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 changelog <changelog>.

596 - Python 3.9 Release Schedule

Summary -- Release highlights

New syntax features:

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

New built-in features:

616, string methods to remove prefixes and suffixes.

New features in the standard library:

593, flexible function and variable annotations;
os.pidfd_open added that allows process management without races and signals.

Interpreter improvements:

573, fast access to module state from methods of C extension types;
617, CPython now uses a new parser based on PEG;
a number of Python builtins (range, tuple, set, frozenset, list, dict) are now sped up using 590 vectorcall;
garbage collection does not block on resurrected objects;
a number of Python modules (!_abc, !audioop, !_bz2, !_codecs, !_contextvars, !_crypt, !_functools, !_json, !_locale, math, operator, resource, time, !_weakref) now use multiphase initialization as defined by PEP 489;
a number of standard library modules (!audioop, ast, grp, !_hashlib, pwd, !_posixsubprocess, random, select, struct, termios, zlib) are now using the stable ABI defined by PEP 384.

New library modules:

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

Release process changes:

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 DeprecationWarning warning for several years. For example, using collections.Mapping instead of collections.abc.Mapping emits a DeprecationWarning since Python 3.3, released in 2012.

Test your application with the -W default command-line option to see DeprecationWarning and PendingDeprecationWarning, or even with -W error to treat them as errors. 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 Abstract Base Classes <collections-abstract-base-classes> in the collections module, like collections.Mapping alias to 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 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 removed-in-python-39 section.

New Features

Dictionary Merge & Update Operators

Merge (|) and update (|=) operators have been added to the built-in 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 584 for a full description. (Contributed by Brandt Bucher in 36144.)

New String Methods to Remove Prefixes and Suffixes

str.removeprefix(prefix)<str.removeprefix> and 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 616 for a full description. (Contributed by Dennis Sweeney in 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 585 for more details. (Contributed by Guido van Rossum, Ethan Smith, and Batuhan Taşkaya in 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 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 !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 617 for more details. (Contributed by Guido van Rossum, Pablo Galindo and Lysandros Nikolaou in 40334.)

Other Language Changes

__import__ now raises ImportError instead of ValueError, which used to occur when a relative import went past its top-level package. (Contributed by Ngalim Siregar in 37444.)
Python now gets the absolute path of the script filename specified on the command line (ex: python3 script.py): the __file__ attribute of the __main__ module became an absolute path, rather than a relative path. These paths now remain valid after the current directory is changed by os.chdir. As a side effect, the traceback also displays the absolute path for __main__ module frames in this case. (Contributed by Victor Stinner in 20443.)
In the Python Development Mode <devmode> and in debug build <debug-build>, the encoding and errors arguments are now checked for string encoding and decoding operations. Examples: open, str.encode and bytes.decode.

By default, for best performance, the errors argument is only checked at the first encoding/decoding error and the encoding argument is sometimes ignored for empty strings. (Contributed by Victor Stinner in 37388.)
"".replace("", s, n) now returns s instead of an empty string for all non-zero n. It is now consistent with "".replace("", s). There are similar changes for bytes and bytearray objects. (Contributed by Serhiy Storchaka in 28029.)
Any valid expression can now be used as a decorator. Previously, the grammar was much more restrictive. See 614 for details. (Contributed by Brandt Bucher in 39702.)
Improved help for the typing module. Docstrings are now shown for all special forms and special generic aliases (like Union and List). Using help with generic alias like List[int] will show the help for the correspondent concrete type (list in this case). (Contributed by Serhiy Storchaka in 40257.)
Parallel running of ~agen.aclose / ~agen.asend / ~agen.athrow is now prohibited, and ag_running now reflects the actual running status of the async generator. (Contributed by Yury Selivanov in 30773.)
Unexpected errors in calling the __iter__ method are no longer masked by TypeError in the in operator and functions ~operator.contains, ~operator.indexOf and ~operator.countOf of the operator module. (Contributed by Serhiy Storchaka in 40824.)
Unparenthesized lambda expressions can no longer be the expression part in an if clause in comprehensions and generator expressions. See 41848 and 43755 for details.

New Modules

zoneinfo

The zoneinfo module brings support for the IANA time zone database to the standard library. It adds zoneinfo.ZoneInfo, a concrete 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 tzdata module was released as a first-party package -- distributed via PyPI and maintained by the CPython core team.

