What's New In Python 3.10
*************************

Release:
   3.10.0a2

Date:
   December 07, 2020

This article explains the new features in Python 3.10, compared to
3.9.

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


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


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


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

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

* The "int" type has a new method "int.bit_count()", returning the
  number of ones in the binary expansion of a given integer, also
  known as the population count. (Contributed by Niklas Fiekas in
  bpo-29882.)

* The views returned by "dict.keys()", "dict.values()" and
  "dict.items()" now all have a "mapping" attribute that gives a
  "types.MappingProxyType" object wrapping the original dictionary.
  (Contributed by Dennis Sweeney in bpo-40890.)

* **PEP 618**: The "zip()" function now has an optional "strict" flag,
  used to require that all the iterables have an equal length.


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

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

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

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

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

See **PEP 613** for more details.

(Contributed by Mikhail Golubev in bpo-41923.)


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

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

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

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

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

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

See **PEP 604** for more details.

(Contributed by Maggie Moss and Philippe Prados in bpo-41428.)


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

* Builtin and extension functions that take integer arguments no
  longer accept "Decimal"s, "Fraction"s and other objects that can be
  converted to integers only with a loss (e.g. that have the
  "__int__()" method but do not have the "__index__()" method).
  (Contributed by Serhiy Storchaka in bpo-37999.)

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


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

* None yet.


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


base64
------

Add "base64.b32hexencode()" and "base64.b32hexdecode()" to support the
Base32 Encoding with Extended Hex Alphabet.


codecs
------

Add a "codecs.unregister()" function to unregister a codec search
function. (Contributed by Hai Shi in bpo-41842.)


contextlib
----------

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

Add asynchronous context manager support to
"contextlib.nullcontext()". (Contributed by Tom Gringauz in
bpo-41543.)


curses
------

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


doctest
-------

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


encodings
---------

"encodings.normalize_encoding()" now ignores non-ASCII characters.
(Contributed by Hai Shi in bpo-39337.)


glob
----

Added the *root_dir* and *dir_fd* parameters in "glob()" and "iglob()"
which allow to specify the root directory for searching. (Contributed
by Serhiy Storchaka in bpo-38144.)


inspect
-------

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


linecache
---------

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


os
--

Added "os.cpu_count()" support for VxWorks RTOS. (Contributed by
Peixing Xin in bpo-41440.)

Added a new function "os.eventfd()" and related helpers to wrap the
"eventfd2" syscall on Linux. (Contributed by Christian Heimes in
bpo-41001.)

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


pathlib
-------

Added slice support to "PurePath.parents". (Contributed by Joshua
Cannon in bpo-35498)

Added negative indexing support to "PurePath.parents". (Contributed by
Yaroslav Pankovych in bpo-21041)


platform
--------

Added "platform.freedesktop_os_release()" to retrieve operation system
identification from freedesktop.org os-release standard file.
(Contributed by Christian Heimes in bpo-28468)


py_compile
----------

Added "--quiet" option to command-line interface of "py_compile".
(Contributed by Gregory Schevchenko in bpo-38731.)


shelve
------

The "shelve" module now uses "pickle.DEFAULT_PROTOCOL" by default
instead of "pickle" protocol "3" when creating shelves. (Contributed
by Zackery Spytz in bpo-34204.)


site
----

When a module does not define "__loader__", fall back to
"__spec__.loader". (Contributed by Brett Cannon in bpo-42133.)


socket
------

The exception "socket.timeout" is now an alias of "TimeoutError".
(Contributed by Christian Heimes in bpo-42413.)


sys
---

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


threading
---------

Added "threading.gettrace()" and "threading.getprofile()" to retrieve
the functions set by "threading.settrace()" and
"threading.setprofile()" respectively. (Contributed by Mario Corchero
in bpo-42251.)

Add "threading.__excepthook__" to allow retrieving the original value
of "threading.excepthook()" in case it is set to a broken or a
different value. (Contributed by Mario Corchero in bpo-42308.)


traceback
---------

The "format_exception()", "format_exception_only()", and
"print_exception()" functions can now take an exception object as a
positional-only argument. (Contributed by Zackery Spytz and Matthias
Bussonnier in bpo-26389.)


types
-----

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


typing
------

The behavior of "typing.Literal" was changed to conform with **PEP
586** and to match the behavior of static type checkers specified in
the PEP.

1. "Literal" now de-duplicates parameters.

2. Equality comparisons between "Literal" objects are now order
   independent.

3. "Literal" comparisons now respects types.  For example, "Literal[0]
   == Literal[False]" previously evaluated to "True".  It is now
   "False".  To support this change, the internally used type cache
   now supports differentiating types.

