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What's New In Python 3.9

Editor:Łukasz Langa

This article explains the new features in Python 3.9, compared to 3.8. Python 3.9 was released on October 5, 2020. For full details, see the :ref:`changelog <changelog>`.

.. seealso::

    :pep:`596` - Python 3.9 Release Schedule

Summary -- Release highlights

New syntax features:

  • PEP 584, union operators added to dict;
  • PEP 585, type hinting generics in standard collections;
  • PEP 614, relaxed grammar restrictions on decorators.

New built-in features:

  • PEP 616, string methods to remove prefixes and suffixes.

New features in the standard library:

  • PEP 593, flexible function and variable annotations;
  • :func:`os.pidfd_open` added that allows process management without races and signals.

Interpreter improvements:

New library modules:

  • PEP 615, the IANA Time Zone Database is now present in the standard library in the :mod:`zoneinfo` module;
  • an implementation of a topological sort of a graph is now provided in the new :mod:`graphlib` module.

Release process changes:

  • PEP 602, CPython adopts an annual release cycle.

You should check for DeprecationWarning in your code

When Python 2.7 was still supported, a lot of functionality in Python 3 was kept for backward compatibility with Python 2.7. With the end of Python 2 support, these backward compatibility layers have been removed, or will be removed soon. Most of them emitted a :exc:`DeprecationWarning` warning for several years. For example, using collections.Mapping instead of collections.abc.Mapping emits a :exc:`DeprecationWarning` since Python 3.3, released in 2012.

Test your application with the :option:`-W` default command-line option to see :exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, or even with :option:`-W` error to treat them as errors. :ref:`Warnings Filter <warning-filter>` can be used to ignore warnings from third-party code.

Python 3.9 is the last version providing those Python 2 backward compatibility layers, to give more time to Python projects maintainers to organize the removal of the Python 2 support and add support for Python 3.9.

Aliases to :ref:`Abstract Base Classes <collections-abstract-base-classes>` in the :mod:`collections` module, like collections.Mapping alias to :class:`collections.abc.Mapping`, are kept for one last release for backward compatibility. They will be removed from Python 3.10.

More generally, try to run your tests in the :ref:`Python Development Mode <devmode>` which helps to prepare your code to make it compatible with the next Python version.

Note: a number of pre-existing deprecations were removed in this version of Python as well. Consult the :ref:`removed-in-python-39` section.

New Features

Dictionary Merge & Update Operators

Merge (|) and update (|=) operators have been added to the built-in :class:`dict` class. Those complement the existing dict.update and {**d1, **d2} methods of merging dictionaries.

Example:

>>> x = {"key1": "value1 from x", "key2": "value2 from x"}
>>> y = {"key2": "value2 from y", "key3": "value3 from y"}
>>> x | y
{'key1': 'value1 from x', 'key2': 'value2 from y', 'key3': 'value3 from y'}
>>> y | x
{'key2': 'value2 from x', 'key3': 'value3 from y', 'key1': 'value1 from x'}

See PEP 584 for a full description. (Contributed by Brandt Bucher in :issue:`36144`.)

New String Methods to Remove Prefixes and Suffixes

:meth:`str.removeprefix(prefix)<str.removeprefix>` and :meth:`str.removesuffix(suffix)<str.removesuffix>` have been added to easily remove an unneeded prefix or a suffix from a string. Corresponding bytes, bytearray, and collections.UserString methods have also been added. See PEP 616 for a full description. (Contributed by Dennis Sweeney in :issue:`39939`.)

Type Hinting Generics in Standard Collections

In type annotations you can now use built-in collection types such as list and dict as generic types instead of importing the corresponding capitalized types (e.g. List or Dict) from typing. Some other types in the standard library are also now generic, for example queue.Queue.

Example:

def greet_all(names: list[str]) -> None:
    for name in names:
        print("Hello", name)

See PEP 585 for more details. (Contributed by Guido van Rossum, Ethan Smith, and Batuhan Taşkaya in :issue:`39481`.)

New Parser

Python 3.9 uses a new parser, based on PEG instead of LL(1). The new parser's performance is roughly comparable to that of the old parser, but the PEG formalism is more flexible than LL(1) when it comes to designing new language features. We'll start using this flexibility in Python 3.10 and later.

The :mod:`ast` module uses the new parser and produces the same AST as the old parser.

In Python 3.10, the old parser will be deleted and so will all functionality that depends on it (primarily the :mod:`!parser` module, which has long been deprecated). In Python 3.9 only, you can switch back to the LL(1) parser using a command line switch (-X oldparser) or an environment variable (PYTHONOLDPARSER=1).

