What's New In Python 3.11
*************************

Release:
   3.11.0a7

Date:
   April 25, 2022

This article explains the new features in Python 3.11, compared to
3.10.

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 3.11 moves
  towards release, so it's worth checking back even after reading
  earlier versions.


Summary -- Release highlights
=============================

* Python 3.11 is up to 10-60% faster than Python 3.10. On average, we
  measured a 1.22x speedup on the standard benchmark suite. See Faster
  CPython for details.

New syntax features:

* **PEP 654**: Exception Groups and "except*". (Contributed by Irit
  Katriel in bpo-45292.)

New typing features:

* **PEP 646**: Variadic generics.

* **PEP 655**: Marking individual TypedDict items as required or
  potentially-missing.

* **PEP 673**: "Self" type.

* **PEP 675**: Arbitrary literal string type.


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


Enhanced error locations in tracebacks
--------------------------------------

When printing tracebacks, the interpreter will now point to the exact
expression that caused the error instead of just the line. For
example:

   Traceback (most recent call last):
     File "distance.py", line 11, in <module>
       print(manhattan_distance(p1, p2))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "distance.py", line 6, in manhattan_distance
       return abs(point_1.x - point_2.x) + abs(point_1.y - point_2.y)
                              ^^^^^^^^^
   AttributeError: 'NoneType' object has no attribute 'x'

Previous versions of the interpreter would point to just the line
making it ambiguous which object was "None". These enhanced errors can
also be helpful when dealing with deeply nested dictionary objects and
multiple function calls,

   Traceback (most recent call last):
     File "query.py", line 37, in <module>
       magic_arithmetic('foo')
       ^^^^^^^^^^^^^^^^^^^^^^^
     File "query.py", line 18, in magic_arithmetic
       return add_counts(x) / 25
              ^^^^^^^^^^^^^
     File "query.py", line 24, in add_counts
       return 25 + query_user(user1) + query_user(user2)
                   ^^^^^^^^^^^^^^^^^
     File "query.py", line 32, in query_user
       return 1 + query_count(db, response['a']['b']['c']['user'], retry=True)
                                  ~~~~~~~~~~~~~~~~~~^^^^^
   TypeError: 'NoneType' object is not subscriptable

as well as complex arithmetic expressions:

   Traceback (most recent call last):
     File "calculation.py", line 54, in <module>
       result = (x / y / z) * (a / b / c)
                 ~~~~~~^~~
   ZeroDivisionError: division by zero

See **PEP 657** for more details. (Contributed by Pablo Galindo,
Batuhan Taskaya and Ammar Askar in bpo-43950.)

Note:

  This feature requires storing column positions in code objects which
  may result in a small increase of disk usage of compiled Python
  files or interpreter memory usage. To avoid storing the extra
  information and/or deactivate printing the extra traceback
  information, the "-X" "no_debug_ranges" command line flag or the
  "PYTHONNODEBUGRANGES" environment variable can be used.


Column information for code objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The information used by the enhanced traceback feature is made
available as a general API that can be used to correlate bytecode
instructions with source code. This information can be retrieved
using:

* The "codeobject.co_positions()" method in Python.

* The "PyCode_Addr2Location()" function in the C-API.

The "-X" "no_debug_ranges" option and the environment variable
"PYTHONNODEBUGRANGES" can be used to disable this feature.

See **PEP 657** for more details. (Contributed by Pablo Galindo,
Batuhan Taskaya and Ammar Askar in bpo-43950.)


Exceptions can be enriched with notes (PEP 678)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The "add_note()" method was added to "BaseException". It can be used
to enrich exceptions with context information which is not available
at the time when the exception is raised. The notes added appear in
the default traceback. See **PEP 678** for more details. (Contributed
by Irit Katriel in bpo-45607.)


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

This section covers major changes affecting **PEP 484** type hints and
the "typing" module.


PEP 646: Variadic generics
--------------------------

**PEP 484** introduced "TypeVar", enabling creation of generics
parameterised with a single type. **PEP 646** introduces
"TypeVarTuple", enabling parameterisation with an *arbitrary* number
of types. In other words, a "TypeVarTuple" is a *variadic* type
variable, enabling *variadic* generics. This enables a wide variety of
use cases. In particular, it allows the type of array-like structures
in numerical computing libraries such as NumPy and TensorFlow to be
parameterised with the array *shape*. Static type checkers will now be
able to catch shape-related bugs in code that uses these libraries.

See **PEP 646** for more details.

(Contributed by Matthew Rahtz in bpo-43224, with contributions by
Serhiy Storchaka and Jelle Zijlstra. PEP written by Mark Mendoza,
Matthew Rahtz, Pradeep Kumar Srinivasan, and Vincent Siles.)


PEP 655: Marking individual "TypedDict" items as required or not-required
-------------------------------------------------------------------------

"Required" and "NotRequired" provide a straightforward way to mark
whether individual items in a "TypedDict" must be present. Previously
this was only possible using inheritance.

Fields are still required by default, unless the "total=False"
parameter is set. For example, the following specifies a dictionary
with one required and one not-required key:

   class Movie(TypedDict):
      title: str
      year: NotRequired[int]

   m1: Movie = {"title": "Black Panther", "year": 2018}  # ok
   m2: Movie = {"title": "Star Wars"}  # ok (year is not required)
   m3: Movie = {"year": 2022}  # error (missing required field title)

The following definition is equivalent:

   class Movie(TypedDict, total=False):
      title: Required[str]
      year: int

See **PEP 655** for more details.

(Contributed by David Foster and Jelle Zijlstra in bpo-47087. PEP
written by David Foster.)


PEP 673: "Self" type
--------------------

The new "Self" annotation provides a simple and intuitive way to
annotate methods that return an instance of their class. This behaves
the same as the "TypeVar"-based approach specified in **PEP 484** but
is more concise and easier to follow.

Common use cases include alternative constructors provided as
classmethods and "__enter__()" methods that return "self":

   class MyLock:
       def __enter__(self) -> Self:
           self.lock()
           return self

       ...

   class MyInt:
       @classmethod
       def fromhex(cls, s: str) -> Self:
           return cls(int(s, 16))

       ...

"Self" can also be used to annotate method parameters or attributes of
the same type as their enclosing class.

See **PEP 673** for more details.

(Contributed by James Hilton-Balfe in bpo-46534. PEP written by
Pradeep Kumar Srinivasan and James Hilton-Balfe.)


PEP 675: Arbitrary literal string type
--------------------------------------

The new "LiteralString" annotation may be used to indicate that a
function parameter can be of any literal string type. This allows a
function to accept arbitrary literal string types, as well as strings
created from other literal strings. Type checkers can then enforce
that sensitive functions, such as those that execute SQL statements or
shell commands, are called only with static arguments, providing
protection against injection attacks.

For example, a SQL query function could be annotated as follows:

   def run_query(sql: LiteralString) -> ...
       ...

   def caller(
       arbitrary_string: str,
       query_string: LiteralString,
       table_name: LiteralString,
   ) -> None:
       run_query("SELECT * FROM students")       # ok
       run_query(query_string)                   # ok
       run_query("SELECT * FROM " + table_name)  # ok
       run_query(arbitrary_string)               # type checker error
       run_query(                                # type checker error
           f"SELECT * FROM students WHERE name = {arbitrary_string}"
       )

See **PEP 675** for more details.

(Contributed by Jelle Zijlstra in bpo-47088. PEP written by Pradeep
Kumar Srinivasan and Graham Bleaney.)


