# What’s new in Python 3.15

Editor:

  Hugo van Kemenade

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

For full details, see the 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

- PEP 810: Explicit lazy imports for faster startup times

- PEP 814: Add frozendict built-in type

- PEP 799: A dedicated profiling package for organizing Python profiling tools

- PEP 799: Tachyon: High frequency statistical sampling profiler

- PEP 798: Unpacking in comprehensions

- PEP 686: Python now uses UTF-8 as the default encoding

- PEP 728: `TypedDict` with typed extra items

- PEP 747: Annotating type forms with TypeForm

- PEP 800: Disjoint bases in the type system

- PEP 782: A new PyBytesWriter C API to create a Python bytes object

- PEP 803: Stable ABI for Free-Threaded Builds

- The JIT compiler has been significantly upgraded

- Improved error messages

- The official Windows 64-bit binaries now use the tail-calling interpreter

## New features

### 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 `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 `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:

    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 and debug the failure.

For cases where you want to enable lazy loading globally without modifying source code, Python provides the `-X lazy_imports` command-line option and the `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 `sys.set_lazy_imports()` and `sys.get_lazy_imports()` functions allow changing and querying this mode at runtime.

For more selective control, `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:

    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 `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 `SyntaxError`. Neither star imports nor future imports can be lazy (`lazy from module import *` and `lazy from __future__ import ...` both raise `SyntaxError`).

See also

PEP 810 for the full specification and rationale.

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

### PEP 814: Add frozendict built-in type

A new immutable type, `frozendict`, is added to the `builtins` module. It does not allow modification after creation. A `frozendict` is not a subclass of `dict`; it inherits directly from `object`. A `frozendict` is hashable as long as all of its keys and values are hashable. A `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 `frozendict`: `copy`, `decimal`, `json`, `marshal`, `plistlib` (only for serialization), `pickle`, `pprint` and `xml.etree.ElementTree`.

`eval()` and `exec()` accept `frozendict` for globals, and `type()` and `str.maketrans()` accept `frozendict` for dict.

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

See also

PEP 814 for the full specification and rationale.

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

### PEP 799: A dedicated profiling package

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

- `profiling.tracing`: deterministic function-call tracing (relocated from `cProfile`).

- `profiling.sampling`: a new statistical sampling profiler (named Tachyon).

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

See also

PEP 799 for further details.

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

### Tachyon: High frequency statistical sampling profiler

A new statistical sampling profiler (Tachyon) has been added as `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 `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 `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. 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 `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.)

### 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, `itertools.chain()`, and `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)`.

See also

PEP 798 for further details.

(Contributed by Adam Hartz in gh-143055.)

### PEP 803: `abi3t` – Stable ABI for Free-Threaded Builds

C extensions that target the Stable ABI can now be compiled for the new Stable ABI for Free-Threaded Builds (also known as `abi3t`), which makes them compatible with free-threaded builds of CPython. This usually requires some non-trivial changes to the source code; specifically:

- Switching to API introduced in PEP 697 (Python 3.12), such as negative `basicsize` and `PyObject_GetTypeData()`, rather than making `PyObject` part of the instance struct; and

- Switching from a `PyInit_` function to a new export hook, `PyModExport_*`, introduced for this purpose in PEP 793.

Note that Stable ABI does not offer all the functionality that CPython has to offer. Extensions that cannot switch to `abi3t` should continue to build for the existing Stable ABI (`abi3`) and the version-specific ABI for free-threading (`cp315t`) separately.

Stable ABI for Free-Threaded Builds should typically be selected in a build tool (such as, for example, Setuptools, meson-python, scikit-build-core, or Maturin). At the time of writing, these tools did not support `abi3t`. If this is the case for your tool, compile for `cp315t` separately. If not using a build tool – or when writing such a tool – you can select `abi3t` by setting the macro `Py_TARGET_ABI3T` as discussed in Compiling for Stable ABI.

### Improved error messages

- The interpreter now provides more helpful suggestions in `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:

      @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:

      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'?

- The interpreter now tries to provide a suggestion when `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:

      >>> class A:
      ...     pass
      >>> a = A()
      >>> a.abcde = 1
      >>> del a.abcdf
      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.)

- Several error messages incorrectly using the term “argument” have been corrected. (Contributed by Stan Ulbrych in gh-133382.)

## Other language changes

- 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 opt-in 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 `PYTHONUTF8=0` environment variable or the `-X utf8=0` command-line option.

  See also

  PEP 686 for further details.

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

- Unraisable exceptions are now highlighted with color by default. This can be controlled by environment variables. (Contributed by Peter Bierma in gh-134170.)

- The `__repr__()` of `ImportError` and `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 `__dict__` and `__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 `-W` option and the `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 `Decimal` and `Fraction`), not only integers or floats, although this does not improve precision. (Contributed by Serhiy Storchaka in gh-67795.)

- Added `bytearray.take_bytes(n=None, /)` to take bytes out of a `bytearray` without copying. This enables optimizing code which must return `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 `take_bytes()` are listed below.

