`139871`.)

* Many functions related to compiling or parsing Python code, such as
  :func:`compile`, :func:`ast.parse`, :func:`symtable.symtable`,
  and :func:`importlib.abc.InspectLoader.source_to_code`, now allow the module
  name to be passed. It is needed to unambiguously :ref:`filter <warning-filter>`
  syntax warnings by module name.
  (Contributed by Serhiy Storchaka in :gh:`135801`.)

* Allowed defining the *__dict__* and *__weakref__* :ref:`__slots__ <slots>`
  for any class.
  (Contributed by Serhiy Storchaka in :gh:`41779`.)

* Allowed defining any :ref:`__slots__ <slots>` for a class derived from
  :class:`tuple` (including classes created by :func:`collections.namedtuple`).
  (Contributed by Serhiy Storchaka in :gh:`41779`.)

* The :class:`slice` type now supports subscription,
  making it a :term:`generic type`.
  (Contributed by James Hilton-Balfe in :gh:`128335`.)

* The class :class:`memoryview` now supports the :c:expr:`float complex` and
  :c:expr:`double complex` C types: formatting characters ``'F'`` and ``'D'``
  respectively.
  (Contributed by Sergey B Kirpichev in :gh:`146151`.)

* Allow the *count* argument of :meth:`bytes.replace` to be a keyword.
  (Contributed by Stan Ulbrych in :gh:`147856`.)


New modules
===========

math.integer
------------

This module provides access to the mathematical functions for integer
arguments (:pep:`791`).
(Contributed by Serhiy Storchaka in :gh:`81313`.)


Improved modules
================

argparse
--------

* The :class:`~argparse.BooleanOptionalAction` action supports now single-dash
  long options and alternate prefix characters.
  (Contributed by Serhiy Storchaka in :gh:`138525`.)

* Changed the *suggest_on_error* parameter of :class:`argparse.ArgumentParser` to
  default to ``True``. This enables suggestions for mistyped arguments by default.
  (Contributed by Jakob Schluse in :gh:`140450`.)

* Added backtick markup support in description and epilog text to highlight
  inline code when color output is enabled.
  (Contributed by Savannah Ostrowski in :gh:`142390`.)


array
-----

* Support the :c:expr:`float complex` and :c:expr:`double complex` C types:
  formatting characters ``'F'`` and ``'D'`` respectively.
  (Contributed by Sergey B Kirpichev in :gh:`146151`.)

* Support half-floats (16-bit IEEE 754 binary interchange format): formatting
  character ``'e'``.
  (Contributed by Sergey B Kirpichev in :gh:`146238`.)


base64
------

* Added the *pad* parameter in :func:`~base64.z85encode`.
  (Contributed by Hauke Dämpfling in :gh:`143103`.)

* Added the *padded* parameter in
  :func:`~base64.b32encode`, :func:`~base64.b32decode`,
  :func:`~base64.b32hexencode`, :func:`~base64.b32hexdecode`,
  :func:`~base64.b64encode`, :func:`~base64.b64decode`,
  :func:`~base64.urlsafe_b64encode`, and :func:`~base64.urlsafe_b64decode`.
  (Contributed by Serhiy Storchaka in :gh:`73613`.)

* Added the *wrapcol* parameter in :func:`~base64.b16encode`,
  :func:`~base64.b32encode`, :func:`~base64.b32hexencode`,
  :func:`~base64.b64encode`, :func:`~base64.b85encode`, and
  :func:`~base64.z85encode`.
  (Contributed by Serhiy Storchaka in :gh:`143214` and :gh:`146431`.)

* Added the *ignorechars* parameter in :func:`~base64.b16decode`,
  :func:`~base64.b32decode`,  :func:`~base64.b32hexdecode`,
  :func:`~base64.b64decode`, :func:`~base64.b85decode`, and
  :func:`~base64.z85decode`.
  (Contributed by Serhiy Storchaka in :gh:`144001` and :gh:`146431`.)


binascii
--------

* Added functions for Base32 encoding:

  - :func:`~binascii.b2a_base32` and :func:`~binascii.a2b_base32`

  (Contributed by James Seo in :gh:`146192`.)

* Added functions for Ascii85, Base85, and Z85 encoding:

  - :func:`~binascii.b2a_ascii85` and :func:`~binascii.a2b_ascii85`
  - :func:`~binascii.b2a_base85` and :func:`~binascii.a2b_base85`

  (Contributed by James Seo and Serhiy Storchaka in :gh:`101178`.)

* Added the *padded* parameter in
  :func:`~binascii.b2a_base32`, :func:`~binascii.a2b_base32`,
  :func:`~binascii.b2a_base64`, and :func:`~binascii.a2b_base64`.
  (Contributed by Serhiy Storchaka in :gh:`73613`.)

* Added the *wrapcol* parameter in :func:`~binascii.b2a_base64`.
  (Contributed by Serhiy Storchaka in :gh:`143214`.)

* Added the *alphabet* parameter in :func:`~binascii.b2a_base64` and
  :func:`~binascii.a2b_base64`.
  (Contributed by Serhiy Storchaka in :gh:`145980`.)

* Added the *ignorechars* parameter in :func:`~binascii.a2b_hex`,
  :func:`~binascii.unhexlify`, and :func:`~binascii.a2b_base64`.
  (Contributed by Serhiy Storchaka in :gh:`144001` and :gh:`146431`.)


calendar
--------

* Calendar pages generated by the :class:`calendar.HTMLCalendar` class now support
  dark mode and have been migrated to the HTML5 standard for improved accessibility.
  (Contributed by Jiahao Li and Hugo van Kemenade in :gh:`137634`.)

* The :mod:`calendar`'s :ref:`command-line <calendar-cli>` HTML output now
  accepts the year-month option: ``python -m calendar -t html 2009 06``.
  (Contributed by Pål Grønås Drange in :gh:`140212`.)


collections
-----------

* Added :meth:`!collections.Counter.__xor__` and
  :meth:`!collections.Counter.__ixor__` to compute the symmetric difference
  between :class:`~collections.Counter` objects.
  (Contributed by Raymond Hettinger in :gh:`138682`.)


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

* Improved error reporting when a child process in a
  :class:`concurrent.futures.ProcessPoolExecutor` terminates abruptly.
  The resulting traceback will now tell you the PID and exit code of the
  terminated process.
  (Contributed by Jonathan Berg in :gh:`139486`.)


contextlib
----------

* Added support for arbitrary descriptors :meth:`!__enter__`,
  :meth:`!__exit__`, :meth:`!__aenter__`, and :meth:`!__aexit__` in
  :class:`~contextlib.ExitStack` and :class:`contextlib.AsyncExitStack`, for
  consistency with the :keyword:`with` and :keyword:`async with` statements.
  (Contributed by Serhiy Storchaka in :gh:`144386`.)


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

* Annotations for generated ``__init__`` methods no longer include internal
  type names.


dbm
---

* Added new :meth:`!reorganize` methods to :mod:`dbm.dumb` and :mod:`dbm.sqlite3`
  which allow to recover unused free space previously occupied by deleted entries.
  (Contributed by Andrea Oliveri in :gh:`134004`.)


difflib
-------

  .. _whatsnew315-color-difflib:

* Introduced the optional *color* parameter to :func:`difflib.unified_diff`,
  enabling color output similar to :program:`git diff`.
  This can be controlled by :ref:`environment variables
  <using-on-controlling-color>`.
  (Contributed by Douglas Thor in :gh:`133725`.)