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

* Starred expressions can be used in for statements. (See bpo-46725
  for more details.)

* Asynchronous comprehensions are now allowed inside comprehensions in
  asynchronous functions. Outer comprehensions implicitly become
  asynchronous. (Contributed by Serhiy Storchaka in bpo-33346.)

* A "TypeError" is now raised instead of an "AttributeError" in
  "contextlib.ExitStack.enter_context()" and
  "contextlib.AsyncExitStack.enter_async_context()" for objects which
  do not support the *context manager* or *asynchronous context
  manager* protocols correspondingly. (Contributed by Serhiy Storchaka
  in bpo-44471.)

* A "TypeError" is now raised instead of an "AttributeError" in "with"
  and "async with" statements for objects which do not support the
  *context manager* or *asynchronous context manager* protocols
  correspondingly. (Contributed by Serhiy Storchaka in bpo-12022.)

* Added "object.__getstate__()" which provides the default
  implementation of the "__getstate__()" method.  "Copying" and
  "pickling" instances of subclasses of builtin types "bytearray",
  "set", "frozenset", "collections.OrderedDict", "collections.deque",
  "weakref.WeakSet", and "datetime.tzinfo" now copies and pickles
  instance attributes implemented as *slots*. (Contributed by Serhiy
  Storchaka in bpo-26579.)


Other CPython Implementation Changes
====================================

* Special methods "complex.__complex__()" and "bytes.__bytes__()" are
  implemented to support "typing.SupportsComplex" and
  "typing.SupportsBytes" protocols. (Contributed by Mark Dickinson and
  Dong-hee Na in bpo-24234.)

* "siphash13" is added as a new internal hashing algorithms. It has
  similar security properties as "siphash24" but it is slightly faster
  for long inputs. "str", "bytes", and some other types now use it as
  default algorithm for "hash()". **PEP 552** hash-based pyc files now
  use "siphash13", too. (Contributed by Inada Naoki in bpo-29410.)

* When an active exception is re-raised by a "raise" statement with no
  parameters, the traceback attached to this exception is now always
  "sys.exc_info()[1].__traceback__". This means that changes made to
  the traceback in the current "except" clause are reflected in the
  re-raised exception. (Contributed by Irit Katriel in bpo-45711.)

* The interpreter state's representation of handled exceptions (a.k.a
  exc_info, or _PyErr_StackItem) now has only the "exc_value" field,
  "exc_type" and "exc_traceback" have been removed as their values can
  be derived from "exc_value". (Contributed by Irit Katriel in
  bpo-45711.)

* A new command line option for the Windows installer "AppendPath" has
  been added. It behaves similiar to "PrependPath" but appends the
  install and scripts directories instead of prepending them.
  (Contributed by Bastian Neuburger in bpo-44934.)


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

* A new module, "tomllib", was added for parsing TOML. (Contributed by
  Taneli Hukkinen in bpo-40059.)

* "wsgiref.types", containing WSGI-specific types for static type
  checking, was added. (Contributed by Sebastian Rittau in bpo-42012.)


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


asyncio
-------

* Add raw datagram socket functions to the event loop:
  "sock_sendto()", "sock_recvfrom()" and "sock_recvfrom_into()".
  (Contributed by Alex Grönholm in bpo-46805.)

* Add "start_tls()" method for upgrading existing stream-based
  connections to TLS. (Contributed by Ian Good in bpo-34975.)


fractions
---------

* Support **PEP 515**-style initialization of "Fraction" from string.
  (Contributed by Sergey B Kirpichev in bpo-44258.)

* "Fraction" now implements an "__int__" method, so that an
  "isinstance(some_fraction, typing.SupportsInt)" check passes.
  (Contributed by Mark Dickinson in bpo-44547.)


functools
---------

* "functools.singledispatch()" now supports "types.UnionType" and
  "typing.Union" as annotations to the dispatch argument.:

     >>> from functools import singledispatch
     >>> @singledispatch
     ... def fun(arg, verbose=False):
     ...     if verbose:
     ...         print("Let me just say,", end=" ")
     ...     print(arg)
     ...
     >>> @fun.register
     ... def _(arg: int | float, verbose=False):
     ...     if verbose:
     ...         print("Strength in numbers, eh?", end=" ")
     ...     print(arg)
     ...
     >>> from typing import Union
     >>> @fun.register
     ... def _(arg: Union[list, set], verbose=False):
     ...     if verbose:
     ...         print("Enumerate this:")
     ...     for i, elem in enumerate(arg):
     ...         print(i, elem)
     ...

  (Contributed by Yurii Karabas in bpo-46014.)


hashlib
-------

* "hashlib.blake2b()" and "hashlib.blake2s()" now prefer libb2 over
  Python's vendored copy. (Contributed by Christian Heimes in
  bpo-47095.)

* The internal "_sha3" module with SHA3 and SHAKE algorithms now uses
  *tiny_sha3* instead of the *Keccak Code Package* to reduce code and
  binary size. The "hashlib" module prefers optimized SHA3 and SHAKE
  implementations from OpenSSL. The change affects only installations
  without OpenSSL support. (Contributed by Christian Heimes in
  bpo-47098.)


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

* Apply syntax highlighting to *.pyi* files. (Contributed by Alex
  Waygood and Terry Jan Reedy in bpo-45447.)


inspect
-------

* Add "inspect.getmembers_static()": return all members without
  triggering dynamic lookup via the descriptor protocol. (Contributed
  by Weipeng Hong in bpo-30533.)

* Add "inspect.ismethodwrapper()" for checking if the type of an
  object is a "MethodWrapperType". (Contributed by Hakan Çelik in
  bpo-29418.)

* Change the frame-related functions in the "inspect" module to return
  a regular object (that is backwards compatible with the old tuple-
  like interface) that include the extended **PEP 657** position
  information (end line number, column and end column). The affected
  functions are: "inspect.getframeinfo()", "inspect.getouterframes()",
  "inspect.getinnerframes()", "inspect.stack()" and "inspect.trace()".
  (Contributed by Pablo Galindo in gh-88116)


locale
------

* Add "locale.getencoding()" to get the current locale encoding. It is
  similar to "locale.getpreferredencoding(False)" but ignores the
  Python UTF-8 Mode.


math
----

* Add "math.exp2()": return 2 raised to the power of x. (Contributed
  by Gideon Mitchell in bpo-45917.)

* Add "math.cbrt()": return the cube root of x. (Contributed by Ajith
  Ramachandran in bpo-44357.)

* The behaviour of two "math.pow()" corner cases was changed, for
  consistency with the IEEE 754 specification. The operations
  "math.pow(0.0, -math.inf)" and "math.pow(-0.0, -math.inf)" now
  return "inf". Previously they raised "ValueError". (Contributed by
  Mark Dickinson in bpo-44339.)

* The "math.nan" value is now always available. (Contributed by Victor
  Stinner in bpo-46917.)


operator
--------

* A new function "operator.call" has been added, such that
  "operator.call(obj, *args, **kwargs) == obj(*args, **kwargs)".
  (Contributed by Antony Lee in bpo-44019.)


os
--

* On Windows, "os.urandom()" now uses "BCryptGenRandom()", instead of
  "CryptGenRandom()" which is deprecated. (Contributed by Dong-hee Na
  in bpo-44611.)


re
--

* Atomic grouping ("(?>...)") and possessive quantifiers ("*+", "++",
  "?+", "{m,n}+") are now supported in regular expressions.
  (Contributed by Jeffrey C. Jacobs and Serhiy Storchaka in
  bpo-433030.)


shutil
------

* Add optional parameter *dir_fd* in "shutil.rmtree()". (Contributed
  by Serhiy Storchaka in bpo-46245.)


socket
------

* Add CAN Socket support for NetBSD. (Contributed by Thomas Klausner
  in bpo-30512.)

