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.

See also: **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;

* "os.pidfd_open()" added that allows process management without races
  and signals.

Interpreter improvements:

* **PEP 573**, fast access to module state from methods of C extension
  types;

* **PEP 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 **PEP 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:

* **PEP 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:

* **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
"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 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 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
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 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 **PEP 584** for a full description. (Contributed by Brandt Bucher
in bpo-36144.)


New String Methods to Remove Prefixes and Suffixes
--------------------------------------------------

"str.removeprefix(prefix)" and "str.removesuffix(suffix)" 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 bpo-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 bpo-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 **PEP 617** for more details.  (Contributed by Guido van Rossum,
Pablo Galindo and Lysandros Nikolaou in bpo-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 bpo-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 bpo-20443.)

* In the Python Development Mode and in 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 bpo-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 bpo-28029.)

* Any valid expression can now be used as a *decorator*.  Previously,
  the grammar was much more restrictive.  See **PEP 614** for details.
  (Contributed by Brandt Bucher in bpo-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 bpo-40257.)

* Parallel running of "aclose()" / "asend()" / "athrow()" is now
  prohibited, and "ag_running" now reflects the actual running status
  of the async generator. (Contributed by Yury Selivanov in
  bpo-30773.)

* Unexpected errors in calling the "__iter__" method are no longer
  masked by "TypeError" in the "in" operator and functions
  "contains()", "indexOf()" and "countOf()" of the "operator" module.
  (Contributed by Serhiy Storchaka in bpo-40824.)

* Unparenthesized lambda expressions can no longer be the expression
  part in an "if" clause in comprehensions and generator expressions.
  See bpo-41848 and bpo-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.

See also:

  **PEP 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 bpo-17005.)


Improved Modules
================


ast
---

Added the *indent* option to "dump()" which allows it to produce a
multiline indented output. (Contributed by Serhiy Storchaka in
bpo-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 bpo-38870.)

Added docstrings to AST nodes that contains the ASDL signature used to
construct that node. (Contributed by Batuhan Taskaya in bpo-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 bpo-37228.)

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

Added "asyncio.PidfdChildWatcher", a Linux-specific child watcher
implementation that polls process file descriptors. (bpo-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
"run_in_executor()" that can directly take keyword arguments.
(Contributed by Kyle Stanley and Yury Selivanov in bpo-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 bpo-32751.)

"asyncio" now raises "TyperError" when calling incompatible methods
with an "ssl.SSLSocket" socket. (Contributed by Ido Michael in
bpo-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 bpo-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 bpo-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 bpo-39349.)

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

Workers in "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 bpo-39207.)


curses
------

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


datetime
--------

The "isocalendar()" of "datetime.date" and "isocalendar()" of
"datetime.datetime" methods now returns a "namedtuple()" instead of a
"tuple". (Contributed by Dong-hee Na in bpo-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 bpo-40698.)


fcntl
-----

Added constants "F_OFD_GETLK", "F_OFD_SETLK" and "F_OFD_SETLKW".
(Contributed by Dong-hee Na in bpo-38602.)


ftplib
------

"FTP" and "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 Dong-hee Na in bpo-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 bpo-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 bpo-39322.)


hashlib
-------

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


http
----

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


IDLE and idlelib
----------------

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

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

Added keywords to module name completion list.  (Contributed by Terry
J. Reedy in bpo-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
bpo-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 bpo-40468.)  Move the indent space setting from the
Font tab to the new Windows tab.  (Contributed by Mark Roseman and
Terry Jan Reedy in bpo-33962.)

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


imaplib
-------

"IMAP4" and "IMAP4_SSL" now have an optional *timeout* parameter for
their constructors. Also, the "open()" method now has an optional
*timeout* parameter with this change. The overridden methods of
"IMAP4_SSL" and "IMAP4_stream" were applied to this change.
(Contributed by Dong-hee Na in bpo-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 Dong-hee Na in bpo-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 bpo-37444.)

Import loaders which publish immutable module objects can now publish
immutable packages in addition to individual modules. (Contributed by
Dino Viehland in bpo-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
bpo-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 bpo-36350 and
bpo-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 "scope_id" attribute.
(Contributed by Oleksandr Pavliuk in bpo-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 bpo-36384).


math
----

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

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

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

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


multiprocessing
---------------

The "multiprocessing.SimpleQueue" class has a new "close()" method to
explicitly close the queue. (Contributed by Victor Stinner in
bpo-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 Dong-hee Na in bpo-39259.)


os
--

Added "CLD_KILLED" and "CLD_STOPPED" for "si_code". (Contributed by
Dong-hee Na in bpo-38493.)