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

graphlib

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

Improved Modules

ast

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

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

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

asyncio

Due to significant security concerns, the reuse_address parameter of 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 37228.)

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

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

Added a new coroutine 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 ~asyncio.loop.run_in_executor that can directly take keyword arguments. (Contributed by Kyle Stanley and Yury Selivanov in 32309.)

When cancelling the task due to a timeout, 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 32751.)

asyncio now raises TypeError when calling incompatible methods with an ssl.SSLSocket socket. (Contributed by Ido Michael in 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 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 38112.)

concurrent.futures

Added a new cancel_futures parameter to 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 39349.)

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

Workers in ~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 39207.)

curses

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

datetime

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

distutils

The 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 40698.)

fcntl

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

ftplib

~ftplib.FTP and ~ftplib.FTP_TLS now raise a ValueError if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Donghee Na in 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 38379.)

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

hashlib

The hashlib module can now use SHA3 hashes and SHAKE XOF from OpenSSL when available. (Contributed by Christian Heimes in 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 40479)

http

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

IDLE and idlelib

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

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

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

New in 3.9 maintenance releases

Make IDLE invoke sys.excepthook (when started without '-n'). User hooks were previously ignored. (Contributed by Ken Hilton in 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 40468.) Move the indent space setting from the Font tab to the new Windows tab. (Contributed by Mark Roseman and Terry Jan Reedy in 33962.)

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

imaplib

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

imaplib.IMAP4.unselect is added. 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 imaplib.IMAP4.close, except that no messages are permanently removed from the currently selected mailbox. (Contributed by Donghee Na in 40375.)

importlib

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

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

Added 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 39791.)

Refreshed importlib.metadata from importlib_metadata version 1.6.1.

inspect

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

ipaddress

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

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

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

math

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

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

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

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

multiprocessing

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

nntplib

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

os

Added ~os.CLD_KILLED and ~os.CLD_STOPPED for !si_code. (Contributed by Donghee Na in 38493.)

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

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

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

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

pathlib

Added pathlib.Path.readlink() which acts similarly to os.readlink. (Contributed by Girts Folkmanis in 30618)

pdb

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

poplib

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

pprint

pprint can now pretty-print types.SimpleNamespace. (Contributed by Carl Bordum Hansen in 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 40257.)

random

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

signal

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

smtplib

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

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

socket

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

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

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

time

On AIX, ~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 40192)

sys

Added a new 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 1294959.)

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

tracemalloc

Added 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 40630.)

typing

593 introduced an typing.Annotated type to decorate existing types with context-specific metadata and new include_extras parameter to 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. (39926).

venv

The activation scripts provided by 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 37663.)

xml

White space characters within attributes are now preserved when serializing 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 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 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 40010.)
Optimized the subprocess module on FreeBSD using closefrom(). (Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak and Victor Stinner in 38061.)
:cPyLong_FromDouble is now up to 1.87x faster for values that fit into :clong. (Contributed by Sergey Fedoseev in 37986.)
A number of Python builtins (range, tuple, set, frozenset, list, dict) are now sped up by using 590 vectorcall protocol. (Contributed by Donghee Na, Mark Shannon, Jeroen Demeyer and Petr Viktorin in 37207.)
Optimized ~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 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 37257.)
floor division of float operation now has a better performance. Also the message of ZeroDivisionError for this operation is updated. (Contributed by Donghee Na in 39434.)
Decoding short ASCII strings with UTF-8 and ascii codecs is now about 15% faster. (Contributed by Inada Naoki in 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