* Improved the styling of HTML diff pages generated by the :class:`difflib.HtmlDiff`
  class, and migrated the output to the HTML5 standard.
  (Contributed by Jiahao Li in :gh:`134580`.)


functools
---------

* :func:`~functools.singledispatchmethod` now supports non-:term:`descriptor`
  callables.
  (Contributed by Serhiy Storchaka in :gh:`140873`.)

* :func:`~functools.singledispatchmethod` now dispatches on the second argument
  if it wraps a regular method and is called as a class attribute.
  (Contributed by Bartosz Sławecki in :gh:`143535`.)


hashlib
-------

* Ensure that hash functions guaranteed to be always *available* exist as
  attributes of :mod:`hashlib` even if they will not work at runtime due to
  missing backend implementations. For instance, ``hashlib.md5`` will no
  longer raise :exc:`AttributeError` if OpenSSL is not available and Python
  has been built without MD5 support.
  (Contributed by Bénédikt Tran in :gh:`136929`.)


http.client
-----------

* A new *max_response_headers* keyword-only parameter has been added to
  :class:`~http.client.HTTPConnection` and :class:`~http.client.HTTPSConnection`
  constructors. This parameter overrides the default maximum number of allowed
  response headers.
  (Contributed by Alexander Enrique Urieles Nieto in :gh:`131724`.)


http.cookies
------------

* Allow '``"``' double quotes in cookie values.
  (Contributed by Nick Burns and Senthil Kumaran in :gh:`92936`.)


inspect
-------

* Add parameters *inherit_class_doc* and *fallback_to_class_doc*
  for :func:`~inspect.getdoc`.
  (Contributed by Serhiy Storchaka in :gh:`132686`.)

json
----

* Add the *array_hook* parameter to  :func:`~json.load` and
  :func:`~json.loads` functions:
  allow a callback for JSON literal array types to customize Python lists in
  the resulting decoded object. Passing combined :class:`frozendict` to
  *object_pairs_hook* param and :class:`tuple` to ``array_hook`` will yield a
  deeply nested immutable Python structure representing the JSON data.
  (Contributed by Joao S. O. Bueno in :gh:`146440`)


locale
------

* :func:`~locale.setlocale` now supports language codes with ``@``-modifiers.
  ``@``-modifiers are no longer silently removed in :func:`~locale.getlocale`,
  but included in the language code.
  (Contributed by Serhiy Storchaka in :gh:`137729`.)

* Undeprecate the :func:`locale.getdefaultlocale` function.
  (Contributed by Victor Stinner in :gh:`130796`.)


math
----

* Add :func:`math.isnormal` and :func:`math.issubnormal` functions.
  (Contributed by Sergey B Kirpichev in :gh:`132908`.)

* Add :func:`math.fmax`, :func:`math.fmin` and :func:`math.signbit` functions.
  (Contributed by Bénédikt Tran in :gh:`135853`.)


mimetypes
---------

* Add ``application/dicom`` MIME type for ``.dcm`` extension.
  (Contributed by Benedikt Johannes in :gh:`144217`.)
* Add ``application/efi``. (Contributed by Charlie Lin in :gh:`145720`.)
* Add ``application/node`` MIME type for ``.cjs`` extension.
  (Contributed by John Franey in :gh:`140937`.)
* Add ``application/toml``. (Contributed by Gil Forcada in :gh:`139959`.)
* Add ``application/sql`` and ``application/vnd.sqlite3``.
  (Contributed by Charlie Lin in :gh:`145698`.)
* Add the following MIME types:

  - ``application/vnd.ms-cab-compressed`` for ``.cab`` extension
  - ``application/vnd.ms-htmlhelp`` for ``.chm`` extension
  - ``application/vnd.ms-officetheme`` for ``.thmx`` extension

  (Contributed by Charlie Lin in :gh:`145718`.)

* Add ``image/jxl``. (Contributed by Foolbar in :gh:`144213`.)
* Rename ``application/x-texinfo`` to ``application/texinfo``.
  (Contributed by Charlie Lin in :gh:`140165`.)
* Changed the MIME type for ``.ai`` files to ``application/pdf``.
  (Contributed by Stan Ulbrych in :gh:`141239`.)


mmap
----

* :class:`mmap.mmap` now has a *trackfd* parameter on Windows;
  if it is ``False``, the file handle corresponding to *fileno* will
  not be duplicated.
  (Contributed by Serhiy Storchaka in :gh:`78502`.)

* Added the :meth:`mmap.mmap.set_name` method
  to annotate an anonymous memory mapping
  if Linux kernel supports :manpage:`PR_SET_VMA_ANON_NAME <PR_SET_VMA(2const)>` (Linux 5.17 or newer).
  (Contributed by Donghee Na in :gh:`142419`.)


os
--

* Add :func:`os.statx` on Linux kernel versions 4.11 and later with
  glibc versions 2.28 and later.
  (Contributed by Jeffrey Bosboom and Victor Stinner in :gh:`83714`.)


os.path
-------

* Add support of the all-but-last mode in :func:`~os.path.realpath`.
  (Contributed by Serhiy Storchaka in :gh:`71189`.)

* The *strict* parameter to :func:`os.path.realpath` accepts a new value,
  :data:`os.path.ALLOW_MISSING`.
  If used, errors other than :exc:`FileNotFoundError` will be re-raised;
  the resulting path can be missing but it will be free of symlinks.
  (Contributed by Petr Viktorin for :cve:`2025-4517`.)


pickle
------

* Add support for pickling private methods and nested classes.
  (Contributed by Zackery Spytz and Serhiy Storchaka in :gh:`77188`.)


re
--

* :func:`re.prefixmatch` and a corresponding :meth:`~re.Pattern.prefixmatch`
  have been added as alternate more explicit names for the existing
  :func:`re.match` and :meth:`~re.Pattern.match` APIs. These are intended
  to be used to alleviate confusion around what *match* means by following the
  Zen of Python's *"Explicit is better than implicit"* mantra. Most other
  language regular expression libraries use an API named *match* to mean what
  Python has always called *search*.
  (Contributed by Gregory P. Smith in :gh:`86519`.)


resource
--------

* Add new constants: :data:`~resource.RLIMIT_NTHR`,
  :data:`~resource.RLIMIT_UMTXP`, :data:`~resource.RLIMIT_THREADS`,
  :data:`~resource.RLIM_SAVED_CUR`, and :data:`~resource.RLIM_SAVED_MAX`.
  (Contributed by Serhiy Storchaka in :gh:`137512`.)


shelve
------

* Added new :meth:`!reorganize` method to :mod:`shelve` used to recover unused free
  space previously occupied by deleted entries.
  (Contributed by Andrea Oliveri in :gh:`134004`.)
* Add support for custom serialization and deserialization functions
  in the :mod:`shelve` module.
  (Contributed by Furkan Onder in :gh:`99631`.)


socket
------

* Add constants for the ISO-TP CAN protocol.
  (Contributed by Patrick Menschel and Stefan Tatschner in :gh:`86819`.)


sqlite3
-------

* The :ref:`command-line interface <sqlite3-cli>` has several new features:

  * SQL keyword completion on <tab>.
    (Contributed by Long Tan in :gh:`133393`.)

  * Prompts, error messages, and help text are now colored.
    This is enabled by default, see :ref:`using-on-controlling-color` for
    details.
    (Contributed by Stan Ulbrych and Łukasz Langa in :gh:`133461`.)