* "create_connection()" has an option to raise, in case of failure to
  connect, an "ExceptionGroup" containing all errors instead of only
  raising the last error. (Contributed by Irit Katriel in bpo-29980).


sqlite3
-------

* You can now disable the authorizer by passing "None" to
  "set_authorizer()". (Contributed by Erlend E. Aasland in bpo-44491.)

* Collation name "create_collation()" can now contain any Unicode
  character.  Collation names with invalid characters now raise
  "UnicodeEncodeError" instead of "sqlite3.ProgrammingError".
  (Contributed by Erlend E. Aasland in bpo-44688.)

* "sqlite3" exceptions now include the SQLite extended error code as
  "sqlite_errorcode" and the SQLite error name as "sqlite_errorname".
  (Contributed by Aviv Palivoda, Daniel Shahaf, and Erlend E. Aasland
  in bpo-16379 and bpo-24139.)

* Add "setlimit()" and "getlimit()" to "sqlite3.Connection" for
  setting and getting SQLite limits by connection basis. (Contributed
  by Erlend E. Aasland in bpo-45243.)

* "sqlite3" now sets "sqlite3.threadsafety" based on the default
  threading mode the underlying SQLite library has been compiled with.
  (Contributed by Erlend E. Aasland in bpo-45613.)

* "sqlite3" C callbacks now use unraisable exceptions if callback
  tracebacks are enabled. Users can now register an "unraisable hook
  handler" to improve their debug experience. (Contributed by Erlend
  E. Aasland in bpo-45828.)

* Fetch across rollback no longer raises "InterfaceError". Instead we
  leave it to the SQLite library to handle these cases. (Contributed
  by Erlend E. Aasland in bpo-44092.)

* Add "serialize()" and "deserialize()" to "sqlite3.Connection" for
  serializing and deserializing databases. (Contributed by Erlend E.
  Aasland in bpo-41930.)

* Add "create_window_function()" to "sqlite3.Connection" for creating
  aggregate window functions. (Contributed by Erlend E. Aasland in
  bpo-34916.)

* Add "blobopen()" to "sqlite3.Connection". "sqlite3.Blob" allows
  incremental I/O operations on blobs. (Contributed by Aviv Palivoda
  and Erlend E. Aasland in bpo-24905)


sys
---

* "sys.exc_info()" now derives the "type" and "traceback" fields from
  the "value" (the exception instance), so when an exception is
  modified while it is being handled, the changes are reflected in the
  results of subsequent calls to "exc_info()". (Contributed by Irit
  Katriel in bpo-45711.)

* Add "sys.exception()" which returns the active exception instance
  (equivalent to "sys.exc_info()[1]"). (Contributed by Irit Katriel in
  bpo-46328.)


sysconfig
---------

* Two new installation schemes (*posix_venv*, *nt_venv* and *venv*)
  were added and are used when Python creates new virtual environments
  or when it is running from a virtual environment. The first two
  schemes (*posix_venv* and *nt_venv*) are OS-specific for non-Windows
  and Windows, the *venv* is essentially an alias to one of them
  according to the OS Python runs on. This is useful for downstream
  distributors who modify "sysconfig.get_preferred_scheme()". Third
  party code that creates new virtual environments should use the new
  *venv* installation scheme to determine the paths, as does "venv".
  (Contributed by Miro Hrončok in bpo-45413.)


threading
---------

* On Unix, if the "sem_clockwait()" function is available in the C
  library (glibc 2.30 and newer), the "threading.Lock.acquire()"
  method now uses the monotonic clock ("time.CLOCK_MONOTONIC") for the
  timeout, rather than using the system clock ("time.CLOCK_REALTIME"),
  to not be affected by system clock changes. (Contributed by Victor
  Stinner in bpo-41710.)


time
----

* On Unix, "time.sleep()" now uses the "clock_nanosleep()" or
  "nanosleep()" function, if available, which has a resolution of 1
  nanosecond (10^-9 seconds), rather than using "select()" which has a
  resolution of 1 microsecond (10^-6 seconds). (Contributed by
  Benjamin Szőke and Victor Stinner in bpo-21302.)

* On Windows 8.1 and newer, "time.sleep()" now uses a waitable timer
  based on high-resolution timers which has a resolution of 100
  nanoseconds (10^-7 seconds). Previously, it had a resolution of 1
  millisecond (10^-3 seconds). (Contributed by Benjamin Szőke, Dong-
  hee Na, Eryk Sun and Victor Stinner in bpo-21302 and bpo-45429.)


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

* The Unicode database has been updated to version 14.0.0.
  (bpo-45190).


venv
----

* When new Python virtual environments are created, the *venv*
  sysconfig installation scheme is used to determine the paths inside
  the environment. When Python runs in a virtual environment, the same
  installation scheme is the default. That means that downstream
  distributors can change the default sysconfig install scheme without
  changing behavior of virtual environments. Third party code that
  also creates new virtual environments should do the same.
  (Contributed by Miro Hrončok in bpo-45413.)


warnings
--------

* "warnings.catch_warnings()" now accepts arguments for
  "warnings.simplefilter()", providing a more concise way to locally
  ignore warnings or convert them to errors. (Contributed by Zac
  Hatfield-Dodds in bpo-47074.)


zipfile
-------

* Added support for specifying member name encoding for reading
  metadata in the zipfile's directory and file headers. (Contributed
  by Stephen J. Turnbull and Serhiy Storchaka in bpo-28080.)


fcntl
-----

* On FreeBSD, the "F_DUP2FD" and "F_DUP2FD_CLOEXEC" flags respectively
  are supported, the former equals to "dup2" usage while the latter
  set the "FD_CLOEXEC" flag in addition.


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

* Compiler now optimizes simple C-style formatting with literal format
  containing only format codes "%s", "%r" and "%a" and makes it as
  fast as corresponding f-string expression. (Contributed by Serhiy
  Storchaka in bpo-28307.)

* "Zero-cost" exceptions are implemented. The cost of "try" statements
  is almost eliminated when no exception is raised. (Contributed by
  Mark Shannon in bpo-40222.)

* Pure ASCII strings are now normalized in constant time by
  "unicodedata.normalize()". (Contributed by Dong-hee Na in
  bpo-44987.)

* "math" functions "comb()" and "perm()" are now up to 10 times or
  more faster for large arguments (the speed up is larger for larger
  *k*). (Contributed by Serhiy Storchaka in bpo-37295.)

* Dict don't store hash value when all inserted keys are Unicode
  objects. This reduces dict size. For example,
  "sys.getsizeof(dict.fromkeys("abcdefg"))" becomes 272 bytes from 352
  bytes on 64bit platform. (Contributed by Inada Naoki in bpo-46845.)

* "re"'s regular expression matching engine has been partially
  refactored, and now uses computed gotos (or "threaded code") on
  supported platforms. As a result, Python 3.11 executes the
  pyperformance regular expression benchmarks up to 10% faster than
  Python 3.10.


Faster CPython
==============

CPython 3.11 is on average 1.22x faster than CPython 3.10 when
measured with the pyperformance benchmark suite, and compiled with GCC
on Ubuntu Linux. Depending on your workload, the speedup could be up
to 10-60% faster.