The distutils bdist_msi command is now deprecated, use bdist_wheel (wheel packages) instead. (Contributed by Hugo van Kemenade in 39586.)
Currently math.factorial accepts float instances with non-negative integer values (like 5.0). It raises a ValueError for non-integral and negative floats. It is now deprecated. In future Python versions it will raise a TypeError for all floats. (Contributed by Serhiy Storchaka in 37315.)
The !parser and !symbol modules are deprecated and will be removed in future versions of Python. For the majority of use cases, users can leverage the Abstract Syntax Tree (AST) generation and compilation stage, using the ast module.
The Public C API functions :c!PyParser_SimpleParseStringFlags, :c!PyParser_SimpleParseStringFlagsFilename, :c!PyParser_SimpleParseFileFlags and :c!PyNode_Compile are deprecated and will be removed in Python 3.10 together with the old parser.
Using NotImplemented in a boolean context has been deprecated, as it is almost exclusively the result of incorrect rich comparator implementations. It will be made a TypeError in a future version of Python. (Contributed by Josh Rosenberg in 35712.)
The random module currently accepts any hashable type as a possible seed value. Unfortunately, some of those types are not guaranteed to have a deterministic hash value. After Python 3.9, the module will restrict its seeds to None, int, float, str, bytes, and bytearray.
Opening the ~gzip.GzipFile file for writing without specifying the mode argument is deprecated. In future Python versions it will always be opened for reading by default. Specify the mode argument for opening it for writing and silencing a warning. (Contributed by Serhiy Storchaka in 28286.)
Deprecated the split() method of !_tkinter.TkappType in favour of the splitlist() method which has more consistent and predicable behavior. (Contributed by Serhiy Storchaka in 38371.)
The explicit passing of coroutine objects to asyncio.wait has been deprecated and will be removed in version 3.11. (Contributed by Yury Selivanov and Kyle Stanley in 34790.)
binhex4 and hexbin4 standards are now deprecated. The !binhex module and the following binascii functions are now deprecated:
- !b2a_hqx, !a2b_hqx
- !rlecode_hqx, !rledecode_hqx
(Contributed by Victor Stinner in 39353.)
ast classes slice, Index and ExtSlice are considered deprecated and will be removed in future Python versions. value itself should be used instead of Index(value). Tuple(slices, Load()) should be used instead of ExtSlice(slices). (Contributed by Serhiy Storchaka in 34822.)
ast classes Suite, Param, AugLoad and AugStore are considered deprecated and will be removed in future Python versions. They were not generated by the parser and not accepted by the code generator in Python 3. (Contributed by Batuhan Taskaya in 39639 and 39969 and Serhiy Storchaka in 39988.)
The :c!PyEval_InitThreads and :c!PyEval_ThreadsInitialized functions are now deprecated and will be removed in Python 3.11. Calling :c!PyEval_InitThreads now does nothing. The GIL is initialized by :cPy_Initialize since Python 3.7. (Contributed by Victor Stinner in 39877.)
Passing None as the first argument to the shlex.split function has been deprecated. (Contributed by Zackery Spytz in 33262.)
!smtpd.MailmanProxy is now deprecated as it is unusable without an external module, mailman. (Contributed by Samuel Colvin in 35800.)
The !lib2to3 module now emits a PendingDeprecationWarning. Python 3.9 switched to a PEG parser (see 617), and Python 3.10 may include new language syntax that is not parsable by lib2to3's LL(1) parser. The !lib2to3 module may be removed from the standard library in a future Python version. Consider third-party alternatives such as LibCST or parso. (Contributed by Carl Meyer in 40360.)
The random parameter of random.shuffle has been deprecated. (Contributed by Raymond Hettinger in 40465)