See PEP 617 for more details. (Contributed by Guido van Rossum, Pablo Galindo and Lysandros Nikolaou in :issue:`40334`.)

Other Language Changes

New Modules

zoneinfo

The :mod:`zoneinfo` module brings support for the IANA time zone database to the standard library. It adds :class:`zoneinfo.ZoneInfo`, a concrete :class:`datetime.tzinfo` implementation backed by the system's time zone data.

Example:

>>> from zoneinfo import ZoneInfo
>>> from datetime import datetime, timedelta

>>> # Daylight saving time
>>> dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo("America/Los_Angeles"))
>>> print(dt)
2020-10-31 12:00:00-07:00
>>> dt.tzname()
'PDT'

>>> # Standard time
>>> dt += timedelta(days=7)
>>> print(dt)
2020-11-07 12:00:00-08:00
>>> print(dt.tzname())
PST

As a fall-back source of data for platforms that don't ship the IANA database, the :pypi:`tzdata` module was released as a first-party package -- distributed via PyPI and maintained by the CPython core team.

.. seealso::

    :pep:`615` -- Support for the IANA Time Zone Database in the Standard Library
        PEP written and implemented by Paul Ganssle

graphlib

A new module, :mod:`graphlib`, was added that contains the :class:`graphlib.TopologicalSorter` class to offer functionality to perform topological sorting of graphs. (Contributed by Pablo Galindo, Tim Peters and Larry Hastings in :issue:`17005`.)

Improved Modules

ast

Added the indent option to :func:`~ast.dump` which allows it to produce a multiline indented output. (Contributed by Serhiy Storchaka in :issue:`37995`.)

Added :func:`ast.unparse` as a function in the :mod:`ast` module that can be used to unparse an :class:`ast.AST` object and produce a string with code that would produce an equivalent :class:`ast.AST` object when parsed. (Contributed by Pablo Galindo and Batuhan Taskaya in :issue:`38870`.)

Added docstrings to AST nodes that contains the ASDL signature used to construct that node. (Contributed by Batuhan Taskaya in :issue:`39638`.)

asyncio

Due to significant security concerns, the reuse_address parameter of :meth:`asyncio.loop.create_datagram_endpoint` is no longer supported. This is because of the behavior of the socket option SO_REUSEADDR in UDP. For more details, see the documentation for loop.create_datagram_endpoint(). (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in :issue:`37228`.)

Added a new :term:`coroutine` :meth:`~asyncio.loop.shutdown_default_executor` that schedules a shutdown for the default executor that waits on the :class:`~concurrent.futures.ThreadPoolExecutor` to finish closing. Also, :func:`asyncio.run` has been updated to use the new :term:`coroutine`. (Contributed by Kyle Stanley in :issue:`34037`.)

Added :class:`asyncio.PidfdChildWatcher`, a Linux-specific child watcher implementation that polls process file descriptors. (:issue:`38692`)

Added a new :term:`coroutine` :func:`asyncio.to_thread`. It is mainly used for running IO-bound functions in a separate thread to avoid blocking the event loop, and essentially works as a high-level version of :meth:`~asyncio.loop.run_in_executor` that can directly take keyword arguments. (Contributed by Kyle Stanley and Yury Selivanov in :issue:`32309`.)

When cancelling the task due to a timeout, :meth:`asyncio.wait_for` will now wait until the cancellation is complete also in the case when timeout is <= 0, like it does with positive timeouts. (Contributed by Elvis Pranskevichus in :issue:`32751`.)

:mod:`asyncio` now raises :exc:`TypeError` when calling incompatible methods with an :class:`ssl.SSLSocket` socket. (Contributed by Ido Michael in :issue:`37404`.)

compileall

Added new possibility to use hardlinks for duplicated .pyc files: hardlink_dupes parameter and --hardlink-dupes command line option. (Contributed by Lumír 'Frenzy' Balhar in :issue:`40495`.)

Added new options for path manipulation in resulting .pyc files: stripdir, prependdir, limit_sl_dest parameters and -s, -p, -e command line options. Added the possibility to specify the option for an optimization level multiple times. (Contributed by Lumír 'Frenzy' Balhar in :issue:`38112`.)

concurrent.futures

Added a new cancel_futures parameter to :meth:`concurrent.futures.Executor.shutdown` that cancels all pending futures which have not started running, instead of waiting for them to complete before shutting down the executor. (Contributed by Kyle Stanley in :issue:`39349`.)