This project focuses on two major areas in Python: faster startup and
faster runtime. Other optimizations not under this project are listed
in Optimizations.


Faster Startup
--------------


Frozen imports / Static code objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Python caches bytecode in the __pycache__ directory to speed up module
loading.

Previously in 3.10, Python module execution looked like this:

   Read __pycache__ -> Unmarshal -> Heap allocated code object -> Evaluate

In Python 3.11, the core modules essential for Python startup are
"frozen". This means that their code objects (and bytecode) are
statically allocated by the interpreter. This reduces the steps in
module execution process to this:

   Statically allocated code object -> Evaluate

Interpreter startup is now 10-15% faster in Python 3.11. This has a
big impact for short-running programs using Python.

(Contributed by Eric Snow, Guido van Rossum and Kumar Aditya in
numerous issues.)


Faster Runtime
--------------


Cheaper, lazy Python frames
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Python frames are created whenever Python calls a Python function.
This frame holds execution information. The following are new frame
optimizations:

* Streamlined the frame creation process.

* Avoided memory allocation by generously re-using frame space on the
  C stack.

* Streamlined the internal frame struct to contain only essential
  information. Frames previously held extra debugging and memory
  management information.

Old-style frame objects are now created only when required by
debuggers. For most user code, no frame objects are created at all. As
a result, nearly all Python functions calls have sped up
significantly. We measured a 3-7% speedup in pyperformance.

(Contributed by Mark Shannon in bpo-44590.)


Inlined Python function calls
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

During a Python function call, Python will call an evaluating C
function to interpret that function's code. This effectively limits
pure Python recursion to what's safe for the C stack.

In 3.11, when CPython detects Python code calling another Python
function, it sets up a new frame, and "jumps" to the new code inside
the new frame. This avoids calling the C interpreting function
altogether.

Most Python function calls now consume no C stack space. This speeds
up most of such calls. In simple recursive functions like fibonacci or
factorial, a 1.7x speedup was observed. This also means recursive
functions can recurse significantly deeper (if the user increases the
recursion limit). We measured a 1-3% improvement in pyperformance.

(Contributed by Pablo Galindo and Mark Shannon in bpo-45256.)


PEP 659: Specializing Adaptive Interpreter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

**PEP 659** is one of the key parts of the faster CPython project. The
general idea is that while Python is a dynamic language, most code has
regions where objects and types rarely change. This concept is known
as *type stability*.

At runtime, Python will try to look for common patterns and type
stability in the executing code. Python will then replace the current
operation with a more specialized one. This specialized operation uses
fast paths available only to those use cases/types, which generally
outperform their generic counterparts. This also brings in another
concept called *inline caching*, where Python caches the results of
expensive operations directly in the bytecode.

The specializer will also combine certain common instruction pairs
into one superinstruction. This reduces the overhead during execution.

Python will only specialize when it sees code that is "hot" (executed
multiple times). This prevents Python from wasting time for run-once
code. Python can also de-specialize when code is too dynamic or when
the use changes. Specialization is attempted periodically, and
specialization attempts are not too expensive. This allows
specialization to adapt to new circumstances.

(PEP written by Mark Shannon, with ideas inspired by Stefan
Brunthaler. See **PEP 659** for more information.)

+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Operation       | Form                 | Specialization                                          | Operation speedup   | Contributor(s)      |
|                 |                      |                                                         | (up to)             |                     |
|=================|======================|=========================================================|=====================|=====================|
| Binary          | "x+x; x*x; x-x;"     | Binary add, multiply and subtract for common types such | 10%                 | Mark Shannon, Dong- |
| operations      |                      | as "int", "float", and "str" take custom fast paths for |                     | hee Na, Brandt      |
|                 |                      | their underlying types.                                 |                     | Bucher, Dennis      |
|                 |                      |                                                         |                     | Sweeney             |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Subscript       | "a[i]"               | Subscripting container types such as "list", "tuple"    | 10-25%              | Irit Katriel, Mark  |
|                 |                      | and "dict" directly index the underlying data           |                     | Shannon             |
|                 |                      | structures.  Subscripting custom "__getitem__" is also  |                     |                     |
|                 |                      | inlined similar to Inlined Python function calls.       |                     |                     |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Store subscript | "a[i] = z"           | Similar to subscripting specialization above.           | 10-25%              | Dennis Sweeney      |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Calls           | "f(arg)" "C(arg)"    | Calls to common builtin (C) functions and types such as | 20%                 | Mark Shannon, Ken   |
|                 |                      | "len" and "str" directly call their underlying C        |                     | Jin                 |
|                 |                      | version. This avoids going through the internal calling |                     |                     |
|                 |                      | convention.                                             |                     |                     |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Load global     | "print" "len"        | The object's index in the globals/builtins namespace is | [1]                 | Mark Shannon        |
| variable        |                      | cached. Loading globals and builtins require zero       |                     |                     |
|                 |                      | namespace lookups.                                      |                     |                     |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Load attribute  | "o.attr"             | Similar to loading global variables. The attribute's    | [2]                 | Mark Shannon        |
|                 |                      | index inside the class/object's namespace is cached. In |                     |                     |
|                 |                      | most cases, attribute loading will require zero         |                     |                     |
|                 |                      | namespace lookups.                                      |                     |                     |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Load methods    | "o.meth()"           | The actual address of the method is cached. Method      | 10-20%              | Ken Jin, Mark       |
| for call        |                      | loading now has no namespace lookups -- even for        |                     | Shannon             |
|                 |                      | classes with long inheritance chains.                   |                     |                     |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Store attribute | "o.attr = z"         | Similar to load attribute optimization.                 | 2% in pyperformance | Mark Shannon        |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+
| Unpack Sequence | "*seq"               | Specialized for common containers such as "list" and    | 8%                  | Brandt Bucher       |
|                 |                      | "tuple". Avoids internal calling convention.            |                     |                     |
+-----------------+----------------------+---------------------------------------------------------+---------------------+---------------------+

[1] A similar optimization already existed since Python 3.8.  3.11
    specializes for more forms and reduces some overhead.

[2] A similar optimization already existed since Python 3.10. 3.11
    specializes for more forms. Furthermore, all attribute loads
    should be sped up by bpo-45947.


Misc
----

* Objects now require less memory due to lazily created object
  namespaces. Their namespace dictionaries now also share keys more
  freely. (Contributed Mark Shannon in bpo-45340 and bpo-40116.)

* A more concise representation of exceptions in the interpreter
  reduced the time required for catching an exception by about 10%.
  (Contributed by Irit Katriel in bpo-45711.)


FAQ
---

   Q: How should I write my code to utilize these speedups?

   A: You don't have to change your code. Write Pythonic code that follows common
   best practices. The Faster CPython project optimizes for common code
   patterns we observe.


   Q: Will CPython 3.11 use more memory?

   A: Maybe not. We don't expect memory use to exceed 20% more than 3.10.
   This is offset by memory optimizations for frame objects and object
   dictionaries as mentioned above.


   Q: I don't see any speedups in my workload. Why?

   A: Certain code won't have noticeable benefits. If your code spends most of
   its time on I/O operations, or already does most of its
   computation in a C extension library like numpy, there won't be significant
   speedup. This project currently benefits pure-Python workloads the most.

   Furthermore, the pyperformance figures are a geometric mean. Even within the
   pyperformance benchmarks, certain benchmarks have slowed down slightly, while
   others have sped up by nearly 2x!


   Q: Is there a JIT compiler?

   A: No. We're still exploring other optimizations.