Removed

The erroneous version at !unittest.mock.__version__ has been removed.
!nntplib.NNTP: xpath() and xgtitle() methods have been removed. These methods are deprecated since Python 3.3. Generally, these extensions are not supported or not enabled by NNTP server administrators. For xgtitle(), please use !nntplib.NNTP.descriptions or !nntplib.NNTP.description instead. (Contributed by Donghee Na in 39366.)
array.array: tostring() and fromstring() methods have been removed. They were aliases to tobytes() and frombytes(), deprecated since Python 3.2. (Contributed by Victor Stinner in 38916.)
The undocumented sys.callstats() function has been removed. Since Python 3.7, it was deprecated and always returned None. It required a special build option CALL_PROFILE which was already removed in Python 3.7. (Contributed by Victor Stinner in 37414.)
The sys.getcheckinterval() and sys.setcheckinterval() functions have been removed. They were deprecated since Python 3.2. Use sys.getswitchinterval and sys.setswitchinterval instead. (Contributed by Victor Stinner in 37392.)
The C function PyImport_Cleanup() has been removed. It was documented as: "Empty the module table. For internal use only." (Contributed by Victor Stinner in 36710.)
_dummy_thread and dummy_threading modules have been removed. These modules were deprecated since Python 3.7 which requires threading support. (Contributed by Victor Stinner in 37312.)
aifc.openfp() alias to aifc.open(), sunau.openfp() alias to sunau.open(), and wave.openfp() alias to wave.open() have been removed. They were deprecated since Python 3.7. (Contributed by Victor Stinner in 37320.)
The !isAlive() method of threading.Thread has been removed. It was deprecated since Python 3.8. Use ~threading.Thread.is_alive() instead. (Contributed by Donghee Na in 37804.)
Methods getchildren() and getiterator() of classes ~xml.etree.ElementTree.ElementTree and ~xml.etree.ElementTree.Element in the ~xml.etree.ElementTree module have been removed. They were deprecated in Python 3.2. Use iter(x) or list(x) instead of x.getchildren() and x.iter() or list(x.iter()) instead of x.getiterator(). (Contributed by Serhiy Storchaka in 36543.)
The old plistlib API has been removed, it was deprecated since Python 3.4. Use the ~plistlib.load, ~plistlib.loads, ~plistlib.dump, and ~plistlib.dumps functions. Additionally, the use_builtin_types parameter was removed, standard bytes objects are always used instead. (Contributed by Jon Janzen in 36409.)
The C function PyGen_NeedsFinalizing has been removed. It was not documented, tested, or used anywhere within CPython after the implementation of 442. Patch by Joannah Nanjekye. (Contributed by Joannah Nanjekye in 15088)
base64.encodestring() and base64.decodestring(), aliases deprecated since Python 3.1, have been removed: use base64.encodebytes and base64.decodebytes instead. (Contributed by Victor Stinner in 39351.)
fractions.gcd() function has been removed, it was deprecated since Python 3.5 (22486): use math.gcd instead. (Contributed by Victor Stinner in 39350.)
The buffering parameter of bz2.BZ2File has been removed. Since Python 3.0, it was ignored and using it emitted a DeprecationWarning. Pass an open file object to control how the file is opened. (Contributed by Victor Stinner in 39357.)
The encoding parameter of json.loads has been removed. As of Python 3.1, it was deprecated and ignored; using it has emitted a DeprecationWarning since Python 3.8. (Contributed by Inada Naoki in 39377)
with (await asyncio.lock): and with (yield from asyncio.lock): statements are not longer supported, use async with lock instead. The same is correct for asyncio.Condition and asyncio.Semaphore. (Contributed by Andrew Svetlov in 34793.)
The !sys.getcounts function, the -X showalloccount command line option and the show_alloc_count field of the C structure :cPyConfig have been removed. They required a special Python build by defining COUNT_ALLOCS macro. (Contributed by Victor Stinner in 39489.)
The _field_types attribute of the typing.NamedTuple class has been removed. It was deprecated since Python 3.8. Use the __annotations__ attribute instead. (Contributed by Serhiy Storchaka in 40182.)
The !symtable.SymbolTable.has_exec method has been removed. It was deprecated since 2006, and only returning False when it's called. (Contributed by Batuhan Taskaya in 40208)
The !asyncio.Task.current_task and !asyncio.Task.all_tasks have been removed. They were deprecated since Python 3.7 and you can use asyncio.current_task and asyncio.all_tasks instead. (Contributed by Rémi Lapeyre in 40967)
The unescape() method in the html.parser.HTMLParser class has been removed (it was deprecated since Python 3.4). html.unescape should be used for converting character references to the corresponding unicode characters.