  Suggested optimizing refactors
  Description | Old | New
  --- | --- | ---
  Return `bytes` after working with `bytearray` |
      def read() -> bytes:
          buffer = bytearray(1024)
          ...
          return bytes(buffer)
      def read() -> bytes:
          buffer = bytearray(1024)
          ...
          return buffer.take_bytes()
  Empty a buffer getting the bytes |
      buffer = bytearray(1024)
      ...
      data = bytes(buffer)
      buffer.clear()
      buffer = bytearray(1024)
      ...
      data = buffer.take_bytes()
  Split a buffer at a specific separator |
      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')
      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 |
      buffer = bytearray(b'abc\ndef')
      n = buffer.find(b'\n')
      data = bytes(buffer[:n])
      buffer.clear()
      assert data == b'abc'
      assert len(buffer) == 0
      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 `compile()`, `ast.parse()`, `symtable.symtable()`, and `importlib.abc.InspectLoader.source_to_code()`, now allow the module name to be passed. It is needed to unambiguously filter syntax warnings by module name. (Contributed by Serhiy Storchaka in gh-135801.)

- Allowed defining the __dict__ and __weakref__ __slots__ for any class. (Contributed by Serhiy Storchaka in gh-41779.)

- Allowed defining any __slots__ for a class derived from `tuple` (including classes created by `collections.namedtuple()`). (Contributed by Serhiy Storchaka in gh-41779.)

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

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

- Allow the count argument of `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 `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 `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 float complex and 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 `z85encode()`. (Contributed by Hauke Dämpfling in gh-143103.)

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

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

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

### binascii

- Added functions for Base32 encoding:

  - `b2a_base32()` and `a2b_base32()`

  (Contributed by James Seo in gh-146192.)

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

  - `b2a_ascii85()` and `a2b_ascii85()`

  - `b2a_base85()` and `a2b_base85()`

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

- Added the padded parameter in `b2a_base32()`, `a2b_base32()`, `b2a_base64()`, and `a2b_base64()`. (Contributed by Serhiy Storchaka in gh-73613.)

- Added the wrapcol parameter in `b2a_base64()`. (Contributed by Serhiy Storchaka in gh-143214.)

- Added the alphabet parameter in `b2a_base64()` and `a2b_base64()`. (Contributed by Serhiy Storchaka in gh-145980.)

- Added the ignorechars parameter in `a2b_hex()`, `unhexlify()`, and `a2b_base64()`. (Contributed by Serhiy Storchaka in gh-144001 and gh-146431.)

### calendar

- Calendar pages generated by the `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 `calendar`’s command-line 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 `collections.Counter.__xor__()` and `collections.Counter.__ixor__()` to compute the symmetric difference between `Counter` objects. (Contributed by Raymond Hettinger in gh-138682.)

### concurrent.futures

- Improved error reporting when a child process in a `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 `__enter__()`, `__exit__()`, `__aenter__()`, and `__aexit__()` in `ExitStack` and `contextlib.AsyncExitStack`, for consistency with the `with` and `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 `reorganize()` methods to `dbm.dumb` and `dbm.sqlite3` which allow to recover unused free space previously occupied by deleted entries. (Contributed by Andrea Oliveri in gh-134004.)

### difflib

- Introduced the optional color parameter to `difflib.unified_diff()`, enabling color output similar to git diff. This can be controlled by environment variables. (Contributed by Douglas Thor in gh-133725.)

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

### functools

- `singledispatchmethod()` now supports non-descriptor callables. (Contributed by Serhiy Storchaka in gh-140873.)

- `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 `hashlib` even if they will not work at runtime due to missing backend implementations. For instance, `hashlib.md5` will no longer raise `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 `HTTPConnection` and `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.)

### http.server

- The logging of `BaseHTTPRequestHandler`, as used by the command-line interface, is colored by default. This can be controlled with environment variables. (Contributed by Hugo van Kemenade in gh-146292.)

### inspect

- Add parameters inherit_class_doc and fallback_to_class_doc for `getdoc()`. (Contributed by Serhiy Storchaka in gh-132686.)

### json

- Add the array_hook parameter to `load()` and `loads()` functions: allow a callback for JSON literal array types to customize Python lists in the resulting decoded object. Passing combined `frozendict` to object_pairs_hook param and `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

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

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

### math

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

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

### mimetypes

- Add more MIME types. (Contributed by Benedikt Johannes, Charlie Lin, Foolbar, Gil Forcada and John Franey in gh-144217, gh-145720, gh-140937, gh-139959, gh-145698, gh-145718 and 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

- `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 `mmap.mmap.set_name()` method to annotate an anonymous memory mapping if Linux kernel supports PR_SET_VMA_ANON_NAME (Linux 5.17 or newer). (Contributed by Donghee Na in gh-142419.)

### os

- Add `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 `realpath()`. (Contributed by Serhiy Storchaka in gh-71189.)

- The strict parameter to `os.path.realpath()` accepts a new value, `os.path.ALLOW_MISSING`. If used, errors other than `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.)

### pprint

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

- Add t-string support to `pprint`. (Contributed by Loïc Simon and Hugo van Kemenade in gh-134551.)

### re

- `re.prefixmatch()` and a corresponding `re.Pattern.prefixmatch()` have been added as alternate, more explicit names for the existing and now soft deprecated `re.match()` and `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: `RLIMIT_NTHR`, `RLIMIT_UMTXP`, `RLIMIT_THREADS`, `RLIM_SAVED_CUR`, and `RLIM_SAVED_MAX`. (Contributed by Serhiy Storchaka in gh-137512.)