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

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

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

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


pathlib
-------

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


pdb
---

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


poplib
------

"POP3" and "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 Dong-hee Na in bpo-39259.)


pprint
------

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


random
------

Added a new "random.Random.randbytes" method: generate random bytes.
(Contributed by Victor Stinner in bpo-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.
(bpo-38712)


smtplib
-------

"SMTP" and "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 Dong-hee Na in bpo-39259.)

"LMTP" constructor  now has an optional *timeout* parameter.
(Contributed by Dong-hee Na in bpo-39329.)


socket
------

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

The socket module now supports the "CAN_J1939" protocol on platforms
that support it.  (Contributed by Karl Ding in bpo-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 bpo-28724.)


time
----

On AIX, "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
ms. (Contributed by Batuhan Taskaya in bpo-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 bpo-1294959.)

Previously, "sys.stderr" was block-buffered when non-interactive. Now
"stderr" defaults to always being line-buffered. (Contributed by
Jendrik Seipp in bpo-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 bpo-40630.)


typing
------

**PEP 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. (bpo-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 bpo-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
bpo-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 bpo-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 bpo-40010.)

* Optimized the "subprocess" module on FreeBSD using "closefrom()".
  (Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak
  and Victor Stinner in bpo-38061.)

* "PyLong_FromDouble()" is now up to 1.87x faster for values that fit
  into "long". (Contributed by Sergey Fedoseev in bpo-37986.)

* A number of Python builtins ("range", "tuple", "set", "frozenset",
  "list", "dict") are now sped up by using **PEP 590** vectorcall
  protocol. (Contributed by Dong-hee Na, Mark Shannon, Jeroen Demeyer
  and Petr Viktorin in bpo-37207.)

* Optimized "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 bpo-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 bpo-37257.)

* *floor division* of float operation now has a better performance.
  Also the message of "ZeroDivisionError" for this operation is
  updated. (Contributed by Dong-hee Na in bpo-39434.)

* Decoding short ASCII strings with UTF-8 and ascii codecs is now
  about 15% faster.  (Contributed by Inada Naoki in bpo-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 bpo-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 bpo-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 "PyParser_SimpleParseStringFlags()",
  "PyParser_SimpleParseStringFlagsFilename()",
  "PyParser_SimpleParseFileFlags()" and "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 bpo-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 "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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-39639
  and bpo-39969 and Serhiy Storchaka in bpo-39988.)

* The "PyEval_InitThreads()" and "PyEval_ThreadsInitialized()"
  functions are now deprecated and will be removed in Python 3.11.
  Calling "PyEval_InitThreads()" now does nothing. The *GIL* is
  initialized by "Py_Initialize()" since Python 3.7. (Contributed by
  Victor Stinner in bpo-39877.)

* Passing "None" as the first argument to the "shlex.split()" function
  has been deprecated.  (Contributed by Zackery Spytz in bpo-33262.)

* "smtpd.MailmanProxy()" is now deprecated as it is unusable without
  an external module, "mailman".  (Contributed by Samuel Colvin in
  bpo-35800.)

* The "lib2to3" module now emits a "PendingDeprecationWarning". Python
  3.9 switched to a PEG parser (see **PEP 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
  bpo-40360.)

* The *random* parameter of "random.shuffle()" has been deprecated.
  (Contributed by Raymond Hettinger in bpo-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 Dong-hee Na in bpo-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
  bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-37320.)

* The "isAlive()" method of "threading.Thread" has been removed. It
  was deprecated since Python 3.8. Use "is_alive()" instead.
  (Contributed by Dong-hee Na in bpo-37804.)

* Methods "getchildren()" and "getiterator()" of classes "ElementTree"
  and "Element" in the "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 bpo-36543.)

* The old "plistlib" API has been removed, it was deprecated since
  Python 3.4. Use the "load()", "loads()", "dump()", and "dumps()"
  functions. Additionally, the *use_builtin_types* parameter was
  removed, standard "bytes" objects are always used instead.
  (Contributed by Jon Janzen in bpo-36409.)

