What’s New In Python 3.10




January 25, 2021

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

For full details, see the changelog.


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.)

PEP 612: Parameter Specification Variables

Two new options to improve the information provided to static type checkers for PEP 484‘s Callable have been added to the typing module.

The first is the parameter specification variable. They are used to forward the parameter types of one callable to another callable – a pattern commonly found in higher order functions and decorators. Examples of usage can be found in typing.ParamSpec. Previously, there was no easy way to type annotate dependency of parameter types in such a precise manner.

The second option is the new Concatenate operator. It’s used in conjunction with parameter specification variables to type annotate a higher order callable which adds or removes parameters of another callable. Examples of usage can be found in typing.Concatenate.

See typing.Callable, typing.ParamSpec, typing.Concatenate and PEP 612 for more details.

(Contributed by Ken Jin in bpo-41559.)

Better error messages in the parser

When parsing code that contains unclosed parentheses or brackets the interpreter now includes the location of the unclosed bracket of parentheses instead of displaying SyntaxError: unexpected EOF while parsing or pointing to some incorrect location. For instance, consider the following code (notice the unclosed ‘{‘):

expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
            38: 4, 39: 4, 45: 5, 46: 5, 47: 5, 48: 5, 49: 5, 54: 6,
some_other_code = foo()

previous versions of the interpreter reported confusing places as the location of the syntax error:

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

but in Python3.10 a more informative error is emitted:

File "example.py", line 1
    expected = {9: 1, 18: 2, 19: 2, 27: 3, 28: 3, 29: 3, 36: 4, 37: 4,
SyntaxError: '{' was never closed

In a similar way, errors involving unclosed string literals (single and triple quoted) now point to the start of the string instead of reporting EOF/EOL.

These improvements are inspired by previous work in the PyPy interpreter.

(Contributed by Pablo Galindo in bpo-42864 and Batuhan Taskaya in bpo-40176.)

Other Language Changes

  • Builtin and extension functions that take integer arguments no longer accept Decimals, Fractions 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


Misleading phrase “optional arguments” was replaced with “options” in argparse help. Some tests might require adaptation if they rely on exact output match. (Contributed by Raymond Hettinger in bpo-9694.)


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


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


The __args__ of the parameterized generic for collections.abc.Callable are now consistent with typing.Callable. collections.abc.Callable generic now flattens type parameters, similar to what typing.Callable currently does. This means that collections.abc.Callable[[int, str], str] will have __args__ of (int, str, str); previously this was ([int, str], str). To allow this change, types.GenericAlias can now be subclassed, and a subclass will be returned when subscripting the collections.abc.Callable type. Note that a TypeError may be raised for invalid forms of parameterizing collections.abc.Callable which may have passed silently in Python 3.9. (Contributed by Ken Jin in bpo-42195.)


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.)


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.)

The BUTTON5_* constants are now exposed in the curses module if they are provided by the underlying curses library. (Contributed by Zackery Spytz in bpo-39273.)


The bdist_wininst command deprecated in Python 3.8 has been removed. The bdist_wheel command is now recommended to distribute binary packages on Windows. (Contributed by Victor Stinner in bpo-42802.)


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


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


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.)


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

Added globalns and localns parameters in signature() and inspect.Signature.from_callable() to retrieve the annotations in given local and global namespaces. (Contributed by Batuhan Taskaya in bpo-41960.)


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


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.)


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)


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


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


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.)


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


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


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.)

Add sys.module_names, containing the list of the standard library module names. (Contributed by Victor Stinner in bpo-42955.)


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.)


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.)


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.)


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.)


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


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


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


  • 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. This makes optimized LOAD_ATTR instructions the current most performance attribute access method (faster than slots). (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)


  • 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.)

  • The undocumented built-in function sqlite3.enable_shared_cache is now deprecated, scheduled for removal in Python 3.12. Its use is strongly discouraged by the SQLite3 documentation. See the SQLite3 docs for more details. If shared cache must be used, open the database in URI mode using the cache=shared query parameter. (Contributed by Erlend E. Aasland in bpo-24464.)


  • 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.)

  • Removed the PyModule_GetWarningsModule() function that was useless now due to the _warnings module was converted to a builtin module in 2.6. (Contributed by Hai Shi in bpo-42599.)

  • Remove deprecated aliases to Collections Abstract Base Classes from the collections module. (Contributed by Victor Stinner in bpo-37324.)

  • The loop parameter has been removed from most of asyncio‘s high-level API following deprecation in Python 3.8. The motivation behind this change is multifold:

    1. This simplifies the high-level API.

    2. The functions in the high-level API have been implicitly getting the current thread’s running event loop since Python 3.7. There isn’t a need to pass the event loop to the API in most normal use cases.

    3. Event loop passing is error-prone especially when dealing with loops running in different threads.

    Note that the low-level API will still accept loop. See Changes in the Python API for examples of how to replace existing code.

    (Contributed by Yurii Karabas, Andrew Svetlov, Yury Selivanov and Kyle Stanley in bpo-42392.)

Porting to Python 3.10

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

Changes in the Python 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.)

  • atexit: At Python exit, if a callback registered with atexit.register() fails, its exception is now logged. Previously, only some exceptions were logged, and the last exception was always silently ignored. (Contributed by Victor Stinner in bpo-42639.)

  • collections.abc.Callable generic now flattens type parameters, similar to what typing.Callable currently does. This means that collections.abc.Callable[[int, str], str] will have __args__ of (int, str, str); previously this was ([int, str], str). Code which accesses the arguments via typing.get_args() or __args__ need to account for this change. Furthermore, TypeError may be raised for invalid forms of parameterizing collections.abc.Callable which may have passed silently in Python 3.9. (Contributed by Ken Jin in bpo-42195.)

  • socket.htons() and socket.ntohs() now raise OverflowError instead of DeprecationWarning if the given parameter will not fit in a 16-bit unsigned integer. (Contributed by Erlend E. Aasland in bpo-42393.)

  • The loop parameter has been removed from most of asyncio‘s high-level API following deprecation in Python 3.8.

    A coroutine that currently look like this:

    async def foo(loop):
        await asyncio.sleep(1, loop=loop)

    Should be replaced with this:

    async def foo():
        await asyncio.sleep(1)

    If foo() was specifically designed not to run in the current thread’s running event loop (e.g. running in another thread’s event loop), consider using asyncio.run_coroutine_threadsafe() instead.

    (Contributed by Yurii Karabas, Andrew Svetlov, Yury Selivanov and Kyle Stanley in bpo-42392.)

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 3.7.15 or higher. (Contributed by Sergey Fedoseev and Erlend E. Aasland bpo-40744 and bpo-40810.)

  • The atexit module must now always be built as a built-in module. (Contributed by Victor Stinner in bpo-42639.)

  • Added --disable-test-modules option to the configure script: don’t build nor install test modules. (Contributed by Xavier de Gaye, Thomas Petazzoni and Peixing Xin in bpo-27640.)

  • Add --with-wheel-pkg-dir=PATH option to the ./configure script. If specified, the ensurepip module looks for setuptools and pip wheel packages in this directory: if both are present, these wheel packages are used instead of ensurepip bundled wheel packages.

    Some Linux distribution packaging policies recommend against bundling dependencies. For example, Fedora installs wheel packages in the /usr/share/python-wheels/ directory and don’t install the ensurepip._bundled package.

    (Contributed by Victor Stinner in bpo-42856.)

C API Changes

New Features

Porting to Python 3.10


  • 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.)