### shelve

- Added new `reorganize()` method to `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 `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 command-line interface 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 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 `ssl.HAS_PSK_TLS13` whether the `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

  - `ssl.SSLContext.set_groups()` sets the groups allowed for doing key agreement, extending the previous `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.

  - `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.

  - `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 `ssl.SSLContext.set_ciphersuites()` for setting TLS 1.3 ciphers. For TLS 1.2 or earlier, `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:

  - `ssl.get_sigalgs()` returns a list of all available TLS signature algorithms. This call requires OpenSSL 3.4 or later.

  - `ssl.SSLContext.set_client_sigalgs()` sets the signature algorithms allowed for certificate-based client authentication.

  - `ssl.SSLContext.set_server_sigalgs()` sets the signature algorithms allowed for the server to complete the TLS handshake.

  - `ssl.SSLSocket.client_sigalg()` returns the signature algorithm selected for client authentication on the current connection. This call requires OpenSSL 3.5 or later.

  - `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

- `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 `os.pidfd_open()` + `select.poll()`.

  - macOS and other BSD variants use `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 `symtable.Function.get_cells()` and `symtable.Symbol.is_cell()` methods. (Contributed by Yashp002 in gh-143504.)

### sys

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

### sys.monitoring

- The other events (`PY_THROW`, `PY_UNWIND`, `RAISE`, `EXCEPTION_HANDLED`, and `RERAISE`) can now be turned on and disabled on a per code object basis. Returning `DISABLE` from a callback for one of these events disables the event for the entire code object (for the current tool), rather than raising `ValueError` as in prior versions. (Contributed by Gabriele N. Tornetta in gh-146182.)

### 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.)

- `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.)

- `extract()` and `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, `LinkFallbackError`. (Contributed by Petr Viktorin for CVE 2025-4330 and CVE 2024-12718.)

- `extract()` and `extractall()` no longer extract rejected members when `errorlevel()` is zero. (Contributed by Matt Prodani and Petr Viktorin in gh-112887 and CVE 2025-4435.)

- `extract()` and `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 output of the `timeit` command-line interface is colored by default. This can be controlled with environment variables. (Contributed by Hugo van Kemenade in gh-146609.)

- The command-line interface now colorizes error tracebacks by default. This can be controlled with environment variables. (Contributed by Yi Hong in gh-139374.)

- Make the target time of `timeit.Timer.autorange()` configurable and add `--target-time` option to the command-line interface. (Contributed by Alessandro Cucci and Miikka Koskinen in gh-80642.)

### tkinter

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

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

### tomllib

- The `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:

        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:

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

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

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

  (Contributed by Taneli Hukkinen in gh-142956.)

### types

- Expose the write-through `locals()` proxy type as `types.FrameLocalsProxyType`. This represents the type of the `frame.f_locals` attribute, as described in PEP 667.

### typing

- PEP 747: Add `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`, `TypedDict` classes, or `list[int]`) expose precise signatures:

      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 `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.)

- PEP 800: Add `@typing.disjoint_base`, a new decorator marking a class as a disjoint base. This is an advanced feature primarily intended to allow type checkers to faithfully reflect the runtime semantics of types defined as builtins or in compiled extensions. If a class `C` is a disjoint base, then child classes of that class cannot inherit from other disjoint bases that are not parent or child classes of `C`. (Contributed by Jelle Zijlstra in gh-148639.)

- `TypeVarTuple` now accepts `bound`, `covariant`, `contravariant`, and `infer_variance` keyword arguments, matching the interface of `TypeVar` and `ParamSpec`. `bound` semantics remain undefined in the specification.

### unicodedata

- The Unicode database has been updated to Unicode 17.0.0.

- Add `unicodedata.isxidstart()` and `unicodedata.isxidcontinue()` functions to check whether a character can start or continue a Unicode Standard Annex #31 identifier. (Contributed by Stan Ulbrych in gh-129117.)

- Add the `iter_graphemes()` function to iterate over grapheme clusters according to rules defined in Unicode Standard Annex #29, “Unicode Text Segmentation”. Add `grapheme_cluster_break()`, `indic_conjunct_break()` and `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 `block()` function to return the Unicode block assigned to a character. (Contributed by Stan Ulbrych in gh-66802.)

### unittest

- `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 `urlsplit()`, `urlparse()` and `urldefrag()` functions. Add the keep_empty parameter to `urlunsplit()` and `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 `sys.platlibdir` is not equal to `lib`. (Contributed by Rui Xi in gh-133951.)

### warnings

- Improve filtering by module in `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.)

### wave

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

- Added `wave.Wave_read.getformat()`, `wave.Wave_write.getformat()`, and `wave.Wave_write.setformat()` for explicit frame format handling.

- `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.)

### xml.parsers.expat

- Add `SetAllocTrackerActivationThreshold()` and `SetAllocTrackerMaximumAmplification()` to xmlparser objects to tune protections against disproportional amounts of dynamic memory usage from within an Expat parser. (Contributed by Bénédikt Tran in gh-90949.)

- Add `SetBillionLaughsAttackProtectionActivationThreshold()` and `SetBillionLaughsAttackProtectionMaximumAmplification()` to xmlparser objects to tune protections against billion laughs attacks. (Contributed by Bénédikt Tran in gh-90949.)