  * Table, index, trigger, view, column, function, and schema completion on <tab>.
    (Contributed by Long Tan in :gh:`136101`.)


ssl
---

* Indicate through :data:`ssl.HAS_PSK_TLS13` whether the :mod:`ssl` module
  supports "External PSKs" in TLSv1.3, as described in :rfc:`9258`.
  (Contributed by Will Childs-Klein in :gh:`133624`.)

* Added new methods for managing groups used for SSL key agreement

  * :meth:`ssl.SSLContext.set_groups` sets the groups allowed for doing
    key agreement, extending the previous
    :meth:`ssl.SSLContext.set_ecdh_curve` method.
    This new API provides the ability to list multiple groups and
    supports fixed-field and post-quantum groups in addition to ECDH
    curves. This method can also be used to control what key shares
    are sent in the TLS handshake.
  * :meth:`ssl.SSLSocket.group` returns the group selected for doing key
    agreement on the current connection after the TLS handshake completes.
    This call requires OpenSSL 3.2 or later.
  * :meth:`ssl.SSLContext.get_groups` returns a list of all available key
    agreement groups compatible with the minimum and maximum TLS versions
    currently set in the context. This call requires OpenSSL 3.5 or later.

  (Contributed by Ron Frederick in :gh:`136306`.)

* Added a new method :meth:`ssl.SSLContext.set_ciphersuites` for setting TLS 1.3
  ciphers. For TLS 1.2 or earlier, :meth:`ssl.SSLContext.set_ciphers` should
  continue to be used. Both calls can be made on the same context and the
  selected cipher suite will depend on the TLS version negotiated when a
  connection is made.
  (Contributed by Ron Frederick in :gh:`137197`.)

* Added new methods for managing signature algorithms:

  * :func:`ssl.get_sigalgs` returns a list of all available TLS signature
    algorithms. This call requires OpenSSL 3.4 or later.
  * :meth:`ssl.SSLContext.set_client_sigalgs` sets the signature algorithms
    allowed for certificate-based client authentication.
  * :meth:`ssl.SSLContext.set_server_sigalgs` sets the signature algorithms
    allowed for the server to complete the TLS handshake.
  * :meth:`ssl.SSLSocket.client_sigalg` returns the signature algorithm
    selected for client authentication on the current connection. This call
    requires OpenSSL 3.5 or later.
  * :meth:`ssl.SSLSocket.server_sigalg` returns the signature algorithm
    selected for the server to complete the TLS handshake on the current
    connection. This call requires OpenSSL 3.5 or later.

  (Contributed by Ron Frederick in :gh:`138252`.)


subprocess
----------

* :meth:`subprocess.Popen.wait`: when ``timeout`` is not ``None`` and the
  platform supports it, an efficient event-driven mechanism is used to wait for
  process termination:

  - Linux >= 5.3 uses :func:`os.pidfd_open` + :func:`select.poll`.
  - macOS and other BSD variants use :func:`select.kqueue` + ``KQ_FILTER_PROC`` + ``KQ_NOTE_EXIT``.
  - Windows keeps using ``WaitForSingleObject`` (unchanged).

  If none of these mechanisms are available, the function falls back to the
  traditional busy loop (non-blocking call and short sleeps).
  (Contributed by Giampaolo Rodola in :gh:`83069`).


symtable
--------

* Add :meth:`symtable.Function.get_cells` and :meth:`symtable.Symbol.is_cell` methods.
  (Contributed by Yashp002 in :gh:`143504`.)


sys
---

* Add :data:`sys.abi_info` namespace to improve access to ABI information.
  (Contributed by Klaus Zimmermann in :gh:`137476`.)


tarfile
-------

* :func:`~tarfile.data_filter` now normalizes symbolic link targets in order to
  avoid path traversal attacks.
  (Contributed by Petr Viktorin in :gh:`127987` and :cve:`2025-4138`.)
* :func:`~tarfile.TarFile.extractall` now skips fixing up directory attributes
  when a directory was removed or replaced by another kind of file.
  (Contributed by Petr Viktorin in :gh:`127987` and :cve:`2024-12718`.)
* :func:`~tarfile.TarFile.extract` and :func:`~tarfile.TarFile.extractall`
  now (re-)apply the extraction filter when substituting a link (hard or
  symbolic) with a copy of another archive member, and when fixing up
  directory attributes.
  The former raises a new exception, :exc:`~tarfile.LinkFallbackError`.
  (Contributed by Petr Viktorin for :cve:`2025-4330` and :cve:`2024-12718`.)
* :func:`~tarfile.TarFile.extract` and :func:`~tarfile.TarFile.extractall`
  no longer extract rejected members when
  :func:`~tarfile.TarFile.errorlevel` is zero.
  (Contributed by Matt Prodani and Petr Viktorin in :gh:`112887`
  and :cve:`2025-4435`.)
* :func:`~tarfile.TarFile.extract` and :func:`~tarfile.TarFile.extractall`
  now replace slashes by backslashes in symlink targets on Windows to prevent
  creation of corrupted links.
  (Contributed by Christoph Walcher in :gh:`57911`.)


timeit
------

* The command-line interface now colorizes error tracebacks
  by default. This can be controlled with
  :ref:`environment variables <using-on-controlling-color>`.
  (Contributed by Yi Hong in :gh:`139374`.)

* Make the target time of :meth:`timeit.Timer.autorange` configurable
  and add ``--target-time`` option to the command-line interface.
  (Contributed by Alessandro Cucci and Miikka Koskinen in :gh:`140283`.)


tkinter
-------

* The :meth:`!tkinter.Text.search` method now supports two additional
  arguments: *nolinestop* which allows the search to
  continue across line boundaries;
  and *strictlimits* which restricts the search to within the specified range.
  (Contributed by Rihaan Meher in :gh:`130848`.)

* A new method :meth:`!tkinter.Text.search_all` has been introduced.
  This method allows for searching for all matches of a pattern
  using Tcl's ``-all`` and ``-overlap`` options.
  (Contributed by Rihaan Meher in :gh:`130848`.)

* Added new methods :meth:`!pack_content`, :meth:`!place_content` and
  :meth:`!grid_content` which use Tk commands with new names (introduced
  in Tk 8.6) instead of :meth:`!*_slaves` methods which use Tk commands
  with outdated names.
  (Contributed by Serhiy Storchaka in :gh:`143754`.)

* Added :class:`!Event` attributes :attr:`!user_data` for Tk virtual events
  and :attr:`!detail` for ``Enter``, ``Leave``, ``FocusIn``, ``FocusOut``,
  and ``ConfigureRequest`` events.
  (Contributed by Matthias Kievernagel and Serhiy Storchaka in :gh:`47655`.)


.. _whatsnew315-tomllib-1-1-0:

tomllib
-------

* The :mod:`tomllib` module now supports TOML 1.1.0.
  This is a backwards compatible update, meaning that all valid TOML 1.0.0
  documents are parsed the same way.

  The changes, according to the `official TOML changelog`_, are:

  - Allow newlines and trailing commas in inline tables.

    Previously an inline table had to be on a single line and couldn't end
    with a trailing comma. This is now relaxed so that the following is valid:

    .. syntax highlighting needs TOML 1.1.0 support in Pygments,
       see https://github.com/pygments/pygments/issues/3026

    .. code-block:: text

       tbl = {
          key      = "a string",
          moar-tbl =  {
             key = 1,
          },
       }

  - Add ``\xHH`` notation to basic strings for codepoints under 255,
    and the ``\e`` escape for the escape character:

    .. code-block:: text

       null = "null byte: \x00; letter a: \x61"
       csi = "\e["

  - Seconds in datetime and time values are now optional.
    The following are now valid:

    .. code-block:: text

       dt = 2010-02-03 14:15
       t  = 14:15

  (Contributed by Taneli Hukkinen in :gh:`142956`.)