About
-----

Faster CPython explores optimizations for *CPython*. The main team is
funded by Microsoft to work on this full-time. Pablo Galindo Salgado
is also funded by Bloomberg LP to work on the project part-time.
Finally, many contributors are volunteers from the community.


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

* Replaced all numeric "BINARY_*" and "INPLACE_*" instructions with a
  single "BINARY_OP" implementation.

* Replaced the three call instructions: "CALL_FUNCTION",
  "CALL_FUNCTION_KW" and "CALL_METHOD" with "PUSH_NULL", "PRECALL",
  "CALL", and "KW_NAMES". This decouples the argument shifting for
  methods from the handling of keyword arguments and allows better
  specialization of calls.

* Removed "COPY_DICT_WITHOUT_KEYS" and "GEN_START".

* "MATCH_CLASS" and "MATCH_KEYS" no longer push an additional boolean
  value indicating whether the match succeeded or failed. Instead,
  they indicate failure with "None" (where a tuple of extracted values
  would otherwise be).

* Replace several stack manipulation instructions ("DUP_TOP",
  "DUP_TOP_TWO", "ROT_TWO", "ROT_THREE", "ROT_FOUR", and "ROT_N") with
  new "COPY" and "SWAP" instructions.

* Replaced "JUMP_IF_NOT_EXC_MATCH" by "CHECK_EXC_MATCH" which performs
  the check but does not jump.

* Replaced "JUMP_IF_NOT_EG_MATCH" by "CHECK_EG_MATCH" which performs
  the check but does not jump.

* Replaced "JUMP_ABSOLUTE" by the relative "JUMP_BACKWARD".

* Added "JUMP_BACKWARD_NO_INTERRUPT", which is used in certain loops
  where it is undesirable to handle interrupts.

* Replaced "POP_JUMP_IF_TRUE" and "POP_JUMP_IF_FALSE" by the relative
  "POP_JUMP_FORWARD_IF_TRUE", "POP_JUMP_BACKWARD_IF_TRUE",
  "POP_JUMP_FORWARD_IF_FALSE" and "POP_JUMP_BACKWARD_IF_FALSE".

* Added "POP_JUMP_FORWARD_IF_NOT_NONE",
  "POP_JUMP_BACKWARD_IF_NOT_NONE", "POP_JUMP_FORWARD_IF_NONE" and
  "POP_JUMP_BACKWARD_IF_NONE" opcodes to speed up conditional jumps.

* "JUMP_IF_TRUE_OR_POP" and "JUMP_IF_FALSE_OR_POP" are now relative
  rather than absolute.


Deprecated
==========

* The "lib2to3" package and "2to3" tool are now deprecated and may not
  be able to parse Python 3.10 or newer. See the **PEP 617** (New PEG
  parser for CPython).  (Contributed by Victor Stinner in bpo-40360.)

* Undocumented modules "sre_compile", "sre_constants" and "sre_parse"
  are now deprecated. (Contributed by Serhiy Storchaka in bpo-47152.)

* "webbrowser.MacOSX" is deprecated and will be removed in Python
  3.13. It is untested and undocumented and also not used by
  webbrowser itself. (Contributed by Dong-hee Na in bpo-42255.)

* The behavior of returning a value from a "TestCase" and
  "IsolatedAsyncioTestCase" test methods (other than the default
  "None" value), is now deprecated.

* Deprecated the following "unittest" functions, scheduled for removal
  in Python 3.13:

  * "unittest.findTestCases()"

  * "unittest.makeSuite()"

  * "unittest.getTestCaseNames()"

  Use "TestLoader" method instead:

  * "unittest.TestLoader.loadTestsFromModule()"

  * "unittest.TestLoader.loadTestsFromTestCase()"

  * "unittest.TestLoader.getTestCaseNames()"

  (Contributed by Erlend E. Aasland in bpo-5846.)

* The "turtle.RawTurtle.settiltangle()" is deprecated since Python
  3.1, it now emits a deprecation warning and will be removed in
  Python 3.13. Use "turtle.RawTurtle.tiltangle()" instead (it was
  earlier incorrectly marked as deprecated, its docstring is now
  corrected). (Contributed by Hugo van Kemenade in bpo-45837.)

* The delegation of "int()" to "__trunc__()" is now deprecated.
  Calling "int(a)" when "type(a)" implements "__trunc__()" but not
  "__int__()" or "__index__()" now raises a "DeprecationWarning".
  (Contributed by Zackery Spytz in bpo-44977.)

* The following have been deprecated in "configparser" since Python
  3.2. Their deprecation warnings have now been updated to note they
  will removed in Python 3.12:

  * the "configparser.SafeConfigParser" class

  * the "configparser.ParsingError.filename" property

  * the "configparser.ParsingError.readfp()" method

  (Contributed by Hugo van Kemenade in bpo-45173.)

* "configparser.LegacyInterpolation" has been deprecated in the
  docstring since Python 3.2. It now emits a "DeprecationWarning" and
  will be removed in Python 3.13. Use
  "configparser.BasicInterpolation" or
  "configparser.ExtendedInterpolation" instead. (Contributed by Hugo
  van Kemenade in bpo-46607.)

* The "locale.getdefaultlocale()" function is deprecated and will be
  removed in Python 3.13. Use "locale.setlocale()",
  "locale.getpreferredencoding(False)" and "locale.getlocale()"
  functions instead. (Contributed by Victor Stinner in bpo-46659.)

* The "asynchat", "asyncore" and  "smtpd" modules have been deprecated
  since at least Python 3.6. Their documentation and deprecation
  warnings have now been updated to note they will removed in Python
  3.12 (**PEP 594**). (Contributed by Hugo van Kemenade in bpo-47022.)

* **PEP 594** led to the deprecations of the following modules which
  are slated for removal in Python 3.13:

  * "aifc"

  * "audioop"

  * "cgi"

  * "cgitb"

  * "chunk"

  * "crypt"

  * "imghdr"

  * "msilib"

  * "nis"

  * "nntplib"

  * "ossaudiodev"

  * "pipes"

  * "sndhdr"

  * "spwd"

  (Contributed by Brett Cannon in bpo-47061.)


Removed
=======

* "smtpd.MailmanProxy" is now removed as it is unusable without an
  external module, "mailman". (Contributed by Dong-hee Na in
  bpo-35800.)

* The "binhex" module, deprecated in Python 3.9, is now removed. The
  following "binascii" functions, deprecated in Python 3.9, are now
  also removed:

  * "a2b_hqx()", "b2a_hqx()";

  * "rlecode_hqx()", "rledecode_hqx()".

  The "binascii.crc_hqx()" function remains available.

  (Contributed by Victor Stinner in bpo-45085.)

* The distutils "bdist_msi" command, deprecated in Python 3.9, is now
  removed. Use "bdist_wheel" (wheel packages) instead. (Contributed by
  Hugo van Kemenade in bpo-45124.)

* Due to significant security concerns, the *reuse_address* parameter
  of "asyncio.loop.create_datagram_endpoint()", disabled in Python
  3.9, is now entirely removed. This is because of the behavior of the
  socket option "SO_REUSEADDR" in UDP. (Contributed by Hugo van
  Kemenade in bpo-45129.)

* Removed "__getitem__()" methods of "xml.dom.pulldom.DOMEventStream",
  "wsgiref.util.FileWrapper" and "fileinput.FileInput", deprecated
  since Python 3.9. (Contributed by Hugo van Kemenade in bpo-45132.)