Removed daemon threads from :class:`~concurrent.futures.ThreadPoolExecutor` and :class:`~concurrent.futures.ProcessPoolExecutor`. This improves compatibility with subinterpreters and predictability in their shutdown processes. (Contributed by Kyle Stanley in :issue:`39812`.)

Workers in :class:`~concurrent.futures.ProcessPoolExecutor` are now spawned on demand, only when there are no available idle workers to reuse. This optimizes startup overhead and reduces the amount of lost CPU time to idle workers. (Contributed by Kyle Stanley in :issue:`39207`.)

curses

Added :func:`curses.get_escdelay`, :func:`curses.set_escdelay`, :func:`curses.get_tabsize`, and :func:`curses.set_tabsize` functions. (Contributed by Anthony Sottile in :issue:`38312`.)

datetime

The :meth:`~datetime.date.isocalendar()` of :class:`datetime.date` and :meth:`~datetime.datetime.isocalendar()` of :class:`datetime.datetime` methods now returns a :func:`~collections.namedtuple` instead of a :class:`tuple`. (Contributed by Donghee Na in :issue:`24416`.)

distutils

The :command:`upload` command now creates SHA2-256 and Blake2b-256 hash digests. It skips MD5 on platforms that block MD5 digest. (Contributed by Christian Heimes in :issue:`40698`.)

fcntl

Added constants :const:`~fcntl.F_OFD_GETLK`, :const:`~fcntl.F_OFD_SETLK` and :const:`~fcntl.F_OFD_SETLKW`. (Contributed by Donghee Na in :issue:`38602`.)

ftplib

:class:`~ftplib.FTP` and :class:`~ftplib.FTP_TLS` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Donghee Na in :issue:`39259`.)

gc

When the garbage collector makes a collection in which some objects resurrect (they are reachable from outside the isolated cycles after the finalizers have been executed), do not block the collection of all objects that are still unreachable. (Contributed by Pablo Galindo and Tim Peters in :issue:`38379`.)

Added a new function :func:`gc.is_finalized` to check if an object has been finalized by the garbage collector. (Contributed by Pablo Galindo in :issue:`39322`.)

hashlib

The :mod:`hashlib` module can now use SHA3 hashes and SHAKE XOF from OpenSSL when available. (Contributed by Christian Heimes in :issue:`37630`.)

Builtin hash modules can now be disabled with ./configure --without-builtin-hashlib-hashes or selectively enabled with e.g. ./configure --with-builtin-hashlib-hashes=sha3,blake2 to force use of OpenSSL based implementation. (Contributed by Christian Heimes in :issue:`40479`)

http

HTTP status codes 103 EARLY_HINTS, 418 IM_A_TEAPOT and 425 TOO_EARLY are added to :class:`http.HTTPStatus`. (Contributed by Donghee Na in :issue:`39509` and Ross Rhodes in :issue:`39507`.)

IDLE and idlelib

Added option to toggle cursor blink off. (Contributed by Zackery Spytz in :issue:`4603`.)

Escape key now closes IDLE completion windows. (Contributed by Johnny Najera in :issue:`38944`.)

Added keywords to module name completion list. (Contributed by Terry J. Reedy in :issue:`37765`.)

New in 3.9 maintenance releases

Make IDLE invoke :func:`sys.excepthook` (when started without '-n'). User hooks were previously ignored. (Contributed by Ken Hilton in :issue:`43008`.)

The changes above have been backported to 3.8 maintenance releases.

Rearrange the settings dialog. Split the General tab into Windows and Shell/Ed tabs. Move help sources, which extend the Help menu, to the Extensions tab. Make space for new options and shorten the dialog. The latter makes the dialog better fit small screens. (Contributed by Terry Jan Reedy in :issue:`40468`.) Move the indent space setting from the Font tab to the new Windows tab. (Contributed by Mark Roseman and Terry Jan Reedy in :issue:`33962`.)

Apply syntax highlighting to .pyi files. (Contributed by Alex Waygood and Terry Jan Reedy in :issue:`45447`.)

imaplib

:class:`~imaplib.IMAP4` and :class:`~imaplib.IMAP4_SSL` now have an optional timeout parameter for their constructors. Also, the :meth:`~imaplib.IMAP4.open` method now has an optional timeout parameter with this change. The overridden methods of :class:`~imaplib.IMAP4_SSL` and :class:`~imaplib.IMAP4_stream` were applied to this change. (Contributed by Donghee Na in :issue:`38615`.)