### zlib

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

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

## Optimizations

- `mimalloc` is now used as the default allocator for raw memory allocations such as via `PyMem_RawMalloc()` for better performance on free-threaded builds. (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

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

### Upgraded JIT compiler

Results from the pyperformance benchmark suite report 6-7% 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% speedup over the tail calling 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

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 for all supported platforms. (Contributed by Savannah Ostrowski in gh-140973.)

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.)

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.)

More JIT optimizations

More constant-propagation 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.)

Reference counts are avoided whenever it is safe to do so. 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.)

By tracking unique references to objects, the JIT optimizer can now eliminate reference count updates and perform inplace operations on ints and floats. (Contributed by Reiden Ong, and Pieter Eendebak in gh-143414 and gh-146306.)

The JIT optimizer now supports significantly more operations than in 3.14. (Contributed by Kumar Aditya, Ken Jin, Jiahao Li, and Sacul in gh-131798.)

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 3.14. (Contributed by Brandt Bucher in gh-136528 and gh-135905. 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.)

Maintainability

The JIT optimizer’s operations have been simplified. This was made possible by a refactoring of JIT data structures. (Contributed by Zhongtian Zheng in gh-148211 and Hai Zhu in gh-143421.)

## Removed

### collections.abc

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

### ctypes

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

### datetime

- `strptime()` now raises `ValueError` when the format string contains `%d` (day of month) without a year directive. This has been deprecated since Python 3.13. (Contributed by Stan Ulbrych and Gregory P. Smith in gh-70647.)

### glob

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

### http.server

- Removed the `CGIHTTPRequestHandler` class and the `--cgi` flag from the 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 `importlib.resources.files()` function. (Contributed by Semyon Moroz in gh-138044.)

### pathlib

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

### platform

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

### sre_*

- Removed `sre_compile`, `sre_constants` and `sre_parse` modules. (Contributed by Stan Ulbrych in gh-135994.)

### sysconfig

- Removed the check_home parameter of `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 `RLock` objects. This was deprecated in Python 3.14. (Contributed by Bénédikt Tran in gh-134087.)

### typing

- `typing.ByteString` has been removed from `typing.__all__`. `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 `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 `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.)

- Deprecated `typing.no_type_check_decorator()` has been removed. (Contributed by Nikita Sobolev in gh-133601.)

### wave

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

### zipimport

- Remove deprecated `zipimport.zipimporter.load_module()`. Use `zipimport.zipimporter.exec_module()` instead. (Contributed by Jiahao Li in gh-133656.)

## Deprecated

### New deprecations

- `base64`:

  - Accepting the `+` and `/` characters with an alternative alphabet in `b64decode()` and `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 `-b` and `-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 `BytesWarning` will be raised for these cases; use a type checker instead.

    (Contributed by Nikita Sobolev in gh-136355.)

- `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 `collections.abc.ByteString` was subclassed or used as the second argument to `isinstance()` or `issubclass()`, but warnings were not previously emitted if it was merely imported or accessed from the `collections.abc` module.

- `hashlib`:

  - In hash function constructors such as `new()` or the direct hash-named constructors such as `md5()` and `sha256()`, the optional initial data parameter could also be passed as a keyword argument named `data=` or `string=` in various `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.)

- `re`:

  - `re.match()` and `re.Pattern.match()` are now soft deprecated in favor of the new `re.prefixmatch()` and `re.Pattern.prefixmatch()` APIs, which have been added as alternate, more explicit names. 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.

    We do not plan to remove the older `match()` name, as it has been used in code for over 30 years. Code supporting older versions of Python should continue to use `match()`, while new code should prefer `prefixmatch()`. See prefixmatch() vs. match().

    (Contributed by Gregory P. Smith in gh-86519 and Hugo van Kemenade in gh-148100.)

- `struct`:

  - Calling the `Struct.__new__()` without required argument now is deprecated and will be removed in Python 3.20. Calling `__init__()` method on initialized `Struct` objects is deprecated and will be removed in Python 3.20.

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

- `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 `typing.ByteString` was subclassed or used as the second argument to `isinstance()` or `issubclass()`, but warnings were not previously emitted if it was merely imported or accessed from the `typing` module.

- `__version__`

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

    - `argparse`

    - `csv`

    - `ctypes`

    - `ctypes.macholib`

    - `decimal` (use `decimal.SPEC_VERSION` instead)

    - `http.server`

    - `imaplib`

    - `ipaddress`

    - `json`

    - `logging` (`__date__` also deprecated)

    - `optparse`

    - `pickle`

    - `platform`

    - `re`

    - `socketserver`

    - `tabnanny`

    - `tarfile`

    - `tkinter.font`

    - `tkinter.ttk`

    - `wsgiref.simple_server`

    - `xml.etree.ElementTree`

    - `xml.sax.expatreader`

    - `xml.sax.handler`

    - `zlib`

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

### Pending removal in Python 3.16

- The import system:

  - Setting `__loader__` on a module while failing to set `__spec__.loader` is deprecated. In Python 3.16, `__loader__` will cease to be set or taken into consideration by the import system or the standard library.

- `array`:

  - The `'u'` format code (`wchar_t`) has been deprecated in documentation since Python 3.3 and at runtime since Python 3.13. Use the `'w'` format code (`Py_UCS4`) for Unicode characters instead.

- `asyncio`:

  - `asyncio.iscoroutinefunction()` is deprecated and will be removed in Python 3.16; use `inspect.iscoroutinefunction()` instead. (Contributed by Jiahao Li and Kumar Aditya in gh-122875.)