.. _official TOML changelog: https://github.com/toml-lang/toml/blob/main/CHANGELOG.md


types
-----

* Expose the write-through :func:`locals` proxy type
  as :data:`types.FrameLocalsProxyType`.
  This represents the type of the :attr:`frame.f_locals` attribute,
  as described in :pep:`667`.


typing
------

.. _whatsnew315-typeform:

* :pep:`747`: Add :data:`~typing.TypeForm`, a new special form for annotating
  values that are themselves type expressions.
  ``TypeForm[T]`` means "a type form object describing ``T`` (or a type
  assignable to ``T``)". At runtime, ``TypeForm(x)`` simply returns ``x``,
  which allows explicit annotation of type-form values without changing
  behavior.

  This helps libraries that accept user-provided type expressions
  (for example ``int``, ``str | None``, :class:`~typing.TypedDict`
  classes, or ``list[int]``) expose precise signatures:

  .. code-block:: python

     from typing import Any, TypeForm

     def cast[T](typ: TypeForm[T], value: Any) -> T: ...

  (Contributed by Jelle Zijlstra in :gh:`145033`.)

* Code like ``class ExtraTypeVars(P1[S], Protocol[T, T2]): ...`` now raises
  a :exc:`TypeError`, because ``S`` is not listed in ``Protocol`` parameters.
  (Contributed by Nikita Sobolev in :gh:`137191`.)

* Code like ``class B2(A[T2], Protocol[T1, T2]): ...`` now correctly handles
  type parameters order: it is ``(T1, T2)``, not ``(T2, T1)``
  as it was incorrectly inferred in runtime before.
  (Contributed by Nikita Sobolev in :gh:`137191`.)


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

* The Unicode database has been updated to Unicode 17.0.0.

* Add :func:`unicodedata.isxidstart` and :func:`unicodedata.isxidcontinue`
  functions to check whether a character can start or continue a
  `Unicode Standard Annex #31 <https://www.unicode.org/reports/tr31/>`_ identifier.
  (Contributed by Stan Ulbrych in :gh:`129117`.)

* Add the :func:`~unicodedata.iter_graphemes`
  function to iterate over grapheme clusters according to rules defined in
  `Unicode Standard Annex #29, "Unicode Text Segmentation"
  <https://www.unicode.org/reports/tr29/>`_.
  Add :func:`~unicodedata.grapheme_cluster_break`,
  :func:`~unicodedata.indic_conjunct_break` and
  :func:`~unicodedata.extended_pictographic` functions to get the properties
  of the character which are related to the above algorithm.
  (Contributed by Serhiy Storchaka and Guillaume Sanchez in :gh:`74902`.)

* Add :func:`~unicodedata.block` function to return the `Unicode block
  <https://www.unicode.org/versions/Unicode17.0.0/core-spec/chapter-3/#G64189>`_
  assigned to a character.
  (Contributed by Stan Ulbrych in :gh:`66802`.)


unittest
--------

* :func:`unittest.TestCase.assertLogs` will now accept a formatter
  to control how messages are formatted.
  (Contributed by Garry Cairns in :gh:`134567`.)


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

* Add the *missing_as_none* parameter to :func:`~urllib.parse.urlsplit`,
  :func:`~urllib.parse.urlparse` and :func:`~urllib.parse.urldefrag` functions.
  Add the *keep_empty* parameter to :func:`~urllib.parse.urlunsplit` and
  :func:`~urllib.parse.urlunparse` functions.
  This allows to distinguish between empty and not defined URI components
  and preserve empty components.
  (Contributed by Serhiy Storchaka in :gh:`67041`.)


venv
----

* On POSIX platforms, platlib directories will be created if needed when
  creating virtual environments, instead of using ``lib64 -> lib`` symlink.
  This means purelib and platlib of virtual environments no longer share the
  same ``lib`` directory on platforms where :data:`sys.platlibdir` is not
  equal to ``lib``.
  (Contributed by Rui Xi in :gh:`133951`.)


warnings
--------

* Improve filtering by module in :func:`warnings.warn_explicit` if no *module*
  argument is passed.
  It now tests the module regular expression in the warnings filter not only
  against the filename with ``.py`` stripped, but also against module names
  constructed starting from different parent directories of the filename
  (with ``/__init__.py``, ``.py`` and, on Windows, ``.pyw`` stripped).
  (Contributed by Serhiy Storchaka in :gh:`135801`.)


xml.parsers.expat
-----------------

* Add :meth:`~xml.parsers.expat.xmlparser.SetAllocTrackerActivationThreshold`
  and :meth:`~xml.parsers.expat.xmlparser.SetAllocTrackerMaximumAmplification`
  to :ref:`xmlparser <xmlparser-objects>` objects to tune protections against
  disproportional amounts of dynamic memory usage from within an Expat parser.
  (Contributed by Bénédikt Tran in :gh:`90949`.)

* Add :meth:`~xml.parsers.expat.xmlparser.SetBillionLaughsAttackProtectionActivationThreshold`
  and :meth:`~xml.parsers.expat.xmlparser.SetBillionLaughsAttackProtectionMaximumAmplification`
  to :ref:`xmlparser <xmlparser-objects>` objects to tune protections against
  `billion laughs`_ attacks.
  (Contributed by Bénédikt Tran in :gh:`90949`.)

  .. _billion laughs: https://en.wikipedia.org/wiki/Billion_laughs_attack


zlib
----

* Allow combining two Adler-32 checksums via :func:`~zlib.adler32_combine`.
  (Contributed by Callum Attryde and Bénédikt Tran in :gh:`134635`.)

* Allow combining two CRC-32 checksums via :func:`~zlib.crc32_combine`.
  (Contributed by Bénédikt Tran in :gh:`134635`.)


.. Add improved modules above alphabetically, not here at the end.

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

* ``mimalloc`` is now used as the default allocator for
  for raw memory allocations such as via :c:func:`PyMem_RawMalloc`
  for better performance on :term:`free-threaded builds <free-threaded build>`.
  (Contributed by Kumar Aditya in :gh:`144914`.)


base64 & binascii
-----------------

* CPython's underlying base64 implementation now encodes 2x faster and decodes 3x
  faster thanks to simple CPU pipelining optimizations.
  (Contributed by Gregory P. Smith and Serhiy Storchaka in :gh:`143262`.)

* Implementation for Ascii85, Base85, and Z85 encoding has been rewritten in C.
  Encoding and decoding is now two orders of magnitude faster and consumes
  two orders of magnitude less memory.
  (Contributed by James Seo and Serhiy Storchaka in :gh:`101178`.)

* Implementation for Base32 has been rewritten in C.
  Encoding and decoding is now two orders of magnitude faster.
  (Contributed by James Seo in :gh:`146192`)


csv
---

* :meth:`csv.Sniffer.sniff` delimiter detection is now up to 1.6x faster.
  (Contributed by Maurycy Pawłowski-Wieroński in :gh:`137628`.)