:meth:`imaplib.IMAP4.unselect` is added. :meth:`imaplib.IMAP4.unselect` frees server's resources associated with the selected mailbox and returns the server to the authenticated state. This command performs the same actions as :meth:`imaplib.IMAP4.close`, except that no messages are permanently removed from the currently selected mailbox. (Contributed by Donghee Na in :issue:`40375`.)

importlib

To improve consistency with import statements, :func:`importlib.util.resolve_name` now raises :exc:`ImportError` instead of :exc:`ValueError` for invalid relative import attempts. (Contributed by Ngalim Siregar in :issue:`37444`.)

Import loaders which publish immutable module objects can now publish immutable packages in addition to individual modules. (Contributed by Dino Viehland in :issue:`39336`.)

Added :func:`importlib.resources.files` function with support for subdirectories in package data, matching backport in importlib_resources version 1.5. (Contributed by Jason R. Coombs in :issue:`39791`.)

Refreshed importlib.metadata from importlib_metadata version 1.6.1.

inspect

:attr:`inspect.BoundArguments.arguments` is changed from OrderedDict to regular dict. (Contributed by Inada Naoki in :issue:`36350` and :issue:`39775`.)

ipaddress

:mod:`ipaddress` now supports IPv6 Scoped Addresses (IPv6 address with suffix %<scope_id>).

Scoped IPv6 addresses can be parsed using :class:`ipaddress.IPv6Address`. If present, scope zone ID is available through the :attr:`~ipaddress.IPv6Address.scope_id` attribute. (Contributed by Oleksandr Pavliuk in :issue:`34788`.)

Starting with Python 3.9.5 the :mod:`ipaddress` module no longer accepts any leading zeros in IPv4 address strings. (Contributed by Christian Heimes in :issue:`36384`).

math

Expanded the :func:`math.gcd` function to handle multiple arguments. Formerly, it only supported two arguments. (Contributed by Serhiy Storchaka in :issue:`39648`.)

Added :func:`math.lcm`: return the least common multiple of specified arguments. (Contributed by Mark Dickinson, Ananthakrishnan and Serhiy Storchaka in :issue:`39479` and :issue:`39648`.)

Added :func:`math.nextafter`: return the next floating-point value after x towards y. (Contributed by Victor Stinner in :issue:`39288`.)

Added :func:`math.ulp`: return the value of the least significant bit of a float. (Contributed by Victor Stinner in :issue:`39310`.)

multiprocessing

The :class:`multiprocessing.SimpleQueue` class has a new :meth:`~multiprocessing.SimpleQueue.close` method to explicitly close the queue. (Contributed by Victor Stinner in :issue:`30966`.)

nntplib

:class:`!NNTP` and :class:`!NNTP_SSL` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Donghee Na in :issue:`39259`.)

os

Added :const:`~os.CLD_KILLED` and :const:`~os.CLD_STOPPED` for :attr:`!si_code`. (Contributed by Donghee Na in :issue:`38493`.)

Exposed the Linux-specific :func:`os.pidfd_open` (:issue:`38692`) and :const:`os.P_PIDFD` (:issue:`38713`) for process management with file descriptors.

The :func:`os.unsetenv` function is now also available on Windows. (Contributed by Victor Stinner in :issue:`39413`.)

The :func:`os.putenv` and :func:`os.unsetenv` functions are now always available. (Contributed by Victor Stinner in :issue:`39395`.)

Added :func:`os.waitstatus_to_exitcode` function: convert a wait status to an exit code. (Contributed by Victor Stinner in :issue:`40094`.)

pathlib

Added :meth:`pathlib.Path.readlink()` which acts similarly to :func:`os.readlink`. (Contributed by Girts Folkmanis in :issue:`30618`)

pdb

On Windows now :class:`~pdb.Pdb` supports ~/.pdbrc. (Contributed by Tim Hopper and Dan Lidral-Porter in :issue:`20523`.)

poplib

:class:`~poplib.POP3` and :class:`~poplib.POP3_SSL` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Donghee Na in :issue:`39259`.)

pprint

:mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`. (Contributed by Carl Bordum Hansen in :issue:`37376`.)

pydoc

The documentation string is now shown not only for class, function, method etc, but for any object that has its own __doc__ attribute. (Contributed by Serhiy Storchaka in :issue:`40257`.)

random

Added a new :attr:`random.Random.randbytes` method: generate random bytes. (Contributed by Victor Stinner in :issue:`40286`.)

signal

Exposed the Linux-specific :func:`signal.pidfd_send_signal` for sending to signals to a process using a file descriptor instead of a pid. (:issue:`38712`)

smtplib

:class:`~smtplib.SMTP` and :class:`~smtplib.SMTP_SSL` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Donghee Na in :issue:`39259`.)