4. "Literal" objects will now raise a "TypeError" exception during
   equality comparisons if one of their parameters are not
   *immutable*. Note that declaring "Literal" with mutable parameters
   will not throw an error:

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

(Contributed by Yurii Karabas in bpo-42345.)


unittest
--------

Add new method "assertNoLogs()" to complement the existing
"assertLogs()". (Contributed by Kit Yan Choi in bpo-39385.)


xml
---

Add a "LexicalHandler" class to the "xml.sax.handler" module.
(Contributed by Jonathan Gossage and Zackery Spytz in bpo-35018.)


zipimport
---------

Add methods related to **PEP 451**: "find_spec()",
"zipimport.zipimporter.create_module()", and
"zipimport.zipimporter.exec_module()". (Contributed by Brett Cannon in
bpo-42131.


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

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

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

* The "LOAD_ATTR" instruction now uses new "per opcode cache"
  mechanism. It is about 36% faster now.  (Contributed by Pablo
  Galindo and Yury Selivanov in bpo-42093, based on ideas implemented
  originally in PyPy and MicroPython.)

* When building Python with "--enable-optimizations" now "-fno-
  semantic-interposition" is added to both the compile and link line.
  This speeds builds of the Python interpreter created with "--enable-
  shared" with "gcc" by up to 30%. See this article for more details.
  (Contributed by Victor Stinner and Pablo Galindo in bpo-38980.)

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


Deprecated
==========

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

* The various "load_module()" methods of "importlib" have been
  documented as deprecated since Python 3.6, but will now also trigger
  a "DeprecationWarning". Use "exec_module()" instead. (Contributed by
  Brett Cannon in bpo-26131.)

* "zimport.zipimporter.load_module()" has been deprecated in
  preference for "exec_module()". (Contributed by Brett Cannon in
  bpo-26131.)

* The use of "load_module()" by the import system now triggers an
  "ImportWarning" as "exec_module()" is preferred. (Contributed by
  Brett Cannon in bpo-26131.)

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


Removed
=======

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

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

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

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

* Removed the Public C API functions
  "PyParser_SimpleParseStringFlags()",
  "PyParser_SimpleParseStringFlagsFilename()",
  "PyParser_SimpleParseFileFlags()" and "PyNode_Compile()" that were
  deprecated in 3.9 due to the switch to the new PEG parser.

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


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

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


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

* The *etype* parameters of the "format_exception()",
  "format_exception_only()", and "print_exception()" functions in the
  "traceback" module have been renamed to *exc*. (Contributed by
  Zackery Spytz and Matthias Bussonnier in bpo-26389.)


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

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


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

* The C99 functions "snprintf()" and "vsnprintf()" are now required to
  build Python. (Contributed by Victor Stinner in bpo-36020.)

* "sqlite3" requires SQLite 3.7.3 or higher. (Contributed by Sergey
  Fedoseev and Erlend E. Aasland bpo-40744.)


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


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

* The result of "PyNumber_Index()" now always has exact type "int".
  Previously, the result could have been an instance of a subclass of
  "int". (Contributed by Serhiy Storchaka in bpo-40792.)

* Add a new "orig_argv" member to the "PyConfig" structure: the list
  of the original command line arguments passed to the Python
  executable. (Contributed by Victor Stinner in bpo-23427.)

* The "PyDateTime_DATE_GET_TZINFO()" and
  "PyDateTime_TIME_GET_TZINFO()" macros have been added for accessing
  the "tzinfo" attributes of "datetime.datetime" and "datetime.time"
  objects. (Contributed by Zackery Spytz in bpo-30155.)

* Add a "PyCodec_Unregister()" function to unregister a codec search
  function. (Contributed by Hai Shi in bpo-41842.)

* The "PyIter_Send()" function was added to allow sending value into
  iterator without raising "StopIteration" exception. (Contributed by
  Vladimir Matveev in bpo-41756.)

* Added "PyUnicode_AsUTF8AndSize()" to the limited C API. (Contributed
  by Alex Gaynor in bpo-41784.)

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

* Added "Py_NewRef()" and "Py_XNewRef()" functions to increment the
  reference count of an object and return the object. (Contributed by
  Victor Stinner in bpo-42262.)

* The "PyType_FromSpecWithBases()" and "PyType_FromModuleAndSpec()"
  functions now accept a single class as the *bases* argument.
  (Contributed by Serhiy Storchaka in bpo-42423.)