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

__import__ and importlib.util.resolve_name now raise ImportError where it previously raised ValueError. Callers catching the specific exception type and supporting both Python 3.9 and earlier versions will need to catch both using except (ImportError, ValueError):.
The venv activation scripts no longer special-case when __VENV_PROMPT__ is set to "".
The select.epoll.unregister method no longer ignores the ~errno.EBADF error. (Contributed by Victor Stinner in 39239.)
The compresslevel parameter of bz2.BZ2File became keyword-only, since the buffering parameter has been removed. (Contributed by Victor Stinner in 39357.)
Simplified AST for subscription. Simple indices will be represented by their value, extended slices will be represented as tuples. Index(value) will return a value itself, ExtSlice(slices) will return Tuple(slices, Load()). (Contributed by Serhiy Storchaka in 34822.)
The importlib module now ignores the PYTHONCASEOK environment variable when the -E or -I command line options are being used.
The encoding parameter has been added to the classes ftplib.FTP and ftplib.FTP_TLS as a keyword-only parameter, and the default encoding is changed from Latin-1 to UTF-8 to follow 2640.
asyncio.loop.shutdown_default_executor has been added to ~asyncio.AbstractEventLoop, meaning alternative event loops that inherit from it should have this method defined. (Contributed by Kyle Stanley in 34037.)
The constant values of future flags in the __future__ module is updated in order to prevent collision with compiler flags. Previously PyCF_ALLOW_TOP_LEVEL_AWAIT was clashing with CO_FUTURE_DIVISION. (Contributed by Batuhan Taskaya in 39562)
array('u') now uses :cwchar_t as C type instead of Py_UNICODE. This change doesn't affect to its behavior because Py_UNICODE is alias of :cwchar_t since Python 3.3. (Contributed by Inada Naoki in 34538.)
The logging.getLogger API now returns the root logger when passed the name 'root', whereas previously it returned a non-root logger named 'root'. This could affect cases where user code explicitly wants a non-root logger named 'root', or instantiates a logger using logging.getLogger(__name__) in some top-level module called 'root.py'. (Contributed by Vinay Sajip in 37742.)
Division handling of ~pathlib.PurePath now returns NotImplemented instead of raising a TypeError when passed something other than an instance of str or ~pathlib.PurePath. This allows creating compatible classes that don't inherit from those mentioned types. (Contributed by Roger Aiudi in 34775).
Starting with Python 3.9.5 the ipaddress module no longer accepts any leading zeros in IPv4 address strings. Leading zeros are ambiguous and interpreted as octal notation by some libraries. For example the legacy function socket.inet_aton treats leading zeros as octal notatation. glibc implementation of modern ~socket.inet_pton does not accept any leading zeros. (Contributed by Christian Heimes in 36384).
codecs.lookup now normalizes the encoding name the same way as encodings.normalize_encoding, except that codecs.lookup also converts the name to lower case. For example, "latex+latin1" encoding name is now normalized to "latex_latin1". (Contributed by Jordon Xu in 37751.)

Changes in the C API

Instances of heap-allocated types <heap-types> (such as those created with :cPyType_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~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 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 35810 and 40217 for more information.)
The functions PyEval_CallObject, PyEval_CallFunction, PyEval_CallMethod and PyEval_CallObjectWithKeywords are deprecated. Use :cPyObject_Call and its variants instead. (See more details in 29548.)

CPython bytecode changes

The LOAD_ASSERTION_ERROR opcode was added for handling the assert statement. Previously, the assert statement would not work correctly if the AssertionError exception was being shadowed. (Contributed by Zackery Spytz in 34880.)
The COMPARE_OP opcode was split into four distinct instructions:
- COMPARE_OP for rich comparisons
- IS_OP for 'is' and 'is not' tests
- CONTAINS_OP for 'in' and 'not in' tests
- JUMP_IF_NOT_EXC_MATCH for checking exceptions in 'try-except' statements.
(Contributed by Mark Shannon in 39156.)

Build Changes

Added --with-platlibdir option to the configure script: name of the platform-specific library directory, stored in the new sys.platlibdir attribute. See sys.platlibdir attribute for more information. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in 1294959.)
The COUNT_ALLOCS special build macro has been removed. (Contributed by Victor Stinner in 39489.)
On non-Windows platforms, the :c!setenv and :c!unsetenv functions are now required to build Python. (Contributed by Victor Stinner in 39395.)
On non-Windows platforms, creating bdist_wininst installers is now officially unsupported. (See 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 --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 34956.)
Python can now be built for Windows 10 ARM64. (Contributed by Steve Dower in 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 36044 and 37707 for more details.)