:class:`~smtplib.LMTP` constructor now has an optional timeout parameter. (Contributed by Donghee Na in :issue:`39329`.)

socket

The :mod:`socket` module now exports the :const:`~socket.CAN_RAW_JOIN_FILTERS` constant on Linux 4.1 and greater. (Contributed by Stefan Tatschner and Zackery Spytz in :issue:`25780`.)

The socket module now supports the :const:`~socket.CAN_J1939` protocol on platforms that support it. (Contributed by Karl Ding in :issue:`40291`.)

The socket module now has the :func:`socket.send_fds` and :func:`socket.recv_fds` functions. (Contributed by Joannah Nanjekye, Shinya Okano and Victor Stinner in :issue:`28724`.)

time

On AIX, :func:`~time.thread_time` is now implemented with thread_cputime() which has nanosecond resolution, rather than clock_gettime(CLOCK_THREAD_CPUTIME_ID) which has a resolution of 10 milliseconds. (Contributed by Batuhan Taskaya in :issue:`40192`)

sys

Added a new :data:`sys.platlibdir` attribute: name of the platform-specific library directory. It is used to build the path of standard library and the paths of installed extension modules. It is equal to "lib" on most platforms. On Fedora and SuSE, it is equal to "lib64" on 64-bit platforms. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)

Previously, :data:`sys.stderr` was block-buffered when non-interactive. Now stderr defaults to always being line-buffered. (Contributed by Jendrik Seipp in :issue:`13601`.)

tracemalloc

Added :func:`tracemalloc.reset_peak` to set the peak size of traced memory blocks to the current size, to measure the peak of specific pieces of code. (Contributed by Huon Wilson in :issue:`40630`.)

typing

PEP 593 introduced an :data:`typing.Annotated` type to decorate existing types with context-specific metadata and new include_extras parameter to :func:`typing.get_type_hints` to access the metadata at runtime. (Contributed by Till Varoquaux and Konstantin Kashin.)

unicodedata

The Unicode database has been updated to version 13.0.0. (:issue:`39926`).

venv

The activation scripts provided by :mod:`venv` now all specify their prompt customization consistently by always using the value specified by __VENV_PROMPT__. Previously some scripts unconditionally used __VENV_PROMPT__, others only if it happened to be set (which was the default case), and one used __VENV_NAME__ instead. (Contributed by Brett Cannon in :issue:`37663`.)

xml

White space characters within attributes are now preserved when serializing :mod:`xml.etree.ElementTree` to XML file. EOLNs are no longer normalized to "n". This is the result of discussion about how to interpret section 2.11 of XML spec. (Contributed by Mefistotelis in :issue:`39011`.)

Optimizations

  • Optimized the idiom for assignment a temporary variable in comprehensions. Now for y in [expr] in comprehensions is as fast as a simple assignment y = expr. For example:

    sums = [s for s in [0] for x in data for s in [s + x]]

    Unlike the := operator this idiom does not leak a variable to the outer scope.

    (Contributed by Serhiy Storchaka in :issue:`32856`.)

  • Optimized signal handling in multithreaded applications. If a thread different than the main thread gets a signal, the bytecode evaluation loop is no longer interrupted at each bytecode instruction to check for pending signals which cannot be handled. Only the main thread of the main interpreter can handle signals.

    Previously, the bytecode evaluation loop was interrupted at each instruction until the main thread handles signals. (Contributed by Victor Stinner in :issue:`40010`.)

  • Optimized the :mod:`subprocess` module on FreeBSD using closefrom(). (Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak and Victor Stinner in :issue:`38061`.)

  • :c:func:`PyLong_FromDouble` is now up to 1.87x faster for values that fit into :c:expr:`long`. (Contributed by Sergey Fedoseev in :issue:`37986`.)

  • A number of Python builtins (:class:`range`, :class:`tuple`, :class:`set`, :class:`frozenset`, :class:`list`, :class:`dict`) are now sped up by using PEP 590 vectorcall protocol. (Contributed by Donghee Na, Mark Shannon, Jeroen Demeyer and Petr Viktorin in :issue:`37207`.)

  • Optimized :func:`~set.difference_update` for the case when the other set is much larger than the base set. (Suggested by Evgeny Kapun with code contributed by Michele Orrù in :issue:`8425`.)

  • Python's small object allocator (obmalloc.c) now allows (no more than) one empty arena to remain available for immediate reuse, without returning it to the OS. This prevents thrashing in simple loops where an arena could be created and destroyed anew on each iteration. (Contributed by Tim Peters in :issue:`37257`.)