* The "PyType_FromModuleAndSpec()" function now accepts NULL "tp_doc"
  slot. (Contributed by Hai Shi in bpo-41832.)

* The "PyType_GetSlot()" function can accept static types.
  (Contributed by Hai Shi and Petr Viktorin in bpo-41073.)


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

* The "PY_SSIZE_T_CLEAN" macro must now be defined to use
  "PyArg_ParseTuple()" and "Py_BuildValue()" formats which use "#":
  "es#", "et#", "s#", "u#", "y#", "z#", "U#" and "Z#". See Parsing
  arguments and building values and the **PEP 353**. (Contributed by
  Victor Stinner in bpo-40943.)

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

     #if PY_VERSION_HEX < 0x030900A4
     #  define Py_SET_REFCNT(obj, refcnt) ((Py_REFCNT(obj) = (refcnt)), (void)0)
     #endif

  (Contributed by Victor Stinner in bpo-39573.)

* Calling "PyDict_GetItem()" without *GIL* held had been allowed for
  historical reason. It is no longer allowed. (Contributed by Victor
  Stinner in bpo-40839.)

* "PyUnicode_FromUnicode(NULL, size)" and
  "PyUnicode_FromStringAndSize(NULL, size)" raise "DeprecationWarning"
  now.  Use "PyUnicode_New()" to allocate Unicode object without
  initial data. (Contributed by Inada Naoki in bpo-36346.)

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

* "Py_GetPath()", "Py_GetPrefix()", "Py_GetExecPrefix()",
  "Py_GetProgramFullPath()", "Py_GetPythonHome()" and
  "Py_GetProgramName()" functions now return "NULL" if called before
  "Py_Initialize()" (before Python is initialized). Use the new Python
  Initialization Configuration API to get the Python Path
  Configuration.. (Contributed by Victor Stinner in bpo-42260.)

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


Deprecated
----------

* The "PyUnicode_InternImmortal()" function is now deprecated and will
  be removed in Python 3.12: use "PyUnicode_InternInPlace()" instead.
  (Contributed by Victor Stinner in bpo-41692.)


Removed
-------

* "PyObject_AsCharBuffer()", "PyObject_AsReadBuffer()",
  "PyObject_CheckReadBuffer()", and "PyObject_AsWriteBuffer()" are
  removed. Please migrate to new buffer protocol;
  "PyObject_GetBuffer()" and "PyBuffer_Release()". (Contributed by
  Inada Naoki in bpo-41103.)

* Removed "Py_UNICODE_str*" functions manipulating "Py_UNICODE*"
  strings. (Contributed by Inada Naoki in bpo-41123.)

     * "Py_UNICODE_strlen": use "PyUnicode_GetLength()" or
       "PyUnicode_GET_LENGTH"

     * "Py_UNICODE_strcat": use "PyUnicode_CopyCharacters()" or
       "PyUnicode_FromFormat()"

     * "Py_UNICODE_strcpy", "Py_UNICODE_strncpy": use
       "PyUnicode_CopyCharacters()" or "PyUnicode_Substring()"

     * "Py_UNICODE_strcmp": use "PyUnicode_Compare()"

     * "Py_UNICODE_strncmp": use "PyUnicode_Tailmatch()"

     * "Py_UNICODE_strchr", "Py_UNICODE_strrchr": use
       "PyUnicode_FindChar()"

* Removed "PyUnicode_GetMax()". Please migrate to new (**PEP 393**)
  APIs. (Contributed by Inada Naoki in bpo-41103.)

* Removed "PyLong_FromUnicode()". Please migrate to
  "PyLong_FromUnicodeObject()". (Contributed by Inada Naoki in
  bpo-41103.)

* Removed "PyUnicode_AsUnicodeCopy()". Please use
  "PyUnicode_AsUCS4Copy()" or "PyUnicode_AsWideCharString()"
  (Contributed by Inada Naoki in bpo-41103.)

* Removed "_Py_CheckRecursionLimit" variable: it has been replaced by
  "ceval.recursion_limit" of the "PyInterpreterState" structure.
  (Contributed by Victor Stinner in bpo-41834.)

* Removed undocumented macros "Py_ALLOW_RECURSION" and
  "Py_END_ALLOW_RECURSION" and the "recursion_critical" field of the
  "PyInterpreterState" structure. (Contributed by Serhiy Storchaka in
  bpo-41936.)

* Removed the undocumented "PyOS_InitInterrupts()" function.
  Initializing Python already implicitly installs signal handlers: see
  "PyConfig.install_signal_handlers". (Contributed by Victor Stinner
  in bpo-41713.)
