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¶
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
inttype 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.items()now all have a
mappingattribute that gives a
types.MappingProxyTypeobject wrapping the original dictionary. (Contributed by Dennis Sweeney in bpo-40890.)
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
typing module has a special annotation
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
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¶
The extended color functions added in ncurses 6.1 will be used transparently
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.)
types.NotImplementedType classes, providing a new set
of types readily interpretable by type checkers.
(Contributed by Bas van Beek in bpo-41810.)
LOAD_ATTRinstruction 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
-fno-semantic-interpositionis added to both the compile and link line. This speeds builds of the Python interpreter created with
gccby 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,
module_repr()(which the import system takes care of for you), the
__package__attribute (superseded by
__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
DeprecationWarningwill be raised as appropriate to help identify code which needs updating during this transition.
Removed special methods
complexclass. They always raised a
TypeError. (Contributed by Serhiy Storchaka in bpo-41974.)
ParserBase.error()method from the private and undocumented
_markupbasemodule has been removed.
html.parser.HTMLParseris the only subclass of
error()implementation has already been removed in Python 3.5. (Contributed by Berker Peksag in bpo-31844.)
unicodedata.ucnhash_CAPIattribute which was an internal PyCapsule object. The related private
_PyUnicode_Name_CAPIstructure 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.
C API Changes¶
PyDateTime_TIME_GET_TZINFO()macros have been added for accessing the
datetime.timeobjects. (Contributed by Zackery Spytz in bpo-30155.)
Porting to Python 3.10¶
PY_SSIZE_T_CLEANmacro must now be defined to use
Py_BuildValue()formats which use
Z#. See Parsing arguments and building values and the PEP 353. (Contributed by Victor Stinner in bpo-40943.)
Py_TYPE()is changed to the inline static function,
Py_TYPE(obj) = new_typemust 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) #endif
(Contributed by Dong-hee Na in bpo-39573.)
Py_REFCNT()is changed to the inline static function,
Py_REFCNT(obj) = new_refcntmust 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.)
Py_SIZE()is changed to the inline static function,
Py_SIZE(obj) = new_sizemust 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) #endif
(Contributed by Victor Stinner in bpo-39573.)
PyUnicode_New()to allocate Unicode object without initial data. (Contributed by Inada Naoki in bpo-36346.)
_PyUnicode_Name_CAPIstructure of the PyCapsule API
unicodedata.ucnhash_CAPIhas been moved to the internal C API. (Contributed by Victor Stinner in bpo-42157.)
PyObject_AsWriteBuffer()are removed. Please migrate to new buffer protocol;
PyBuffer_Release(). (Contributed by Inada Naoki in bpo-41103.)
Py_UNICODE*strings. (Contributed by Inada Naoki in bpo-41123.)