  • :term:`floor division` of float operation now has a better performance. Also the message of :exc:`ZeroDivisionError` for this operation is updated. (Contributed by Donghee Na in :issue:`39434`.)

  • Decoding short ASCII strings with UTF-8 and ascii codecs is now about 15% faster. (Contributed by Inada Naoki in :issue:`37348`.)

Here's a summary of performance improvements from Python 3.4 through Python 3.9:

Python version                       3.4     3.5     3.6     3.7     3.8    3.9
--------------                       ---     ---     ---     ---     ---    ---

Variable and attribute read access:
    read_local                       7.1     7.1     5.4     5.1     3.9    3.9
    read_nonlocal                    7.1     8.1     5.8     5.4     4.4    4.5
    read_global                     15.5    19.0    14.3    13.6     7.6    7.8
    read_builtin                    21.1    21.6    18.5    19.0     7.5    7.8
    read_classvar_from_class        25.6    26.5    20.7    19.5    18.4   17.9
    read_classvar_from_instance     22.8    23.5    18.8    17.1    16.4   16.9
    read_instancevar                32.4    33.1    28.0    26.3    25.4   25.3
    read_instancevar_slots          27.8    31.3    20.8    20.8    20.2   20.5
    read_namedtuple                 73.8    57.5    45.0    46.8    18.4   18.7
    read_boundmethod                37.6    37.9    29.6    26.9    27.7   41.1

Variable and attribute write access:
    write_local                      8.7     9.3     5.5     5.3     4.3    4.3
    write_nonlocal                  10.5    11.1     5.6     5.5     4.7    4.8
    write_global                    19.7    21.2    18.0    18.0    15.8   16.7
    write_classvar                  92.9    96.0   104.6   102.1    39.2   39.8
    write_instancevar               44.6    45.8    40.0    38.9    35.5   37.4
    write_instancevar_slots         35.6    36.1    27.3    26.6    25.7   25.8

Data structure read access:
    read_list                       24.2    24.5    20.8    20.8    19.0   19.5
    read_deque                      24.7    25.5    20.2    20.6    19.8   20.2
    read_dict                       24.3    25.7    22.3    23.0    21.0   22.4
    read_strdict                    22.6    24.3    19.5    21.2    18.9   21.5

Data structure write access:
    write_list                      27.1    28.5    22.5    21.6    20.0   20.0
    write_deque                     28.7    30.1    22.7    21.8    23.5   21.7
    write_dict                      31.4    33.3    29.3    29.2    24.7   25.4
    write_strdict                   28.4    29.9    27.5    25.2    23.1   24.5

Stack (or queue) operations:
    list_append_pop                 93.4   112.7    75.4    74.2    50.8   50.6
    deque_append_pop                43.5    57.0    49.4    49.2    42.5   44.2
    deque_append_popleft            43.7    57.3    49.7    49.7    42.8   46.4

Timing loop:
    loop_overhead                    0.5     0.6     0.4     0.3     0.3    0.3

These results were generated from the variable access benchmark script at: Tools/scripts/var_access_benchmark.py. The benchmark script displays timings in nanoseconds. The benchmarks were measured on an Intel® Core™ i7-4960HQ processor running the macOS 64-bit builds found at python.org.

Deprecated

Removed

Porting to Python 3.9

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

Changes in the Python API

Changes in the C API

  • Instances of :ref:`heap-allocated types <heap-types>` (such as those created with :c:func:`PyType_FromSpec` and similar APIs) hold a reference to their type object since Python 3.8. As indicated in the "Changes in the C API" of Python 3.8, for the vast majority of cases, there should be no side effect but for types that have a custom :c:member:`~PyTypeObject.tp_traverse` function, ensure that all custom tp_traverse functions of heap-allocated types visit the object's type.

    Example:

    int
foo_traverse(foo_struct *self, visitproc visit, void *arg) {
// Rest of the traverse function
#if PY_VERSION_HEX >= 0x03090000
    // This was not needed before Python 3.9 (Python issue 35810 and 40217)
    Py_VISIT(Py_TYPE(self));
#endif
}

    If your traverse function delegates to tp_traverse of its base class (or another type), ensure that Py_TYPE(self) is visited only once. Note that only :ref:`heap type <heap-types>` are expected to visit the type in tp_traverse.

    For example, if your tp_traverse function includes:

    base->tp_traverse(self, visit, arg)

    then add:

    #if PY_VERSION_HEX >= 0x03090000
    // This was not needed before Python 3.9 (bpo-35810 and bpo-40217)
    if (base->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        // a heap type's tp_traverse already visited Py_TYPE(self)
    } else {
        Py_VISIT(Py_TYPE(self));
    }
#else

    (See :issue:`35810` and :issue:`40217` for more information.)