* The C function "PyGen_NeedsFinalizing" has been removed. It was not
  documented, tested, or used anywhere within CPython after the
  implementation of **PEP 442**. Patch by Joannah Nanjekye.
  (Contributed by Joannah Nanjekye in bpo-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 bpo-39351.)

* "fractions.gcd()" function has been removed, it was deprecated since
  Python 3.5 (bpo-22486): use "math.gcd()" instead. (Contributed by
  Victor Stinner in bpo-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 bpo-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 bpo-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 bpo-34793.)

* The "sys.getcounts()" function, the "-X showalloccount" command line
  option and the "show_alloc_count" field of the C structure
  "PyConfig" have been removed. They required a special Python build
  by defining "COUNT_ALLOCS" macro. (Contributed by Victor Stinner in
  bpo-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 bpo-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 bpo-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 bpo-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 "EBADF"
  error. (Contributed by Victor Stinner in bpo-39239.)

* The *compresslevel* parameter of "bz2.BZ2File" became keyword-only,
  since the *buffering* parameter has been removed. (Contributed by
  Victor Stinner in bpo-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
  bpo-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 **RFC 2640**.

* "asyncio.loop.shutdown_default_executor()" has been added to
  "AbstractEventLoop", meaning alternative event loops that inherit
  from it should have this method defined. (Contributed by Kyle
  Stanley in bpo-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 bpo-39562)

* "array('u')" now uses "wchar_t" as C type instead of "Py_UNICODE".
  This change doesn’t affect to its behavior because "Py_UNICODE" is
  alias of "wchar_t" since Python 3.3. (Contributed by Inada Naoki in
  bpo-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 bpo-37742.)

* Division handling of "PurePath" now returns "NotImplemented" instead
  of raising a "TypeError" when passed something other than an
  instance of "str" or "PurePath".  This allows creating compatible
  classes that don’t inherit from those mentioned types. (Contributed
  by Roger Aiudi in bpo-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
  "inet_pton()" does not accept any leading zeros. (Contributed by
  Christian Heimes in bpo-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 bpo-37751.)


Changes in the C API
--------------------

* Instances of heap-allocated types (such as those created with
  "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 "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 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 bpo-35810 and bpo-40217 for more information.)

* The functions "PyEval_CallObject", "PyEval_CallFunction",
  "PyEval_CallMethod" and "PyEval_CallObjectWithKeywords" are
  deprecated. Use "PyObject_Call()" and its variants instead. (See
  more details in bpo-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 bpo-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 bpo-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 bpo-1294959.)

* The "COUNT_ALLOCS" special build macro has been removed.
  (Contributed by Victor Stinner in bpo-39489.)

* On non-Windows platforms, the "setenv()" and "unsetenv()" functions
  are now required to build Python. (Contributed by Victor Stinner in
  bpo-39395.)

* On non-Windows platforms, creating "bdist_wininst" installers is now
  officially unsupported.  (See bpo-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
  bpo-34956.)

* Python can now be built for Windows 10 ARM64. (Contributed by Steve
  Dower in bpo-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 bpo-36044 and bpo-37707 for
  more details.)


C API Changes
=============


New Features
------------

* **PEP 573**: Added "PyType_FromModuleAndSpec()" to associate a
  module with a class; "PyType_GetModule()" and
  "PyType_GetModuleState()" to retrieve the module and its state; and
  "PyCMethod" and "METH_METHOD" to allow a method to access the class
  it was defined in. (Contributed by Marcel Plch and Petr Viktorin in
  bpo-38787.)

* Added "PyFrame_GetCode()" function: get a frame code. Added
  "PyFrame_GetBack()" function: get the frame next outer frame.
  (Contributed by Victor Stinner in bpo-40421.)

* Added "PyFrame_GetLineNumber()" to the limited C API. (Contributed
  by Victor Stinner in bpo-40421.)

* Added "PyThreadState_GetInterpreter()" and
  "PyInterpreterState_Get()" functions to get the interpreter. Added
  "PyThreadState_GetFrame()" function to get the current frame of a
  Python thread state. Added "PyThreadState_GetID()" function: get the
  unique identifier of a Python thread state. (Contributed by Victor
  Stinner in bpo-39947.)

* Added a new public "PyObject_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 bpo-37194.)

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

  * Provide "Py_EnterRecursiveCall()" and "Py_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 bpo-38644 and bpo-39542.)