.. _whatsnew315-jit:

Upgraded JIT compiler
---------------------

Results from the `pyperformance <https://github.com/python/pyperformance>`__
benchmark suite report
`6-7% <https://www.doesjitgobrrr.com/run/2026-04-01>`__
geometric mean performance improvement for the JIT over the standard CPython
interpreter built with all optimizations enabled on x86-64 Linux. On AArch64
macOS, the JIT has a
`12-13% <https://www.doesjitgobrrr.com/run/2026-04-01>`__
speedup over the :ref:`tail calling interpreter <whatsnew314-tail-call-interpreter>`
with all optimizations enabled. The speedups for JIT
builds versus no JIT builds range from roughly 15% slowdown to over
100% speedup (ignoring the ``unpack_sequence`` microbenchmark) on
x86-64 Linux and AArch64 macOS systems.

.. attention::
    These results are not yet final.

The major upgrades to the JIT are:

* LLVM 21 build-time dependency
* New tracing frontend
* Basic register allocation in the JIT
* More JIT optimizations
* Better machine code generation

.. rubric:: LLVM 21 build-time dependency

The JIT compiler now uses LLVM 21 for build-time stencil generation. As
always, LLVM is only needed when building CPython with the JIT enabled;
end users running Python do not need LLVM installed. Instructions for
installing LLVM can be found in the `JIT compiler documentation
<https://github.com/python/cpython/blob/main/Tools/jit/README.md>`__
for all supported platforms.

(Contributed by Savannah Ostrowski in :gh:`140973`.)

.. rubric:: A new tracing frontend

The JIT compiler now supports significantly more bytecode operations and
control flow than in Python 3.14, enabling speedups on a wider variety of
code. For example, simple Python object creation is now understood by the
3.15 JIT compiler. Overloaded operations and generators are also partially
supported. This was made possible by an overhauled JIT tracing frontend
that records actual execution paths through code, rather than estimating
them as the previous implementation did.

(Contributed by Ken Jin in :gh:`139109`. Support for Windows added by
Mark Shannon in :gh:`141703`.)

.. rubric:: Basic register allocation in the JIT

A basic form of register allocation has been added to the JIT compiler's
optimizer. This allows the JIT compiler to avoid certain stack operations
altogether and instead operate on registers. This allows the JIT to produce
more efficient traces by avoiding reads and writes to memory.

(Contributed by Mark Shannon in :gh:`135379`.)

.. rubric:: More JIT optimizations

More `constant-propagation <https://en.wikipedia.org/wiki/Constant_folding>`__
is now performed. This means when the JIT compiler detects that certain user
code results in constants, the code can be simplified by the JIT.

(Contributed by Ken Jin and Savannah Ostrowski in :gh:`132732`.)

The JIT avoids :term:`reference count`\ s where possible. This generally
reduces the cost of most operations in Python.

(Contributed by Ken Jin, Donghee Na, Zheao Li, Hai Zhu, Savannah Ostrowski,
Reiden Ong, Noam Cohen, Tomas Roun, PuQing, Cajetan Rodrigues, and Sacul in :gh:`134584`.)

.. rubric:: Better machine code generation

The JIT compiler's machine code generator now produces better machine code
for x86-64 and AArch64 macOS and Linux targets. In general, users should
experience lower memory usage for generated machine code and more efficient
machine code versus the old JIT.

(Contributed by Brandt Bucher in :gh:`136528` and :gh:`136528`.
Implementation for AArch64 contributed by Mark Shannon in :gh:`139855`.
Additional optimizations for AArch64 contributed by Mark Shannon and
Diego Russo in :gh:`140683` and :gh:`142305`.)


Removed
========

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

* :class:`collections.abc.ByteString` has been removed from
  ``collections.abc.__all__``. :class:`!collections.abc.ByteString` has been
  deprecated since Python 3.12, and is scheduled for removal in Python 3.17.


ctypes
------

* Removed the undocumented function :func:`!ctypes.SetPointerType`,
  which has been deprecated since Python 3.13.
  (Contributed by Bénédikt Tran in :gh:`133866`.)


glob
----

* Removed the undocumented :func:`!glob.glob0` and :func:`!glob.glob1`
  functions, which have been deprecated since Python 3.13. Use
  :func:`glob.glob` and pass a directory to its *root_dir* argument instead.
  (Contributed by Barney Gale in :gh:`137466`.)


http.server
-----------

* Removed the :class:`!CGIHTTPRequestHandler` class
  and the ``--cgi`` flag from the :program:`python -m http.server`
  command-line interface. They were deprecated in Python 3.13.
  (Contributed by Bénédikt Tran in :gh:`133810`.)


importlib.resources
-------------------

* Removed deprecated ``package`` parameter
  from :func:`importlib.resources.files` function.
  (Contributed by Semyon Moroz in :gh:`138044`.)


pathlib
-------

* Removed deprecated :meth:`!pathlib.PurePath.is_reserved`.
  Use :func:`os.path.isreserved` to detect reserved paths on Windows.
  (Contributed by Nikita Sobolev in :gh:`133875`.)


platform
--------

* Removed the :func:`!platform.java_ver` function,
  which was deprecated since Python 3.13.
  (Contributed by Alexey Makridenko in :gh:`133604`.)


pprint
------

* Add an *expand* keyword argument for :func:`pprint.pprint`,
  :func:`pprint.pformat`, :func:`pprint.pp`. If true, the output will be
  formatted similar to pretty-printed :func:`json.dumps` when
  *indent* is supplied.
  (Contributed by Stefan Todoran and Semyon Moroz in :gh:`112632`.)


sre_*
-----

* Removed :mod:`!sre_compile`, :mod:`!sre_constants` and :mod:`!sre_parse` modules.
  (Contributed by Stan Ulbrych in :gh:`135994`.)


sysconfig
---------

* Removed the *check_home* parameter of :func:`sysconfig.is_python_build`.
  (Contributed by Filipe Laíns in :gh:`92897`.)


threading
---------

* Remove support for arbitrary positional or keyword arguments in the C
  implementation of :class:`~threading.RLock` objects. This was deprecated
  in Python 3.14.
  (Contributed by Bénédikt Tran in :gh:`134087`.)


typing
------

* :class:`typing.ByteString` has been removed from ``typing.__all__``.
  :class:`!typing.ByteString` has been deprecated since Python 3.9, and is
  scheduled for removal in Python 3.17.

* The undocumented keyword argument syntax for creating
  :class:`~typing.NamedTuple` classes (for example,
  ``Point = NamedTuple("Point", x=int, y=int)``) is no longer supported.
  Use the class-based syntax or the functional syntax instead.
  (Contributed by Bénédikt Tran in :gh:`133817`.)

* Using ``TD = TypedDict("TD")`` or ``TD = TypedDict("TD", None)`` to
  construct a :class:`~typing.TypedDict` type with zero fields is no
  longer supported. Use ``class TD(TypedDict): pass``
  or ``TD = TypedDict("TD", {})`` instead.
  (Contributed by Bénédikt Tran in :gh:`133823`.)


wave
----

* Added support for IEEE floating-point WAVE audio
  (``WAVE_FORMAT_IEEE_FLOAT``) in :mod:`wave`.

* Added :meth:`wave.Wave_read.getformat`, :meth:`wave.Wave_write.getformat`,
  and :meth:`wave.Wave_write.setformat` for explicit frame format handling.

* :meth:`wave.Wave_write.setparams` accepts both 7-item tuples including
  ``format`` and 6-item tuples for backwards compatibility (defaulting to
  ``WAVE_FORMAT_PCM``).

* ``WAVE_FORMAT_IEEE_FLOAT`` output now includes a ``fact`` chunk,
  as required for non-PCM WAVE formats.