C API Changes

New Features

573: Added :cPyType_FromModuleAndSpec to associate a module with a class; :cPyType_GetModule and :cPyType_GetModuleState to retrieve the module and its state; and :cPyCMethod and :cMETH_METHOD to allow a method to access the class it was defined in. (Contributed by Marcel Plch and Petr Viktorin in 38787.)
Added :cPyFrame_GetCode function: get a frame code. Added :cPyFrame_GetBack function: get the frame next outer frame. (Contributed by Victor Stinner in 40421.)
Added :cPyFrame_GetLineNumber to the limited C API. (Contributed by Victor Stinner in 40421.)
Added :cPyThreadState_GetInterpreter and :cPyInterpreterState_Get functions to get the interpreter. Added :cPyThreadState_GetFrame function to get the current frame of a Python thread state. Added :cPyThreadState_GetID function: get the unique identifier of a Python thread state. (Contributed by Victor Stinner in 39947.)
Added a new public :cPyObject_CallNoArgs function to the C API, which calls a callable Python object without any arguments. It is the most efficient way to call a callable Python object without any argument. (Contributed by Victor Stinner in 37194.)
Changes in the limited C API (if Py_LIMITED_API macro is defined):
- Provide :cPy_EnterRecursiveCall and :cPy_LeaveRecursiveCall as regular functions for the limited API. Previously, there were defined as macros, but these macros didn't compile with the limited C API which cannot access PyThreadState.recursion_depth field (the structure is opaque in the limited C API).
- PyObject_INIT() and PyObject_INIT_VAR() become regular "opaque" function to hide implementation details.
(Contributed by Victor Stinner in 38644 and 39542.)
The :cPyModule_AddType function is added to help adding a type to a module. (Contributed by Donghee Na in 40024.)
Added the functions :cPyObject_GC_IsTracked and :cPyObject_GC_IsFinalized to the public API to allow to query if Python objects are being currently tracked or have been already finalized by the garbage collector respectively. (Contributed by Pablo Galindo Salgado in 40241.)
Added :c!_PyObject_FunctionStr to get a user-friendly string representation of a function-like object. (Patch by Jeroen Demeyer in 37645.)
Added :cPyObject_CallOneArg for calling an object with one positional argument (Patch by Jeroen Demeyer in 37483.)

Porting to Python 3.9

PyInterpreterState.eval_frame (523) now requires a new mandatory tstate parameter (PyThreadState*). (Contributed by Victor Stinner in 38500.)
Extension modules: :c~PyModuleDef.m_traverse, :c~PyModuleDef.m_clear and :c~PyModuleDef.m_free functions of :cPyModuleDef are no longer called if the module state was requested but is not allocated yet. This is the case immediately after the module is created and before the module is executed (:cPy_mod_exec function). More precisely, these functions are not called if :c~PyModuleDef.m_size is greater than 0 and the module state (as returned by :cPyModule_GetState) is NULL.

Extension modules without module state (m_size <= 0) are not affected.
If :cPy_AddPendingCall is called in a subinterpreter, the function is now scheduled to be called from the subinterpreter, rather than being called from the main interpreter. Each subinterpreter now has its own list of scheduled calls. (Contributed by Victor Stinner in 39984.)
The Windows registry is no longer used to initialize sys.path when the -E option is used (if :cPyConfig.use_environment is set to 0). This is significant when embedding Python on Windows. (Contributed by Zackery Spytz in 8901.)
The global variable :cPyStructSequence_UnnamedField is now a constant and refers to a constant string. (Contributed by Serhiy Storchaka in 38650.)
The :c!PyGC_Head structure is now opaque. It is only defined in the internal C API (pycore_gc.h). (Contributed by Victor Stinner in 40241.)
The Py_UNICODE_COPY, Py_UNICODE_FILL, PyUnicode_WSTR_LENGTH, :c!PyUnicode_FromUnicode, :c!PyUnicode_AsUnicode, _PyUnicode_AsUnicode, and :c!PyUnicode_AsUnicodeAndSize are marked as deprecated in C. They have been deprecated by 393 since Python 3.3. (Contributed by Inada Naoki in 36346.)
The :cPy_FatalError function is replaced with a macro which logs automatically the name of the current function, unless the Py_LIMITED_API macro is defined. (Contributed by Victor Stinner in 39882.)
The vectorcall protocol now requires that the caller passes only strings as keyword names. (See 37540 for more information.)
Implementation details of a number of macros and functions are now hidden:
- :cPyObject_IS_GC macro was converted to a function.
- The :c!PyObject_NEW macro becomes an alias to the :cPyObject_New macro, and the :c!PyObject_NEW_VAR macro becomes an alias to the :cPyObject_NewVar macro. They no longer access directly the :cPyTypeObject.tp_basicsize member.
- :c!PyObject_GET_WEAKREFS_LISTPTR macro was converted to a function: the macro accessed directly the :cPyTypeObject.tp_weaklistoffset member.
- :cPyObject_CheckBuffer macro was converted to a function: the macro accessed directly the :cPyTypeObject.tp_as_buffer member.
- :cPyIndex_Check is now always declared as an opaque function to hide implementation details: removed the PyIndex_Check() macro. The macro accessed directly the :cPyTypeObject.tp_as_number member.
(See 40170 for more details.)