* The "PyModule_AddType()" function is added to help adding a type to
  a module. (Contributed by Dong-hee Na in bpo-40024.)

* Added the functions "PyObject_GC_IsTracked()" and
  "PyObject_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 bpo-40241.)

* Added "_PyObject_FunctionStr()" to get a user-friendly string
  representation of a function-like object. (Patch by Jeroen Demeyer
  in bpo-37645.)

* Added "PyObject_CallOneArg()" for calling an object with one
  positional argument (Patch by Jeroen Demeyer in bpo-37483.)


Porting to Python 3.9
---------------------

* "PyInterpreterState.eval_frame" (**PEP 523**) now requires a new
  mandatory *tstate* parameter ("PyThreadState*"). (Contributed by
  Victor Stinner in bpo-38500.)

* Extension modules: "m_traverse", "m_clear" and "m_free" functions of
  "PyModuleDef" 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 ("Py_mod_exec"
  function). More precisely, these functions are not called if
  "m_size" is greater than 0 and the module state (as returned by
  "PyModule_GetState()") is "NULL".

  Extension modules without module state ("m_size <= 0") are not
  affected.

* If "Py_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
  bpo-39984.)

* The Windows registry is no longer used to initialize "sys.path" when
  the "-E" option is used (if "PyConfig.use_environment" is set to
  "0"). This is significant when embedding Python on Windows.
  (Contributed by Zackery Spytz in bpo-8901.)

* The global variable "PyStructSequence_UnnamedField" is now a
  constant and refers to a constant string. (Contributed by Serhiy
  Storchaka in bpo-38650.)

* The "PyGC_Head" structure is now opaque. It is only defined in the
  internal C API ("pycore_gc.h"). (Contributed by Victor Stinner in
  bpo-40241.)

* The "Py_UNICODE_COPY", "Py_UNICODE_FILL", "PyUnicode_WSTR_LENGTH",
  "PyUnicode_FromUnicode()", "PyUnicode_AsUnicode()",
  "_PyUnicode_AsUnicode", and "PyUnicode_AsUnicodeAndSize()" are
  marked as deprecated in C.  They have been deprecated by **PEP 393**
  since Python 3.3. (Contributed by Inada Naoki in bpo-36346.)

* The "Py_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
  bpo-39882.)

* The vectorcall protocol now requires that the caller passes only
  strings as keyword names. (See bpo-37540 for more information.)

* Implementation details of a number of macros and functions are now
  hidden:

  * "PyObject_IS_GC()" macro was converted to a function.

  * The "PyObject_NEW()" macro becomes an alias to the
    "PyObject_New()" macro, and the "PyObject_NEW_VAR()" macro becomes
    an alias to the "PyObject_NewVar()" macro. They no longer access
    directly the "PyTypeObject.tp_basicsize" member.

  * "PyObject_GET_WEAKREFS_LISTPTR()" macro was converted to a
    function: the macro accessed directly the
    "PyTypeObject.tp_weaklistoffset" member.

  * "PyObject_CheckBuffer()" macro was converted to a function: the
    macro accessed directly the "PyTypeObject.tp_as_buffer" member.

  * "PyIndex_Check()" is now always declared as an opaque function to
    hide implementation details: removed the "PyIndex_Check()" macro.
    The macro accessed directly the "PyTypeObject.tp_as_number"
    member.

  (See bpo-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
  bpo-38835.)

* The "tp_print" slot of PyTypeObject 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 bpo-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 bpo-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 bpo-38644 and bpo-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 bpo-39946.)

* Removed the following functions from the C API. Call
  "PyGC_Collect()" explicitly to clear all free lists. (Contributed by
  Inada Naoki and Victor Stinner in bpo-37340, bpo-38896 and
  bpo-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 bpo-39465.)

* Removed "Py_UNICODE_MATCH". It has been deprecated by **PEP 393**,
  and broken since Python 3.3. The "PyUnicode_Tailmatch()" function
  can be used instead. (Contributed by Inada Naoki in bpo-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 bpo-39372.)


Notable changes in Python 3.9.1
===============================


typing
------

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

1. "Literal" now de-duplicates parameters.

2. Equality comparisons between "Literal" objects are now order
   independent.

3. "Literal" comparisons now 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 "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 bpo-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 bpo-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 bpo-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 bpo-42967.)