(Contributed by Lionel Koenig and Michiel W. Beijen in :gh:`60729`.)

* Removed the ``getmark()``, ``setmark()`` and ``getmarkers()`` methods
  of the :class:`~wave.Wave_read` and :class:`~wave.Wave_write` classes,
  which were deprecated since Python 3.13.
  (Contributed by Bénédikt Tran in :gh:`133873`.)


zipimport
---------

* Remove deprecated :meth:`!zipimport.zipimporter.load_module`.
  Use :meth:`zipimport.zipimporter.exec_module` instead.
  (Contributed by Jiahao Li in :gh:`133656`.)


Deprecated
==========

New deprecations
----------------

* :mod:`base64`:

  * Accepting the ``+`` and ``/`` characters with an alternative alphabet in
    :func:`~base64.b64decode` and :func:`~base64.urlsafe_b64decode` is now
    deprecated.
    In future Python versions they will be errors in the strict mode and
    discarded in the non-strict mode.
    (Contributed by Serhiy Storchaka in :gh:`125346`.)

* CLI:

  * Deprecate :option:`-b` and :option:`!-bb` command-line options
    and schedule them to become no-ops in Python 3.17.
    These were primarily helpers for the Python 2 -> 3 transition.
    Starting with Python 3.17, no :exc:`BytesWarning` will be raised
    for these cases; use a type checker instead.

    (Contributed by Nikita Sobolev in :gh:`136355`.)

* :mod:`collections.abc`

  * The following statements now cause ``DeprecationWarning``\ s to be emitted
    at runtime:

    * ``from collections.abc import ByteString``
    * ``import collections.abc; collections.abc.ByteString``.

    ``DeprecationWarning``\ s were already emitted if
    :class:`collections.abc.ByteString` was subclassed or used as the second
    argument to :func:`isinstance` or :func:`issubclass`, but warnings were not
    previously emitted if it was merely imported or accessed from the
    :mod:`!collections.abc` module.


* :mod:`hashlib`:

  * In hash function constructors such as :func:`~hashlib.new` or the
    direct hash-named constructors such as :func:`~hashlib.md5` and
    :func:`~hashlib.sha256`, the optional initial data parameter could
    also be passed as a keyword argument named ``data=`` or ``string=`` in
    various :mod:`hashlib` implementations.

    Support for the ``string`` keyword argument name is now deprecated and
    is slated for removal in Python 3.19. Prefer passing the initial data as
    a positional argument for maximum backwards compatibility.

    (Contributed by Bénédikt Tran in :gh:`134978`.)

* :mod:`struct`:

  * Calling the ``Struct.__new__()`` without required argument now is
    deprecated and will be removed in Python 3.20.  Calling
    :meth:`~object.__init__` method on initialized :class:`~struct.Struct`
    objects is deprecated and will be removed in Python 3.20.

    (Contributed by Sergey B Kirpichev and Serhiy Storchaka in :gh:`143715`.)

* :mod:`typing`:

  * The following statements now cause ``DeprecationWarning``\ s to be emitted
    at runtime:

    * ``from typing import ByteString``
    * ``import typing; typing.ByteString``.

    ``DeprecationWarning``\ s were already emitted if :class:`typing.ByteString`
    was subclassed or used as the second argument to :func:`isinstance` or
    :func:`issubclass`, but warnings were not previously emitted if it was
    merely imported or accessed from the :mod:`!typing` module.

  * Deprecated :func:`!typing.no_type_check_decorator` has been removed.
    (Contributed by Nikita Sobolev in :gh:`133601`.)

* ``__version__``

  * The ``__version__``, ``version`` and ``VERSION`` attributes have been
    deprecated in these standard library modules and will be removed in
    Python 3.20. Use :py:data:`sys.version_info` instead.

    - :mod:`argparse`
    - :mod:`csv`
    - :mod:`ctypes`
    - :mod:`!ctypes.macholib`
    - :mod:`decimal` (use :data:`decimal.SPEC_VERSION` instead)
    - :mod:`http.server`
    - :mod:`imaplib`
    - :mod:`ipaddress`
    - :mod:`json`
    - :mod:`logging` (``__date__`` also deprecated)
    - :mod:`optparse`
    - :mod:`pickle`
    - :mod:`platform`
    - :mod:`re`
    - :mod:`socketserver`
    - :mod:`tabnanny`
    - :mod:`tarfile`
    - :mod:`tkinter.font`
    - :mod:`tkinter.ttk`
    - :mod:`wsgiref.simple_server`
    - :mod:`xml.etree.ElementTree`
    - :mod:`!xml.sax.expatreader`
    - :mod:`xml.sax.handler`
    - :mod:`zlib`

    (Contributed by Hugo van Kemenade and Stan Ulbrych in :gh:`76007`.)

.. Add deprecations above alphabetically, not here at the end.

.. include:: ../deprecations/pending-removal-in-3.16.rst

.. include:: ../deprecations/pending-removal-in-3.17.rst

.. include:: ../deprecations/pending-removal-in-3.18.rst

.. include:: ../deprecations/pending-removal-in-3.19.rst

.. include:: ../deprecations/pending-removal-in-3.20.rst

.. include:: ../deprecations/pending-removal-in-future.rst


C API changes
=============

New features
------------

* Add :c:func:`PyArg_ParseArray` and :c:func:`PyArg_ParseArrayAndKeywords`
  functions to parse arguments of functions using the :c:macro:`METH_FASTCALL`
  calling convention.
  (Contributed by Victor Stinner in :gh:`144175`.)

* Add the following functions for the new :class:`frozendict` type:

  * :c:func:`PyAnyDict_Check`
  * :c:func:`PyAnyDict_CheckExact`
  * :c:func:`PyFrozenDict_Check`
  * :c:func:`PyFrozenDict_CheckExact`
  * :c:func:`PyFrozenDict_New`

  (Contributed by Victor Stinner in :gh:`141510`.)

* Add :c:func:`PySys_GetAttr`, :c:func:`PySys_GetAttrString`,
  :c:func:`PySys_GetOptionalAttr`, and :c:func:`PySys_GetOptionalAttrString`
  functions as replacements for :c:func:`PySys_GetObject`.
  (Contributed by Serhiy Storchaka in :gh:`108512`.)

* Add :c:type:`PyUnstable_Unicode_GET_CACHED_HASH` to get the cached hash of
  a string. See the documentation for caveats.
  (Contributed by Petr Viktorin in :gh:`131510`.)

* Add API for checking an extension module's ABI compatibility:
  :c:data:`Py_mod_abi`, :c:func:`PyABIInfo_Check`, :c:macro:`PyABIInfo_VAR`
  and :c:data:`Py_mod_abi`.
  (Contributed by Petr Viktorin in :gh:`137210`.)

.. _whatsnew315-pybyteswriter:

* Implement :pep:`782`, the :ref:`PyBytesWriter API <pybyteswriter>`.
  Add functions:

  * :c:func:`PyBytesWriter_Create`
  * :c:func:`PyBytesWriter_Discard`
  * :c:func:`PyBytesWriter_FinishWithPointer`
  * :c:func:`PyBytesWriter_FinishWithSize`
  * :c:func:`PyBytesWriter_Finish`
  * :c:func:`PyBytesWriter_Format`
  * :c:func:`PyBytesWriter_GetData`
  * :c:func:`PyBytesWriter_GetSize`
  * :c:func:`PyBytesWriter_GrowAndUpdatePointer`
  * :c:func:`PyBytesWriter_Grow`
  * :c:func:`PyBytesWriter_Resize`
  * :c:func:`PyBytesWriter_WriteBytes`

  (Contributed by Victor Stinner in :gh:`129813`.)