* The following deprecated functions and methods are removed in the
  "gettext" module: "lgettext()", "ldgettext()", "lngettext()" and
  "ldngettext()".

  Function "bind_textdomain_codeset()", methods "output_charset()" and
  "set_output_charset()", and the *codeset* parameter of functions
  "translation()" and "install()" are also removed, since they are
  only used for the "l*gettext()" functions. (Contributed by Dong-hee
  Na and Serhiy Storchaka in bpo-44235.)

* The "@asyncio.coroutine" *decorator* enabling legacy generator-based
  coroutines to be compatible with async/await code. The function has
  been deprecated since Python 3.8 and the removal was initially
  scheduled for Python 3.10. Use "async def" instead. (Contributed by
  Illia Volochii in bpo-43216.)

* "asyncio.coroutines.CoroWrapper" used for wrapping legacy generator-
  based coroutine objects in the debug mode. (Contributed by Illia
  Volochii in bpo-43216.)

* Removed the deprecated "split()" method of "_tkinter.TkappType".
  (Contributed by Erlend E. Aasland in bpo-38371.)

* Removed from the "inspect" module:

  * the "getargspec" function, deprecated since Python 3.0; use
    "inspect.signature()" or "inspect.getfullargspec()" instead.

  * the "formatargspec" function, deprecated since Python 3.5; use the
    "inspect.signature()" function and "Signature" object directly.

  * the undocumented "Signature.from_builtin" and
    "Signature.from_function" functions, deprecated since Python 3.5;
    use the "Signature.from_callable()" method instead.

  (Contributed by Hugo van Kemenade in bpo-45320.)

* Remove namespace package support from unittest discovery. It was
  introduced in Python 3.4 but has been broken since Python 3.7.
  (Contributed by Inada Naoki in bpo-23882.)

* Remove "__class_getitem__" method from "pathlib.PurePath", because
  it was not used and added by mistake in previous versions.
  (Contributed by Nikita Sobolev in bpo-46483.)

* Remove the undocumented private "float.__set_format__()" method,
  previously known as "float.__setformat__()" in Python 3.7. Its
  docstring said: "You probably don't want to use this function. It
  exists mainly to be used in Python's test suite." (Contributed by
  Victor Stinner in bpo-46852.)


Porting to Python 3.11
======================

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


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

* Prohibited passing non-"concurrent.futures.ThreadPoolExecutor"
  executors to "loop.set_default_executor()" following a deprecation
  in Python 3.8. (Contributed by Illia Volochii in bpo-43234.)

* "open()", "io.open()", "codecs.open()" and "fileinput.FileInput" no
  longer accept "'U'" ("universal newline") in the file mode. This
  flag was deprecated since Python 3.3. In Python 3, the "universal
  newline" is used by default when a file is open in text mode.  The
  newline parameter of "open()" controls how universal newlines works.
  (Contributed by Victor Stinner in bpo-37330.)

* The "pdb" module now reads the ".pdbrc" configuration file with the
  "'utf-8'" encoding. (Contributed by Srinivas Reddy Thatiparthy (శ్
  రీనివాస్  రెడ్డి తాటిపర్తి) in bpo-41137.)

* When sorting using tuples as keys, the order of the result may
  differ from earlier releases if the tuple elements don't define a
  total ordering (see Value comparisons for information on total
  ordering).  It's generally true that the result of sorting simply
  isn't well-defined in the absence of a total ordering on list
  elements.

* "calendar": The "calendar.LocaleTextCalendar" and
  "calendar.LocaleHTMLCalendar" classes now use "locale.getlocale()",
  instead of using "locale.getdefaultlocale()", if no locale is
  specified. (Contributed by Victor Stinner in bpo-46659.)

* Global inline flags (e.g. "(?i)") can now only be used at the start
  of the regular expressions.  Using them not at the start of
  expression was deprecated since Python 3.6. (Contributed by Serhiy
  Storchaka in bpo-47066.)

* "re" module: Fix a few long-standing bugs where, in rare cases,
  capturing group could get wrong result. So the result may be
  different than before. (Contributed by Ma Lin in bpo-35859.)

* The *population* parameter of "random.sample()" must be a sequence.
  Automatic conversion of sets to lists is no longer supported. If the
  sample size is larger than the population size, a "ValueError" is
  raised. (Contributed by Raymond Hettinger in bpo-40465.)


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

* Building Python now requires a C11 compiler without optional C11
  features. (Contributed by Victor Stinner in bpo-46656.)

* Building Python now requires support of IEEE 754 floating point
  numbers. (Contributed by Victor Stinner in bpo-46917.)

* CPython can now be built with the ThinLTO option via "--with-
  lto=thin". (Contributed by Dong-hee Na and Brett Holman in
  bpo-44340.)

* libpython is no longer linked against libcrypt. (Contributed by Mike
  Gilbert in bpo-45433.)

* Building Python now requires a C99 "<math.h>" header file providing
  the following functions: "copysign()", "hypot()", "isfinite()",
  "isinf()", "isnan()", "round()". (Contributed by Victor Stinner in
  bpo-45440.)

* Building Python now requires a C99 "<math.h>" header file providing
  a "NAN" constant, or the "__builtin_nan()" built-in function.
  (Contributed by Victor Stinner in bpo-46640.)

* Building Python now requires support for floating point Not-a-Number
  (NaN): remove the "Py_NO_NAN" macro. (Contributed by Victor Stinner
  in bpo-46656.)

* Freelists for object structs can now be disabled. A new
  **configure** option "--without-freelists" can be used to disable
  all freelists except empty tuple singleton. (Contributed by
  Christian Heimes in bpo-45522)

* "Modules/Setup" and "Modules/makesetup" have been improved and tied
  up. Extension modules can now be built through "makesetup". All
  except some test modules can be linked statically into main binary
  or library. (Contributed by Brett Cannon and Christian Heimes in
  bpo-45548, bpo-45570, bpo-45571, and bpo-43974.)

* Build dependencies, compiler flags, and linker flags for most stdlib
  extension modules are now detected by **configure**. libffi, libnsl,
  libsqlite3, zlib, bzip2, liblzma, libcrypt, Tcl/Tk libs, and uuid
  flags are detected by "pkg-config" (when available). (Contributed by
  Christian Heimes and Erlend Egeberg Aasland in bpo-45847, bpo-45747,
  and bpo-45763.)

  Note:

    Use the environment variables "TCLTK_CFLAGS" and "TCLTK_LIBS" to
    manually specify the location of Tcl/Tk headers and libraries. The
    **configure** options "--with-tcltk-includes" and "--with-tcltk-
    libs" have been removed.

* CPython now has experimental support for cross compiling to
  WebAssembly platform "wasm32-emscripten". The effort is inspired by
  previous work like Pyodide. (Contributed by Christian Heimes and
  Ethan Smith in bpo-40280.)

* CPython will now use 30-bit digits by default for the Python "int"
  implementation. Previously, the default was to use 30-bit digits on
  platforms with "SIZEOF_VOID_P >= 8", and 15-bit digits otherwise.
  It's still possible to explicitly request use of 15-bit digits via
  either the "--enable-big-digits" option to the configure script or
  (for Windows) the "PYLONG_BITS_IN_DIGIT" variable in
  "PC/pyconfig.h", but this option may be removed at some point in the
  future. (Contributed by Mark Dickinson in bpo-45569.)

* The "tkinter" package now requires Tcl/Tk version 8.5.12 or newer.
  (Contributed by Serhiy Storchaka in bpo-46996.)