  - `asyncio` policy system is deprecated and will be removed in Python 3.16. In particular, the following classes and functions are deprecated:

    - `asyncio.AbstractEventLoopPolicy`

    - `asyncio.DefaultEventLoopPolicy`

    - `asyncio.WindowsSelectorEventLoopPolicy`

    - `asyncio.WindowsProactorEventLoopPolicy`

    - `asyncio.get_event_loop_policy()`

    - `asyncio.set_event_loop_policy()`

    Users should use `asyncio.run()` or `asyncio.Runner` with loop_factory to use the desired event loop implementation.

    For example, to use `asyncio.SelectorEventLoop` on Windows:

        import asyncio

        async def main():
            ...

        asyncio.run(main(), loop_factory=asyncio.SelectorEventLoop)

    (Contributed by Kumar Aditya in gh-127949.)

- `builtins`:

  - Bitwise inversion on boolean types, `~True` or `~False` has been deprecated since Python 3.12, as it produces surprising and unintuitive results (`-2` and `-1`). Use `not x` instead for the logical negation of a Boolean. In the rare case that you need the bitwise inversion of the underlying integer, convert to `int` explicitly (`~int(x)`).

- `functools`:

  - Calling the Python implementation of `functools.reduce()` with function or sequence as keyword arguments has been deprecated since Python 3.14.

- `logging`:

  - Support for custom logging handlers with the strm argument is deprecated and scheduled for removal in Python 3.16. Define handlers with the stream argument instead. (Contributed by Mariusz Felisiak in gh-115032.)

- `mimetypes`:

  - Valid extensions start with a ‘.’ or are empty for `mimetypes.MimeTypes.add_type()`. Undotted extensions are deprecated and will raise a `ValueError` in Python 3.16. (Contributed by Hugo van Kemenade in gh-75223.)

- `shutil`:

  - The `ExecError` exception has been deprecated since Python 3.14. It has not been used by any function in `shutil` since Python 3.4, and is now an alias of `RuntimeError`.

- `symtable`:

  - The `Class.get_methods` method has been deprecated since Python 3.14.

- `sys`:

  - The `_enablelegacywindowsfsencoding()` function has been deprecated since Python 3.13. Use the `PYTHONLEGACYWINDOWSFSENCODING` environment variable instead.

- `sysconfig`:

  - The `sysconfig.expand_makefile_vars()` function has been deprecated since Python 3.14. Use the `vars` argument of `sysconfig.get_paths()` instead.

- `tarfile`:

  - The undocumented and unused `TarFile.tarfile` attribute has been deprecated since Python 3.13.

### Pending removal in Python 3.17

- `datetime`:

  - `strptime()` calls using a format string containing `%e` (day of month) without a year. This has been deprecated since Python 3.15. (Contributed by Stan Ulbrych in gh-70647.)

- `collections.abc`:

  - `collections.abc.ByteString` is scheduled for removal in Python 3.17.

    Use `isinstance(obj, collections.abc.Buffer)` to test if `obj` implements the buffer protocol at runtime. For use in type annotations, either use `Buffer` or a union that explicitly specifies the types your code supports (e.g., `bytes | bytearray | memoryview`).

    `ByteString` was originally intended to be an abstract class that would serve as a supertype of both `bytes` and `bytearray`. However, since the ABC never had any methods, knowing that an object was an instance of `ByteString` never actually told you anything useful about the object. Other common buffer types such as `memoryview` were also never understood as subtypes of `ByteString` (either at runtime or by static type checkers).

    See PEP 688 for more details. (Contributed by Shantanu Jain in gh-91896.)

- `encodings`:

  - Passing non-ascii encoding names to `encodings.normalize_encoding()` is deprecated and scheduled for removal in Python 3.17. (Contributed by Stan Ulbrych in gh-136702.)

- `typing`:

  - Before Python 3.14, old-style unions were implemented using the private class `typing._UnionGenericAlias`. This class is no longer needed for the implementation, but it has been retained for backward compatibility, with removal scheduled for Python 3.17. Users should use documented introspection helpers like `typing.get_origin()` and `typing.get_args()` instead of relying on private implementation details.

  - `typing.ByteString`, deprecated since Python 3.9, is scheduled for removal in Python 3.17.

    Use `isinstance(obj, collections.abc.Buffer)` to test if `obj` implements the buffer protocol at runtime. For use in type annotations, either use `Buffer` or a union that explicitly specifies the types your code supports (e.g., `bytes | bytearray | memoryview`).

    `ByteString` was originally intended to be an abstract class that would serve as a supertype of both `bytes` and `bytearray`. However, since the ABC never had any methods, knowing that an object was an instance of `ByteString` never actually told you anything useful about the object. Other common buffer types such as `memoryview` were also never understood as subtypes of `ByteString` (either at runtime or by static type checkers).

    See PEP 688 for more details. (Contributed by Shantanu Jain in gh-91896.)

### Pending removal in Python 3.18

- No longer accept a boolean value when a file descriptor is expected. (Contributed by Serhiy Storchaka in gh-82626.)

- `decimal`:

  - The non-standard and undocumented `Decimal` format specifier `'N'`, which is only supported in the `decimal` module’s C implementation, has been deprecated since Python 3.13. (Contributed by Serhiy Storchaka in gh-89902.)