* Add a new :c:func:`PyImport_CreateModuleFromInitfunc` C-API for creating
  a module from a *spec* and *initfunc*.
  (Contributed by Itamar Oren in :gh:`116146`.)

* Add :c:func:`PyTuple_FromArray` to create a :class:`tuple` from an array.
  (Contributed by Victor Stinner in :gh:`111489`.)

* Add :c:func:`PyObject_Dump` to dump an object to ``stderr``.
  It should only be used for debugging.
  (Contributed by Victor Stinner in :gh:`141070`.)

* Add :c:func:`PyUnstable_ThreadState_SetStackProtection` and
  :c:func:`PyUnstable_ThreadState_ResetStackProtection` functions to set
  the stack protection base address and stack protection size of a Python
  thread state.
  (Contributed by Victor Stinner in :gh:`139653`.)

* Add :c:func:`PyUnstable_SetImmortal` C-API function to mark objects as :term:`immortal`.
  (Contributed by Kumar Aditya in :gh:`143300`.)

* Restore private provisional ``_Py_InitializeMain()`` function removed in
  Python 3.14.
  (Contributed by Victor Stinner in :gh:`142417`.)

Changed C APIs
--------------

* If the :c:macro:`Py_TPFLAGS_MANAGED_DICT` or :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF`
  flag is set then :c:macro:`Py_TPFLAGS_HAVE_GC` must be set too.
  (Contributed by Sergey Miryanov in :gh:`134786`.)

* :c:macro:`PyDateTime_IMPORT` is now thread safe. Code that directly checks ``PyDateTimeAPI``
  for ``NULL`` should be updated to call :c:macro:`PyDateTime_IMPORT` instead.
  (Contributed by Kumar Aditya in :gh:`141563`.)

Porting to Python 3.15
----------------------

* Private functions promoted to public C APIs:

  The |pythoncapi_compat_project| can be used to get most of these new
  functions on Python 3.14 and older.


Removed C APIs
--------------

* Remove deprecated ``PyUnicode`` functions:

  * :c:func:`!PyUnicode_AsDecodedObject`:
    Use :c:func:`PyCodec_Decode` instead.
  * :c:func:`!PyUnicode_AsDecodedUnicode`:
    Use :c:func:`PyCodec_Decode` instead; Note that some codecs (for example, "base64")
    may return a type other than :class:`str`, such as :class:`bytes`.
  * :c:func:`!PyUnicode_AsEncodedObject`:
    Use :c:func:`PyCodec_Encode` instead.
  * :c:func:`!PyUnicode_AsEncodedUnicode`:
    Use :c:func:`PyCodec_Encode` instead; Note that some codecs (for example, "base64")
    may return a type other than :class:`bytes`, such as :class:`str`.

  (Contributed by Stan Ulbrych in :gh:`133612`.)

* :c:func:`!PyImport_ImportModuleNoBlock`: deprecated alias
  of :c:func:`PyImport_ImportModule`.
  (Contributed by Bénédikt Tran in :gh:`133644`.)

* :c:func:`!PyWeakref_GetObject` and :c:macro:`!PyWeakref_GET_OBJECT`:
  use :c:func:`PyWeakref_GetRef` instead. The |pythoncapi_compat_project|
  can be used to get :c:func:`!PyWeakref_GetRef` on Python 3.12 and older.
  (Contributed by Bénédikt Tran in :gh:`133644`.)

* Remove deprecated :c:func:`!PySys_ResetWarnOptions`.
  Clear :data:`sys.warnoptions` and :data:`!warnings.filters` instead.

  (Contributed by Nikita Sobolev in :gh:`138886`.)

The following functions are removed in favor of :c:func:`PyConfig_Get`.
The |pythoncapi_compat_project| can be used to get :c:func:`!PyConfig_Get`
on Python 3.13 and older.

* Python initialization functions:

  * :c:func:`!Py_GetExecPrefix`:
    use :c:func:`PyConfig_Get("base_exec_prefix") <PyConfig_Get>`
    (:data:`sys.base_exec_prefix`) instead.
    Use :c:func:`PyConfig_Get("exec_prefix") <PyConfig_Get>`
    (:data:`sys.exec_prefix`) if :ref:`virtual environments <venv-def>`
    need to be handled.
  * :c:func:`!Py_GetPath`:
    use :c:func:`PyConfig_Get("module_search_paths") <PyConfig_Get>`
    (:data:`sys.path`) instead.
  * :c:func:`!Py_GetPrefix`:
    use :c:func:`PyConfig_Get("base_prefix") <PyConfig_Get>`
    (:data:`sys.base_prefix`) instead.
    Use :c:func:`PyConfig_Get("prefix") <PyConfig_Get>`
    (:data:`sys.prefix`) if :ref:`virtual environments <venv-def>`
    need to be handled.
  * :c:func:`!Py_GetProgramFullPath`:
    use :c:func:`PyConfig_Get("executable") <PyConfig_Get>`
    (:data:`sys.executable`) instead.
  * :c:func:`!Py_GetProgramName`:
    use :c:func:`PyConfig_Get("executable") <PyConfig_Get>`
    (:data:`sys.executable`) instead.
  * :c:func:`!Py_GetPythonHome`:
    use :c:func:`PyConfig_Get("home") <PyConfig_Get>` or the
    :envvar:`PYTHONHOME` environment variable instead.

  (Contributed by Bénédikt Tran in :gh:`133644`.)

.. |pythoncapi_compat_project| replace:: |pythoncapi_compat_project_link|_
.. |pythoncapi_compat_project_link| replace:: pythoncapi-compat project
.. _pythoncapi_compat_project_link: https://github.com/python/pythoncapi-compat


Deprecated C APIs
-----------------

* Deprecate :pep:`456` support for providing an external definition
  of the string hashing scheme. Removal is scheduled for Python 3.19.

  Previously, embedders could define :c:macro:`Py_HASH_ALGORITHM` to be
  ``Py_HASH_EXTERNAL`` to indicate that the hashing scheme was provided
  externally but this feature was undocumented, untested and most likely
  unused.

  (Contributed by Bénédikt Tran in :gh:`141226`.)

* For unsigned integer formats in :c:func:`PyArg_ParseTuple`,
  accepting Python integers with value that is larger than the maximal value
  for the C type or less than the minimal value for the corresponding
  signed integer type of the same size is now deprecated.
  (Contributed by Serhiy Storchaka in :gh:`132629`.)

* :c:func:`PyBytes_FromStringAndSize(NULL, len) <PyBytes_FromStringAndSize>`
  and :c:func:`_PyBytes_Resize` are :term:`soft deprecated`,
  use the :c:type:`PyBytesWriter` API instead.
  (Contributed by Victor Stinner in :gh:`129813`.)

* :c:func:`!_PyObject_CallMethodId`, :c:func:`!_PyObject_GetAttrId` and
  :c:func:`!_PyUnicode_FromId` are deprecated since 3.15 and will be removed in
  3.20. Instead, use :c:func:`PyUnicode_InternFromString()` and cache the result in
  the module state, then call :c:func:`PyObject_CallMethod` or
  :c:func:`PyObject_GetAttr`.
  (Contributed by Victor Stinner in :gh:`141049`.)