Removed

Excluded PyFPE_START_PROTECT() and PyFPE_END_PROTECT() macros of pyfpe.h from the limited C API. (Contributed by Victor Stinner in 38835.)
The tp_print slot of 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 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 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 38644 and 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 39946.)
Removed the following functions from the C API. Call :cPyGC_Collect explicitly to clear all free lists. (Contributed by Inada Naoki and Victor Stinner in 37340, 38896 and 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 39465.)
Removed Py_UNICODE_MATCH. It has been deprecated by 393, and broken since Python 3.3. The :cPyUnicode_Tailmatch function can be used instead. (Contributed by Inada Naoki in 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 39372.)

Notable changes in Python 3.9.1

typing

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

Literal now de-duplicates parameters.
Equality comparisons between Literal objects are now order independent.
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.
Literal objects will now raise a TypeError exception during equality comparisons if any of their parameters are not 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 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 41100.)

Notable changes in Python 3.9.2

collections.abc

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. Code which accesses the arguments via typing.get_args or __args__ need to account for this change. A DeprecationWarning may be emitted for invalid forms of parameterizing collections.abc.Callable which may have passed silently in Python 3.9.1. This DeprecationWarning will become a TypeError in Python 3.10. (Contributed by Ken Jin in 42195.)

urllib.parse

Earlier Python versions allowed using both ; and & as query parameter separators in urllib.parse.parse_qs and 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 !cgi.parse and !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 42967.)

Notable changes in Python 3.9.3

A security fix alters the 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 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 3986, ASCII newline \n, \r and tab \t characters are stripped from the URL by the parser in urllib.parse preventing such attacks. The removal characters are controlled by a new module level variable urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE. (See 88048)

Notable security feature in 3.9.14

Converting between int and str in bases other than 2 (binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal) now raises a 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 2020-10735. This limit can be configured or disabled by environment variable, command line flag, or sys APIs. See the 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

tarfile

The extraction methods in tarfile, and shutil.unpack_archive, have a new a filter argument that allows limiting tar features than may be surprising or dangerous, such as creating files outside the destination directory. See tarfile-extraction-filter for details. In Python 3.12, use without the filter argument will show a DeprecationWarning. In Python 3.14, the default will switch to 'data'. (Contributed by Petr Viktorin in 706.)

Files

3.9.rst

Latest commit

History

3.9.rst

File metadata and controls

What's New In Python 3.9

Summary -- Release highlights

You should check for DeprecationWarning in your code

New Features

Dictionary Merge & Update Operators

New String Methods to Remove Prefixes and Suffixes

Type Hinting Generics in Standard Collections

New Parser

Other Language Changes

New Modules

zoneinfo

graphlib

Improved Modules

ast

asyncio

compileall

concurrent.futures

curses

datetime

distutils

fcntl

ftplib

gc

hashlib

http

IDLE and idlelib

imaplib

importlib

inspect

ipaddress

math

multiprocessing

nntplib

os

pathlib

pdb

poplib

pprint

pydoc

random

signal

smtplib

socket

time

sys

tracemalloc

typing

unicodedata

venv

xml

Optimizations

Deprecated

Removed

Porting to Python 3.9

Changes in the Python API

Changes in the C API

CPython bytecode changes

Build Changes

C API Changes

New Features

Porting to Python 3.9

Removed

Notable changes in Python 3.9.1

typing

macOS 11.0 (Big Sur) and Apple Silicon Mac support

Notable changes in Python 3.9.2

collections.abc

urllib.parse

Notable changes in Python 3.9.3

Notable changes in Python 3.9.5

urllib.parse

Notable security feature in 3.9.14

Notable changes in 3.9.17

tarfile