  • The functions PyEval_CallObject, PyEval_CallFunction, PyEval_CallMethod and PyEval_CallObjectWithKeywords are deprecated. Use :c:func:`PyObject_Call` and its variants instead. (See more details in :issue:`29548`.)

CPython bytecode changes

Build Changes

  • Added --with-platlibdir option to the configure script: name of the platform-specific library directory, stored in the new :data:`sys.platlibdir` attribute. See :data:`sys.platlibdir` attribute for more information. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)
  • The COUNT_ALLOCS special build macro has been removed. (Contributed by Victor Stinner in :issue:`39489`.)
  • On non-Windows platforms, the :c:func:`!setenv` and :c:func:`!unsetenv` functions are now required to build Python. (Contributed by Victor Stinner in :issue:`39395`.)
  • On non-Windows platforms, creating bdist_wininst installers is now officially unsupported. (See :issue:`10945` for more details.)
  • When building Python on macOS from source, _tkinter now links with non-system Tcl and Tk frameworks if they are installed in /Library/Frameworks, as had been the case on older releases of macOS. If a macOS SDK is explicitly configured, by using :option:`--enable-universalsdk` or -isysroot, only the SDK itself is searched. The default behavior can still be overridden with --with-tcltk-includes and --with-tcltk-libs. (Contributed by Ned Deily in :issue:`34956`.)
  • Python can now be built for Windows 10 ARM64. (Contributed by Steve Dower in :issue:`33125`.)
  • Some individual tests are now skipped when --pgo is used. The tests in question increased the PGO task time significantly and likely didn't help improve optimization of the final executable. This speeds up the task by a factor of about 15x. Running the full unit test suite is slow. This change may result in a slightly less optimized build since not as many code branches will be executed. If you are willing to wait for the much slower build, the old behavior can be restored using ./configure [..] PROFILE_TASK="-m test --pgo-extended". We make no guarantees as to which PGO task set produces a faster build. Users who care should run their own relevant benchmarks as results can depend on the environment, workload, and compiler tool chain. (See :issue:`36044` and :issue:`37707` for more details.)

C API Changes

New Features

Porting to Python 3.9

Removed

  • Excluded PyFPE_START_PROTECT() and PyFPE_END_PROTECT() macros of pyfpe.h from the limited C API. (Contributed by Victor Stinner in :issue:`38835`.)

  • The tp_print slot of :ref:`PyTypeObject <type-structs>` has been removed. It was used for printing objects to files in Python 2.7 and before. Since Python 3.0, it has been ignored and unused. (Contributed by Jeroen Demeyer in :issue:`36974`.)

  • Changes in the limited C API (if Py_LIMITED_API macro is defined):

    • Excluded the following functions from the limited C API:
      • PyThreadState_DeleteCurrent() (Contributed by Joannah Nanjekye in :issue:`37878`.)
      • _Py_CheckRecursionLimit
      • _Py_NewReference()
      • _Py_ForgetReference()
      • _PyTraceMalloc_NewReference()
      • _Py_GetRefTotal()
      • The trashcan mechanism which never worked in the limited C API.
      • PyTrash_UNWIND_LEVEL
      • Py_TRASHCAN_BEGIN_CONDITION
      • Py_TRASHCAN_BEGIN
      • Py_TRASHCAN_END
      • Py_TRASHCAN_SAFE_BEGIN
      • Py_TRASHCAN_SAFE_END
    • Moved following functions and definitions to the internal C API:
      • _PyDebug_PrintTotalRefs()
      • _Py_PrintReferences()
      • _Py_PrintReferenceAddresses()
      • _Py_tracemalloc_config
      • _Py_AddToAllObjects() (specific to Py_TRACE_REFS build)

    (Contributed by Victor Stinner in :issue:`38644` and :issue:`39542`.)

  • Removed _PyRuntime.getframe hook and removed _PyThreadState_GetFrame macro which was an alias to _PyRuntime.getframe. They were only exposed by the internal C API. Removed also PyThreadFrameGetter type. (Contributed by Victor Stinner in :issue:`39946`.)

  • Removed the following functions from the C API. Call :c:func:`PyGC_Collect` explicitly to clear all free lists. (Contributed by Inada Naoki and Victor Stinner in :issue:`37340`, :issue:`38896` and :issue:`40428`.)

