What’s New In Python 3.10




October 27, 2020

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.)

PEP604: 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

Now, type hints can 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 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.)

New Modules

  • None yet.

Improved Modules


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 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.)


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.)


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


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


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.)


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.)


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.)


  • 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.)

  • 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)


  • 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.


  • 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.)

Porting to Python 3.10

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

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

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

    #if PY_VERSION_HEX < 0x030900A4
    #  define Py_SET_TYPE(obj, type) ((Py_TYPE(obj) = (type)), (void)0)

    (Contributed by Dong-hee Na in bpo-39573.)

  • 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)

    (Contributed by Victor Stinner in bpo-39573.)

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

    #if PY_VERSION_HEX < 0x030900A4
    #  define Py_SET_SIZE(obj, size) ((Py_SIZE(obj) = (size)), (void)0)

    (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.)


  • 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.)