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

* "PyErr_SetExcInfo()" no longer uses the "type" and "traceback"
  arguments, the interpreter now derives those values from the
  exception instance (the "value" argument). The function still steals
  references of all three arguments. (Contributed by Irit Katriel in
  bpo-45711.)

* "PyErr_GetExcInfo()" now derives the "type" and "traceback" fields
  of the result from the exception instance (the "value" field).
  (Contributed by Irit Katriel in bpo-45711.)

* "_frozen" has a new "is_package" field to indicate whether or not
  the frozen module is a package.  Previously, a negative value in the
  "size" field was the indicator.  Now only non-negative values be
  used for "size". (Contributed by Kumar Aditya in bpo-46608.)

* "_PyFrameEvalFunction()" now takes "_PyInterpreterFrame*" as its
  second parameter, instead of "PyFrameObject*". See **PEP 523** for
  more details of how to use this function pointer type.

* "PyCode_New()" and "PyCode_NewWithPosOnlyArgs()" now take an
  additional "exception_table" argument. Using these functions should
  be avoided, if at all possible. To get a custom code object: create
  a code object using the compiler, then get a modified version with
  the "replace" method.


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

* Add a new "PyType_GetName()" function to get type's short name.
  (Contributed by Hai Shi in bpo-42035.)

* Add a new "PyType_GetQualName()" function to get type's qualified
  name. (Contributed by Hai Shi in bpo-42035.)

* Add new "PyThreadState_EnterTracing()" and
  "PyThreadState_LeaveTracing()" functions to the limited C API to
  suspend and resume tracing and profiling. (Contributed by Victor
  Stinner in bpo-43760.)

* Added the "Py_Version" constant which bears the same value as
  "PY_VERSION_HEX". (Contributed by  Gabriele N. Tornetta in
  bpo-43931.)

* "Py_buffer" and APIs are now part of the limited API and the stable
  ABI:

  * "PyObject_CheckBuffer()"

  * "PyObject_GetBuffer()"

  * "PyBuffer_GetPointer()"

  * "PyBuffer_SizeFromFormat()"

  * "PyBuffer_ToContiguous()"

  * "PyBuffer_FromContiguous()"

  * "PyBuffer_CopyData()"

  * "PyBuffer_IsContiguous()"

  * "PyBuffer_FillContiguousStrides()"

  * "PyBuffer_FillInfo()"

  * "PyBuffer_Release()"

  * "PyMemoryView_FromBuffer()"

  * "bf_getbuffer" and "bf_releasebuffer" type slots

  (Contributed by Christian Heimes in bpo-45459.)

* Added the "PyType_GetModuleByDef" function, used to get the module
  in which a method was defined, in cases where this information is
  not available directly (via "PyCMethod"). (Contributed by Petr
  Viktorin in bpo-46613.)

* Add new functions to pack and unpack C double (serialize and
  deserialize): "PyFloat_Pack2()", "PyFloat_Pack4()",
  "PyFloat_Pack8()", "PyFloat_Unpack2()", "PyFloat_Unpack4()" and
  "PyFloat_Unpack8()". (Contributed by Victor Stinner in bpo-46906.)

* Add new functions to get frame object attributes:
  "PyFrame_GetBuiltins()", "PyFrame_GetGenerator()",
  "PyFrame_GetGlobals()", "PyFrame_GetLasti()".

* Added two new functions to get and set the active exception
  instance: "PyErr_GetHandledException()" and
  "PyErr_SetHandledException()". These are alternatives to
  "PyErr_SetExcInfo()" and "PyErr_GetExcInfo()" which work with the
  legacy 3-tuple representation of exceptions. (Contributed by Irit
  Katriel in bpo-46343.)


Porting to Python 3.11
----------------------

* The old trashcan macros
  ("Py_TRASHCAN_SAFE_BEGIN"/"Py_TRASHCAN_SAFE_END") are now
  deprecated. They should be replaced by the new macros
  "Py_TRASHCAN_BEGIN" and "Py_TRASHCAN_END".

  A tp_dealloc function that has the old macros, such as:

     static void
     mytype_dealloc(mytype *p)
     {
         PyObject_GC_UnTrack(p);
         Py_TRASHCAN_SAFE_BEGIN(p);
         ...
         Py_TRASHCAN_SAFE_END
     }

  should migrate to the new macros as follows:

     static void
     mytype_dealloc(mytype *p)
     {
         PyObject_GC_UnTrack(p);
         Py_TRASHCAN_BEGIN(p, mytype_dealloc)
         ...
         Py_TRASHCAN_END
     }

  Note that "Py_TRASHCAN_BEGIN" has a second argument which should be
  the deallocation function it is in.

  To support older Python versions in the same codebase, you can
  define the following macros and use them throughout the code
  (credit: these were copied from the "mypy" codebase):

     #if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 8
     #  define CPy_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_BEGIN(op, dealloc)
     #  define CPy_TRASHCAN_END(op) Py_TRASHCAN_END
     #else
     #  define CPy_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_SAFE_BEGIN(op)
     #  define CPy_TRASHCAN_END(op) Py_TRASHCAN_SAFE_END(op)
     #endif

* The "PyType_Ready()" function now raises an error if a type is
  defined with the "Py_TPFLAGS_HAVE_GC" flag set but has no traverse
  function ("PyTypeObject.tp_traverse"). (Contributed by Victor
  Stinner in bpo-44263.)

* Heap types with the "Py_TPFLAGS_IMMUTABLETYPE" flag can now inherit
  the **PEP 590** vectorcall protocol.  Previously, this was only
  possible for static types. (Contributed by Erlend E. Aasland in
  bpo-43908)

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

     #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_TYPE)
     static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type)
     { ob->ob_type = type; }
     #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type)
     #endif

  (Contributed by Victor Stinner in bpo-39573.)

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

     #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_SIZE)
     static inline void _Py_SET_SIZE(PyVarObject *ob, Py_ssize_t size)
     { ob->ob_size = size; }
     #define Py_SET_SIZE(ob, size) _Py_SET_SIZE((PyVarObject*)(ob), size)
     #endif

  (Contributed by Victor Stinner in bpo-39573.)

* "<Python.h>" no longer includes the header files "<stdlib.h>",
  "<stdio.h>", "<errno.h>" and "<string.h>" when the "Py_LIMITED_API"
  macro is set to "0x030b0000" (Python 3.11) or higher. C extensions
  should explicitly include the header files after "#include
  <Python.h>". (Contributed by Victor Stinner in bpo-45434.)

* The non-limited API files "cellobject.h", "classobject.h", "code.h",
  "context.h", "funcobject.h", "genobject.h" and "longintrepr.h" have
  been moved to the "Include/cpython" directory. Moreover, the
  "eval.h" header file was removed. These files must not be included
  directly, as they are already included in "Python.h": Include Files.
  If they have been included directly, consider including "Python.h"
  instead. (Contributed by Victor Stinner in bpo-35134.)

* The "PyUnicode_CHECK_INTERNED()" macro has been excluded from the
  limited C API. It was never usable there, because it used internal
  structures which are not available in the limited C API.
  (Contributed by Victor Stinner in bpo-46007.)

* The "PyFrameObject" structure members have been removed from the
  public C API.

  While the documentation notes that the "PyFrameObject" fields are
  subject to change at any time, they have been stable for a long time
  and were used in several popular extensions.

  In Python 3.11, the frame struct was reorganized to allow
  performance optimizations. Some fields were removed entirely, as
  they were details of the old implementation.