    • PyAsyncGen_ClearFreeLists()
    • PyContext_ClearFreeList()
    • PyDict_ClearFreeList()
    • PyFloat_ClearFreeList()
    • PyFrame_ClearFreeList()
    • PyList_ClearFreeList()
    • PyMethod_ClearFreeList() and PyCFunction_ClearFreeList(): the free lists of bound method objects have been removed.
    • PySet_ClearFreeList(): the set free list has been removed in Python 3.4.
    • PyTuple_ClearFreeList()
    • PyUnicode_ClearFreeList(): the Unicode free list has been removed in Python 3.3.

  • Removed _PyUnicode_ClearStaticStrings() function. (Contributed by Victor Stinner in :issue:`39465`.)

  • Removed Py_UNICODE_MATCH. It has been deprecated by PEP 393, and broken since Python 3.3. The :c:func:`PyUnicode_Tailmatch` function can be used instead. (Contributed by Inada Naoki in :issue:`36346`.)

  • Cleaned header files of interfaces defined but with no implementation. The public API symbols being removed are: _PyBytes_InsertThousandsGroupingLocale, _PyBytes_InsertThousandsGrouping, _Py_InitializeFromArgs, _Py_InitializeFromWideArgs, _PyFloat_Repr, _PyFloat_Digits, _PyFloat_DigitsInit, PyFrame_ExtendStack, _PyAIterWrapper_Type, PyNullImporter_Type, PyCmpWrapper_Type, PySortWrapper_Type, PyNoArgsFunction. (Contributed by Pablo Galindo Salgado in :issue:`39372`.)

Notable changes in Python 3.9.1

typing

The behavior of :class:`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 respect 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 :exc:`TypeError` exception during equality comparisons if any of their parameters are not :term:`hashable`. 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 :issue:`42345`.)

macOS 11.0 (Big Sur) and Apple Silicon Mac support

As of 3.9.1, Python now fully supports building and running on macOS 11.0 (Big Sur) and on Apple Silicon Macs (based on the ARM64 architecture). A new universal build variant, universal2, is now available to natively support both ARM64 and Intel 64 in one set of executables. Binaries can also now be built on current versions of macOS to be deployed on a range of older macOS versions (tested to 10.9) while making some newer OS functions and options conditionally available based on the operating system version in use at runtime ("weaklinking").

(Contributed by Ronald Oussoren and Lawrence D'Anna in :issue:`41100`.)

Notable changes in Python 3.9.2

collections.abc

:class:`collections.abc.Callable` generic now flattens type parameters, similar to what :data:`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, :class:`types.GenericAlias` can now be subclassed, and a subclass will be returned when subscripting the :class:`collections.abc.Callable` type. Code which accesses the arguments via :func:`typing.get_args` or __args__ need to account for this change. A :exc:`DeprecationWarning` may be emitted for invalid forms of parameterizing :class:`collections.abc.Callable` which may have passed silently in Python 3.9.1. This :exc:`DeprecationWarning` will become a :exc:`TypeError` in Python 3.10. (Contributed by Ken Jin in :issue:`42195`.)

urllib.parse

Earlier Python versions allowed using both ; and & as query parameter separators in :func:`urllib.parse.parse_qs` and :func:`urllib.parse.parse_qsl`. Due to security concerns, and to conform with newer W3C recommendations, this has been changed to allow only a single separator key, with & as the default. This change also affects :func:`!cgi.parse` and :func:`!cgi.parse_multipart` as they use the affected functions internally. For more details, please see their respective documentation. (Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)

Notable changes in Python 3.9.3

A security fix alters the :class:`ftplib.FTP` behavior to not trust the IPv4 address sent from the remote server when setting up a passive data channel. We reuse the ftp server IP address instead. For unusual code requiring the old behavior, set a trust_server_pasv_ipv4_address attribute on your FTP instance to True. (See :gh:`87451`)

Notable changes in Python 3.9.5

urllib.parse

The presence of newline or tab characters in parts of a URL allows for some forms of attacks. Following the WHATWG specification that updates RFC 3986, ASCII newline \n, \r and tab \t characters are stripped from the URL by the parser in :mod:`urllib.parse` preventing such attacks. The removal characters are controlled by a new module level variable urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE. (See :gh:`88048`)

Notable security feature in 3.9.14

Converting between :class:`int` and :class:`str` in bases other than 2 (binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal) now raises a :exc:`ValueError` if the number of digits in string form is above a limit to avoid potential denial of service attacks due to the algorithmic complexity. This is a mitigation for :cve:`2020-10735`. This limit can be configured or disabled by environment variable, command line flag, or :mod:`sys` APIs. See the :ref:`integer string conversion length limitation <int_max_str_digits>` documentation. The default limit is 4300 digits in string form.

Notable changes in 3.9.17

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