### Pending removal in Python 3.19

- `ctypes`:

  - Implicitly switching to the MSVC-compatible struct layout by setting `_pack_` but not `_layout_` on non-Windows platforms.

- `hashlib`:

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

    Support for the `string` keyword argument name is now deprecated and slated for removal in Python 3.19.

    Before Python 3.13, the `string` keyword parameter was not correctly supported depending on the backend implementation of hash functions. Prefer passing the initial data as a positional argument for maximum backwards compatibility.

### Pending removal in Python 3.20

- Calling the `__new__()` method of `struct.Struct` without the format argument is deprecated and will be removed in Python 3.20. Calling `__init__()` method on initialized `Struct` objects is deprecated and will be removed in Python 3.20.

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

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

  - `argparse`

  - `csv`

  - `ctypes`

  - `ctypes.macholib`

  - `decimal` (use `decimal.SPEC_VERSION` instead)

  - `http.server`

  - `imaplib`

  - `ipaddress`

  - `json`

  - `logging` (`__date__` also deprecated)

  - `optparse`

  - `pickle`

  - `platform`

  - `re`

  - `socketserver`

  - `tabnanny`

  - `tarfile`

  - `tkinter.font`

  - `tkinter.ttk`

  - `wsgiref.simple_server`

  - `xml.etree.ElementTree`

  - `xml.sax.expatreader`

  - `xml.sax.handler`

  - `zlib`

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

### Pending removal in future versions

The following APIs will be removed in the future, although there is currently no date scheduled for their removal.

- `argparse`:

  - Nesting argument groups and nesting mutually exclusive groups are deprecated.

  - Passing the undocumented keyword argument prefix_chars to `add_argument_group()` is now deprecated.

  - The `argparse.FileType` type converter is deprecated.

- `builtins`:

  - Generators: `throw(type, exc, tb)` and `athrow(type, exc, tb)` signature is deprecated: use `throw(exc)` and `athrow(exc)` instead, the single argument signature.

  - Currently Python accepts numeric literals immediately followed by keywords, for example `0in x`, `1or x`, `0if 1else 2`. It allows confusing and ambiguous expressions like `[0x1for x in y]` (which can be interpreted as `[0x1 for x in y]` or `[0x1f or x in y]`). A syntax warning is raised if the numeric literal is immediately followed by one of keywords `and`, `else`, `for`, `if`, `in`, `is` and `or`. In a future release it will be changed to a syntax error. (gh-87999)

  - Support for `__index__()` and `__int__()` method returning non-int type: these methods will be required to return an instance of a strict subclass of `int`.

  - Support for `__float__()` method returning a strict subclass of `float`: these methods will be required to return an instance of `float`.

  - Support for `__complex__()` method returning a strict subclass of `complex`: these methods will be required to return an instance of `complex`.

  - Passing a complex number as the real or imag argument in the `complex()` constructor is now deprecated; it should only be passed as a single positional argument. (Contributed by Serhiy Storchaka in gh-109218.)

- `calendar`: `calendar.January` and `calendar.February` constants are deprecated and replaced by `calendar.JANUARY` and `calendar.FEBRUARY`. (Contributed by Prince Roshan in gh-103636.)

- `codecs`: use `open()` instead of `codecs.open()`. (gh-133038)

- `codeobject.co_lnotab`: use the `codeobject.co_lines()` method instead.

- `datetime`:

  - `utcnow()`: use `datetime.datetime.now(tz=datetime.UTC)`.

  - `utcfromtimestamp()`: use `datetime.datetime.fromtimestamp(timestamp, tz=datetime.UTC)`.

- `gettext`: Plural value must be an integer.

- `importlib`:

  - `cache_from_source()` debug_override parameter is deprecated: use the optimization parameter instead.

- `importlib.metadata`:

  - `EntryPoints` tuple interface.

  - Implicit `None` on return values.

- `logging`: the `warn()` method has been deprecated since Python 3.3, use `warning()` instead.

- `mailbox`: Use of StringIO input and text mode is deprecated, use BytesIO and binary mode instead.

- `os`: Calling `os.register_at_fork()` in a multi-threaded process.

- `os.path`: `os.path.commonprefix()` is deprecated, use `os.path.commonpath()` for path prefixes. The `os.path.commonprefix()` function is being deprecated due to having a misleading name and module. The function is not safe to use for path prefixes despite being included in a module about path manipulation, meaning it is easy to accidentally introduce path traversal vulnerabilities into Python programs by using this function.

- `pydoc.ErrorDuringImport`: A tuple value for exc_info parameter is deprecated, use an exception instance.

- `re`: More strict rules are now applied for numerical group references and group names in regular expressions. Only sequence of ASCII digits is now accepted as a numerical reference. The group name in bytes patterns and replacement strings can now only contain ASCII letters and digits and underscore. (Contributed by Serhiy Storchaka in gh-91760.)

- `shutil`: `rmtree()`’s onerror parameter is deprecated in Python 3.12; use the onexc parameter instead.

- `ssl` options and protocols:

  - `ssl.SSLContext` without protocol argument is deprecated.

  - `ssl.SSLContext`: `set_npn_protocols()` and `selected_npn_protocol()` are deprecated: use ALPN instead.