* Deprecate :c:member:`~PyComplexObject.cval` field of the
  :c:type:`PyComplexObject` type.
  Use :c:func:`PyComplex_AsCComplex` and :c:func:`PyComplex_FromCComplex`
  to convert a Python complex number to/from the C :c:type:`Py_complex`
  representation.
  (Contributed by Sergey B Kirpichev in :gh:`128813`.)

* Functions :c:func:`_Py_c_sum`, :c:func:`_Py_c_diff`, :c:func:`_Py_c_neg`,
  :c:func:`_Py_c_prod`, :c:func:`_Py_c_quot`, :c:func:`_Py_c_pow` and
  :c:func:`_Py_c_abs` are :term:`soft deprecated`.
  (Contributed by Sergey B Kirpichev in :gh:`128813`.)

* :c:member:`~PyConfig.bytes_warning` is deprecated
  since 3.15 and will be removed in 3.17.
  (Contributed by Nikita Sobolev in :gh:`136355`.)

* :c:macro:`!Py_INFINITY` macro is :term:`soft deprecated`,
  use the C11 standard ``<math.h>`` :c:macro:`!INFINITY` instead.
  (Contributed by Sergey B Kirpichev in :gh:`141004`.)

* The following macros are :term:`soft deprecated`:

  - :c:macro:`Py_ALIGNED`: Prefer ``alignas`` instead.
  - :c:macro:`PY_FORMAT_SIZE_T`: Use ``"z"`` directly.
  - :c:macro:`Py_LL` & :c:macro:`Py_ULL`:
    Use standard suffixes, ``LL`` & ``ULL``.
  - :c:macro:`PY_LONG_LONG`, :c:macro:`PY_LLONG_MIN`, :c:macro:`PY_LLONG_MAX`,
    :c:macro:`PY_ULLONG_MAX`, :c:macro:`PY_INT32_T`, :c:macro:`PY_UINT32_T`,
    :c:macro:`PY_INT64_T`, :c:macro:`PY_UINT64_T`, :c:macro:`PY_SIZE_MAX`:
    Use C99 types/limits.
  - :c:macro:`Py_UNICODE_SIZE`: Use ``sizeof(wchar_t)`` directly.
  - :c:macro:`Py_VA_COPY`: Use ``va_copy`` directly.

  The macro :c:macro:`Py_UNICODE_WIDE`, which was scheduled for removal,
  is :term:`soft deprecated` instead.

  (Contributed by Petr Viktorin in :gh:`146175`.)

* :c:macro:`!Py_MATH_El` and :c:macro:`!Py_MATH_PIl` are deprecated
  since 3.15 and will be removed in 3.20.
  (Contributed by Sergey B Kirpichev in :gh:`141004`.)


.. Add C API deprecations above alphabetically, not here at the end.


Build changes
=============

* Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
  Now this should be explicitly enabled with :option:`--with-system-libmpdec`
  set to ``no`` or with :option:`!--without-system-libmpdec`.
  (Contributed by Sergey B Kirpichev in :gh:`115119`.)

* The new configure option :option:`--with-missing-stdlib-config=FILE` allows
  distributors to pass a `JSON <https://www.json.org/json-en.html>`_
  configuration file containing custom error messages for :term:`standard library`
  modules that are missing or packaged separately.
  (Contributed by Stan Ulbrych and Petr Viktorin in :gh:`139707`.)

* The new configure option :option:`--with-pymalloc-hugepages` enables huge
  page support for :ref:`pymalloc <pymalloc>` arenas. When enabled, arena size
  increases to 2 MiB and allocation uses ``MAP_HUGETLB`` (Linux) or
  ``MEM_LARGE_PAGES`` (Windows) with automatic fallback to regular pages.
  On Windows, use ``build.bat --pymalloc-hugepages``.
  At runtime, huge pages must be explicitly enabled by setting the
  :envvar:`PYTHON_PYMALLOC_HUGEPAGES` environment variable to ``1``.

* Annotating anonymous mmap usage is now supported if Linux kernel supports
  :manpage:`PR_SET_VMA_ANON_NAME <PR_SET_VMA(2const)>` (Linux 5.17 or newer).
  Annotations are visible in ``/proc/<pid>/maps`` if the kernel supports the feature
  and :option:`-X dev <-X>` is passed to the Python or Python is built in :ref:`debug mode <debug-build>`.
  (Contributed by Donghee Na in :gh:`141770`.)

.. _whatsnew315-windows-tail-calling-interpreter:

* 64-bit builds using Visual Studio 2026 (MSVC 18) may now use the new
  :ref:`tail-calling interpreter <whatsnew314-tail-call-interpreter>`.
  Results on Visual Studio 18.1.1 report between
  `15-20% <https://github.com/faster-cpython/ideas/blob/main/results/5800X-msvc.pgo2-vs-msvc.pgo.tc.svg>`__
  speedup on the geometric mean of pyperformance on Windows x86-64 over
  the switch-case interpreter on an AMD Ryzen 7 5800X. We have
  observed speedups ranging from 14% for large pure-Python libraries
  to 40% for long-running small pure-Python scripts on Windows.
  This was made possible by a new feature introduced in MSVC 18,
  which the official Windows 64-bit binaries on python.org__ now use.
  (Contributed by Chris Eibl, Ken Jin, and Brandt Bucher in :gh:`143068`.
  Special thanks to Steve Dower, and the MSVC team including Hulon Jenkins.)

  __ https://www.python.org/downloads/windows/


Porting to Python 3.15
======================

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

* :class:`sqlite3.Connection` APIs have been cleaned up.

  * All parameters of :func:`sqlite3.connect` except *database* are now keyword-only.
  * The first three parameters of methods :meth:`~sqlite3.Connection.create_function`
    and :meth:`~sqlite3.Connection.create_aggregate` are now positional-only.
  * The first parameter of methods :meth:`~sqlite3.Connection.set_authorizer`,
    :meth:`~sqlite3.Connection.set_progress_handler` and
    :meth:`~sqlite3.Connection.set_trace_callback` is now positional-only.

  (Contributed by Serhiy Storchaka in :gh:`133595`.)

* :data:`resource.RLIM_INFINITY` is now always positive.
  Passing a negative integer value that corresponded to its old value
  (such as ``-1`` or ``-3``, depending on platform) to
  :func:`resource.setrlimit` and :func:`resource.prlimit` is now deprecated.
  (Contributed by Serhiy Storchaka in :gh:`137044`.)

* :meth:`mmap.mmap.resize` has been removed on platforms that don't support the
  underlying syscall, instead of raising a :exc:`SystemError`.

* A resource warning is now emitted for an unclosed
  :func:`xml.etree.ElementTree.iterparse` iterator if it opened a file.
  Use its :meth:`!close` method or the :func:`contextlib.closing` context
  manager to close it.
  (Contributed by Osama Abdelkader and Serhiy Storchaka in :gh:`140601`.)

* If a short option and a single-dash long option are passed to
  :meth:`argparse.ArgumentParser.add_argument`, *dest* is now inferred from
  the single-dash long option. For example, in ``add_argument('-f', '-foo')``,
  *dest* is now ``'foo'`` instead of ``'f'``.
  Pass an explicit *dest* argument to preserve the old behavior.
  (Contributed by Serhiy Storchaka in :gh:`138697`.)

* Padding of input no longer required in :func:`base64.urlsafe_b64decode`.
  Pass a new argument ``padded=True`` or use :func:`base64.b64decode`
  with argument ``altchars=b'-_'`` (this works with older Python versions)
  to make padding required.
  (Contributed by Serhiy Storchaka in :gh:`73613`.)