  "PyFrameObject" fields:

  * "f_back": use "PyFrame_GetBack()".

  * "f_blockstack": removed.

  * "f_builtins": use "PyFrame_GetBuiltins()".

  * "f_code": use "PyFrame_GetCode()".

  * "f_gen": use "PyFrame_GetGenerator()".

  * "f_globals": use "PyFrame_GetGlobals()".

  * "f_iblock": removed.

  * "f_lasti": use "PyFrame_GetLasti()". Code using "f_lasti" with
    "PyCode_Addr2Line()" should use "PyFrame_GetLineNumber()" instead;
    it may be faster.

  * "f_lineno": use "PyFrame_GetLineNumber()"

  * "f_locals": use "PyFrame_GetLocals()".

  * "f_stackdepth": removed.

  * "f_state": no public API (renamed to "f_frame.f_state").

  * "f_trace": no public API.

  * "f_trace_lines": use "PyObject_GetAttrString((PyObject*)frame,
    "f_trace_lines")".

  * "f_trace_opcodes": use "PyObject_GetAttrString((PyObject*)frame,
    "f_trace_opcodes")".

  * "f_localsplus": no public API (renamed to "f_frame.localsplus").

  * "f_valuestack": removed.

  The Python frame object is now created lazily. A side effect is that
  the "f_back" member must not be accessed directly, since its value
  is now also computed lazily. The "PyFrame_GetBack()" function must
  be called instead.

  Debuggers that accessed the "f_locals" directly *must* call
  "PyFrame_GetLocals()" instead. They no longer need to call
  "PyFrame_FastToLocalsWithError()" or "PyFrame_LocalsToFast()", in
  fact they should not call those functions. The necessary updating of
  the frame is now managed by the virtual machine.

  Code defining "PyFrame_GetCode()" on Python 3.8 and older:

     #if PY_VERSION_HEX < 0x030900B1
     static inline PyCodeObject* PyFrame_GetCode(PyFrameObject *frame)
     {
         Py_INCREF(frame->f_code);
         return frame->f_code;
     }
     #endif

  Code defining "PyFrame_GetBack()" on Python 3.8 and older:

     #if PY_VERSION_HEX < 0x030900B1
     static inline PyFrameObject* PyFrame_GetBack(PyFrameObject *frame)
     {
         Py_XINCREF(frame->f_back);
         return frame->f_back;
     }
     #endif

  Or use the pythoncapi_compat project to get these two functions on
  older Python versions.

* Changes of the "PyThreadState" structure members:

  * "frame": removed, use "PyThreadState_GetFrame()" (function added
    to Python 3.9 by bpo-40429). Warning: the function returns a
    *strong reference*, need to call "Py_XDECREF()".

  * "tracing": changed, use "PyThreadState_EnterTracing()" and
    "PyThreadState_LeaveTracing()" (functions added to Python 3.11 by
    bpo-43760).

  * "recursion_depth": removed, use "(tstate->recursion_limit -
    tstate->recursion_remaining)" instead.

  * "stackcheck_counter": removed.

  Code defining "PyThreadState_GetFrame()" on Python 3.8 and older:

     #if PY_VERSION_HEX < 0x030900B1
     static inline PyFrameObject* PyThreadState_GetFrame(PyThreadState *tstate)
     {
         Py_XINCREF(tstate->frame);
         return tstate->frame;
     }
     #endif

  Code defining "PyThreadState_EnterTracing()" and
  "PyThreadState_LeaveTracing()" on Python 3.10 and older:

     #if PY_VERSION_HEX < 0x030B00A2
     static inline void PyThreadState_EnterTracing(PyThreadState *tstate)
     {
         tstate->tracing++;
     #if PY_VERSION_HEX >= 0x030A00A1
         tstate->cframe->use_tracing = 0;
     #else
         tstate->use_tracing = 0;
     #endif
     }

     static inline void PyThreadState_LeaveTracing(PyThreadState *tstate)
     {
         int use_tracing = (tstate->c_tracefunc != NULL || tstate->c_profilefunc != NULL);
         tstate->tracing--;
     #if PY_VERSION_HEX >= 0x030A00A1
         tstate->cframe->use_tracing = use_tracing;
     #else
         tstate->use_tracing = use_tracing;
     #endif
     }
     #endif

  Or use the pythoncapi_compat project to get these functions on old
  Python functions.

* Distributors are encouraged to build Python with the optimized
  Blake2 library libb2.


Deprecated
----------

* Deprecate the following functions to configure the Python
  initialization:

  * "PySys_AddWarnOptionUnicode()"

  * "PySys_AddWarnOption()"

  * "PySys_AddXOption()"

  * "PySys_HasWarnOptions()"

  * "Py_SetPath()"

  * "Py_SetProgramName()"

  * "Py_SetPythonHome()"

  * "Py_SetStandardStreamEncoding()"

  * "_Py_SetProgramFullPath()"

  Use the new "PyConfig" API of the Python Initialization
  Configuration instead (**PEP 587**). (Contributed by Victor Stinner
  in bpo-44113.)

* Deprecate the "ob_shash" member of the "PyBytesObject". Use
  "PyObject_Hash()" instead. (Contributed by Inada Naoki in
  bpo-46864.)


Removed
-------

* "PyFrame_BlockSetup()" and "PyFrame_BlockPop()" have been removed.
  (Contributed by Mark Shannon in bpo-40222.)

* Remove the following math macros using the "errno" variable:

  * "Py_ADJUST_ERANGE1()"

  * "Py_ADJUST_ERANGE2()"

  * "Py_OVERFLOWED()"

  * "Py_SET_ERANGE_IF_OVERFLOW()"

  * "Py_SET_ERRNO_ON_MATH_ERROR()"

  (Contributed by Victor Stinner in bpo-45412.)

* Remove "Py_UNICODE_COPY()" and "Py_UNICODE_FILL()" macros,
  deprecated since Python 3.3. Use "PyUnicode_CopyCharacters()" or
  "memcpy()" ("wchar_t*" string), and "PyUnicode_Fill()" functions
  instead. (Contributed by Victor Stinner in bpo-41123.)

* Remove the "pystrhex.h" header file. It only contains private
  functions. C extensions should only include the main "<Python.h>"
  header file. (Contributed by Victor Stinner in bpo-45434.)

* Remove the "Py_FORCE_DOUBLE()" macro. It was used by the
  "Py_IS_INFINITY()" macro. (Contributed by Victor Stinner in
  bpo-45440.)

* The following items are no longer available when "Py_LIMITED_API" is
  defined:

  * "PyMarshal_WriteLongToFile()"

  * "PyMarshal_WriteObjectToFile()"

  * "PyMarshal_ReadObjectFromString()"

  * "PyMarshal_WriteObjectToString()"

  * the "Py_MARSHAL_VERSION" macro

  These are not part of the limited API.

  (Contributed by Victor Stinner in bpo-45474.)

* Exclude "PyWeakref_GET_OBJECT()" from the limited C API. It never
  worked since the "PyWeakReference" structure is opaque in the
  limited C API. (Contributed by Victor Stinner in bpo-35134.)

* Remove the "PyHeapType_GET_MEMBERS()" macro. It was exposed in the
  public C API by mistake, it must only be used by Python internally.
  Use the "PyTypeObject.tp_members" member instead. (Contributed by
  Victor Stinner in bpo-40170.)

* Remove the "HAVE_PY_SET_53BIT_PRECISION" macro (moved to the
  internal C API). (Contributed by Victor Stinner in bpo-45412.)