  - `ssl.OP_NO_SSL*` options

  - `ssl.OP_NO_TLS*` options

  - `ssl.PROTOCOL_SSLv3`

  - `ssl.PROTOCOL_TLS`

  - `ssl.PROTOCOL_TLSv1`

  - `ssl.PROTOCOL_TLSv1_1`

  - `ssl.PROTOCOL_TLSv1_2`

  - `ssl.TLSVersion.SSLv3`

  - `ssl.TLSVersion.TLSv1`

  - `ssl.TLSVersion.TLSv1_1`

- `threading` methods:

  - `threading.Condition.notifyAll()`: use `notify_all()`.

  - `threading.Event.isSet()`: use `is_set()`.

  - `threading.Thread.isDaemon()`, `threading.Thread.setDaemon()`: use `threading.Thread.daemon` attribute.

  - `threading.Thread.getName()`, `threading.Thread.setName()`: use `threading.Thread.name` attribute.

  - `threading.currentThread()`: use `threading.current_thread()`.

  - `threading.activeCount()`: use `threading.active_count()`.

- `typing.Text` (gh-92332).

- The internal class `typing._UnionGenericAlias` is no longer used to implement `typing.Union`. To preserve compatibility with users using this private class, a compatibility shim will be provided until at least Python 3.17. (Contributed by Jelle Zijlstra in gh-105499.)

- `unittest.IsolatedAsyncioTestCase`: it is deprecated to return a value that is not `None` from a test case.

- `urllib.parse` deprecated functions: `urlparse()` instead

  - `splitattr()`

  - `splithost()`

  - `splitnport()`

  - `splitpasswd()`

  - `splitport()`

  - `splitquery()`

  - `splittag()`

  - `splittype()`

  - `splituser()`

  - `splitvalue()`

  - `to_bytes()`

- `wsgiref`: `SimpleHandler.stdout.write()` should not do partial writes.

- `xml.etree.ElementTree`: Testing the truth value of an `Element` is deprecated. In a future release it will always return `True`. Prefer explicit `len(elem)` or `elem is not None` tests instead.

- `sys._clear_type_cache()` is deprecated: use `sys._clear_internal_caches()` instead.

### Soft deprecations

There are no plans to remove soft deprecated APIs.

- `re.match()` and `re.Pattern.match()` are now soft deprecated in favor of the new `re.prefixmatch()` and `re.Pattern.prefixmatch()` APIs, which have been added as alternate, more explicit names. 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.

  We do not plan to remove the older `match()` name, as it has been used in code for over 30 years. Code supporting older versions of Python should continue to use `match()`, while new code should prefer `prefixmatch()`. See prefixmatch() vs. match().

  (Contributed by Gregory P. Smith in gh-86519 and Hugo van Kemenade in gh-148100.)

## C API changes

### New features

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

- Add the following functions for the new `frozendict` type:

  - `PyAnyDict_Check()`

  - `PyAnyDict_CheckExact()`

  - `PyFrozenDict_Check()`

  - `PyFrozenDict_CheckExact()`

  - `PyFrozenDict_New()`

  (Contributed by Victor Stinner in gh-141510.)

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

- Add `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: `Py_mod_abi`, `PyABIInfo_Check()`, `PyABIInfo_VAR` and `Py_mod_abi`. (Contributed by Petr Viktorin in gh-137210.)

- Implement PEP 782, the PyBytesWriter API. Add functions:

  - `PyBytesWriter_Create()`

  - `PyBytesWriter_Discard()`

  - `PyBytesWriter_FinishWithPointer()`

  - `PyBytesWriter_FinishWithSize()`

  - `PyBytesWriter_Finish()`

  - `PyBytesWriter_Format()`

  - `PyBytesWriter_GetData()`

  - `PyBytesWriter_GetSize()`

  - `PyBytesWriter_GrowAndUpdatePointer()`

  - `PyBytesWriter_Grow()`

  - `PyBytesWriter_Resize()`

  - `PyBytesWriter_WriteBytes()`

  (Contributed by Victor Stinner in gh-129813.)

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

- Add `PyTuple_FromArray()` to create a `tuple` from an array. (Contributed by Victor Stinner in gh-111489.)

- Add functions that are guaranteed to be safe for use in `tp_traverse` handlers: `PyObject_GetTypeData_DuringGC()`, `PyObject_GetItemData_DuringGC()`, `PyType_GetModuleState_DuringGC()`, `PyModule_GetState_DuringGC()`, `PyModule_GetToken_DuringGC()`, `PyType_GetBaseByToken_DuringGC()`, `PyType_GetModule_DuringGC()`, `PyType_GetModuleByToken_DuringGC()`. (Contributed by Petr Viktorin in gh-145925.)

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

- Add `PyUnstable_ThreadState_SetStackProtection()` and `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 `PyUnstable_SetImmortal()` C-API function to mark objects as 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 `Py_TPFLAGS_MANAGED_DICT` or `Py_TPFLAGS_MANAGED_WEAKREF` flag is set then `Py_TPFLAGS_HAVE_GC` must be set too. (Contributed by Sergey Miryanov in gh-134786.)

- `PyDateTime_IMPORT` is now thread safe. Code that directly checks `PyDateTimeAPI` for `NULL` should be updated to call `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:

  - `PyUnicode_AsDecodedObject()`: Use `PyCodec_Decode()` instead.

  - `PyUnicode_AsDecodedUnicode()`: Use `PyCodec_Decode()` instead; Note that some codecs (for example, “base64”) may return a type other than `str`, such as `bytes`.

  - `PyUnicode_AsEncodedObject()`: Use `PyCodec_Encode()` instead.

  - `PyUnicode_AsEncodedUnicode()`: Use `PyCodec_Encode()` instead; Note that some codecs (for example, “base64”) may return a type other than `bytes`, such as `str`.

  (Contributed by Stan Ulbrych in gh-133612.)

- `PyImport_ImportModuleNoBlock()`: deprecated alias of `PyImport_ImportModule()`. (Contributed by Bénédikt Tran in gh-133644.)

- `PyWeakref_GetObject()` and `PyWeakref_GET_OBJECT`: use `PyWeakref_GetRef()` instead. The pythoncapi-compat project can be used to get `PyWeakref_GetRef()` on Python 3.12 and older. (Contributed by Bénédikt Tran in gh-133644.)

- Remove deprecated `PySys_ResetWarnOptions()`. Clear `sys.warnoptions` and `warnings.filters` instead.

  (Contributed by Nikita Sobolev in gh-138886.)

The following functions are removed in favor of `PyConfig_Get()`. The pythoncapi-compat project can be used to get `PyConfig_Get()` on Python 3.13 and older.

- Python initialization functions:

  - `Py_GetExecPrefix()`: use `PyConfig_Get("base_exec_prefix")` (`sys.base_exec_prefix`) instead. Use `PyConfig_Get("exec_prefix")` (`sys.exec_prefix`) if virtual environments need to be handled.

  - `Py_GetPath()`: use `PyConfig_Get("module_search_paths")` (`sys.path`) instead.

  - `Py_GetPrefix()`: use `PyConfig_Get("base_prefix")` (`sys.base_prefix`) instead. Use `PyConfig_Get("prefix")` (`sys.prefix`) if virtual environments need to be handled.

  - `Py_GetProgramFullPath()`: use `PyConfig_Get("executable")` (`sys.executable`) instead.

  - `Py_GetProgramName()`: use `PyConfig_Get("executable")` (`sys.executable`) instead.

  - `Py_GetPythonHome()`: use `PyConfig_Get("home")` or the `PYTHONHOME` environment variable instead.

  (Contributed by Bénédikt Tran in gh-133644.)

### 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 `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 `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.)

- `PyBytes_FromStringAndSize(NULL, len)` and `_PyBytes_Resize()` are soft deprecated, use the `PyBytesWriter` API instead. (Contributed by Victor Stinner in gh-129813.)

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

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

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

- `bytes_warning` is deprecated since 3.15 and will be removed in 3.17. (Contributed by Nikita Sobolev in gh-136355.)

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

- The following macros are soft deprecated:

  - `Py_ALIGNED`: Prefer `alignas` instead.

  - `PY_FORMAT_SIZE_T`: Use `"z"` directly.

  - `Py_LL` and `Py_ULL`: Use standard suffixes, `LL` and `ULL`.

  - `PY_LONG_LONG`, `PY_LLONG_MIN`, `PY_LLONG_MAX`, `PY_ULLONG_MAX`, `PY_INT32_T`, `PY_UINT32_T`, `PY_INT64_T`, `PY_UINT64_T`, `PY_SIZE_MAX`: Use C99 types/limits.

  - `Py_UNICODE_SIZE`: Use `sizeof(wchar_t)` directly.

  - `Py_VA_COPY`: Use `va_copy` directly.

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

  (Contributed by Petr Viktorin in gh-146175.)

- `Py_MATH_El` and `Py_MATH_PIl` are deprecated since 3.15 and will be removed in 3.20. (Contributed by Sergey B Kirpichev in gh-141004.)

## Build changes

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

- The new configure option `--with-missing-stdlib-config=FILE` allows distributors to pass a JSON configuration file containing custom error messages for standard library modules that are missing or packaged separately. (Contributed by Stan Ulbrych and Petr Viktorin in gh-139707.)

- The new configure option `--with-pymalloc-hugepages` enables huge page support for 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 `PYTHON_PYMALLOC_HUGEPAGES` environment variable to `1`.

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

- 64-bit builds using Visual Studio 2026 (MSVC 18) may now use the new tail-calling interpreter. Results on Visual Studio 18.1.1 report between 15-20% 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.)

## Porting to Python 3.15

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

- `sqlite3.Connection` APIs have been cleaned up.

  - All parameters of `sqlite3.connect()` except database are now keyword-only.

  - The first three parameters of methods `create_function()` and `create_aggregate()` are now positional-only.

  - The first parameter of methods `set_authorizer()`, `set_progress_handler()` and `set_trace_callback()` is now positional-only.

  (Contributed by Serhiy Storchaka in gh-133595.)

- `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 `resource.setrlimit()` and `resource.prlimit()` is now deprecated. (Contributed by Serhiy Storchaka in gh-137044.)

- `mmap.mmap.resize()` has been removed on platforms that don’t support the underlying syscall, instead of raising a `SystemError`.

- A resource warning is now emitted for an unclosed `xml.etree.ElementTree.iterparse()` iterator if it opened a file. Use its `close()` method or the `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 `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 `base64.urlsafe_b64decode()`. Pass a new argument `padded=True` or use `base64.b64decode()` with argument `altchars=b'-_'` (this works with older Python versions) to make padding required. (Contributed by Serhiy Storchaka in gh-73613.)