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What's New In Python 3.9

Release:|release|
Date: |today|

This article explains the new features in Python 3.9, compared to 3.8.

For full details, see the :ref:`changelog <changelog>`.

Note

Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.9 moves towards release, so it's worth checking back even after reading earlier versions.

Summary -- Release highlights

You should check for DeprecationWarning in your code

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

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

It has been decided to keep a few backward compatibility layers for one last release, to give more time to Python projects maintainers to organize the removal of the Python 2 support and add support for Python 3.9.

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

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

New Features

Dictionary Merge & Update Operators

Merge (|) and update (|=) operators have been added to the built-in :class:`dict` class. See PEP 584 for a full description. (Contributed by Brandt Bucher in :issue:`36144`.)

PEP 616: New removeprefix() and removesuffix() string methods

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

PEP 585: Builtin Generic Types

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

Example:

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

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

PEP 617: New Parser

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

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

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

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

Other Language Changes

  • :func:`__import__` now raises :exc:`ImportError` instead of :exc:`ValueError`, which used to occur when a relative import went past its top-level package. (Contributed by Ngalim Siregar in :issue:`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 :mod:`__main__` module became an absolute path, rather than a relative path. These paths now remain valid after the current directory is changed by :func:`os.chdir`. As a side effect, the traceback also displays the absolute path for :mod:`__main__` module frames in this case. (Contributed by Victor Stinner in :issue:`20443`.)

  • In the :ref:`Python Development Mode <devmode>` and in debug build, the encoding and errors arguments are now checked for string encoding and decoding operations. Examples: :func:`open`, :meth:`str.encode` and :meth:`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 :issue:`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 :class:`bytes` and :class:`bytearray` objects. (Contributed by Serhiy Storchaka in :issue:`28029`.)

  • Any valid expression can now be used as a :term:`decorator`. Previously, the grammar was much more restrictive. See PEP 614 for details. (Contributed by Brandt Bucher in :issue:`39702`.)

  • Improved help for the :mod:`typing` module. Docstrings are now shown for all special forms and special generic aliases (like Union and List). Using :func:`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 :issue:`40257`.)

New Modules

zoneinfo

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

Example:

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

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

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

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

.. seealso::

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

graphlib

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

Improved Modules

ast

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

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

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

asyncio

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

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

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

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

compileall

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

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

concurrent.futures

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

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

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

curses

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

datetime

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

distutils

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

fcntl

Added constants :data:`~fcntl.F_OFD_GETLK`, :data:`~fcntl.F_OFD_SETLK` and :data:`~fcntl.F_OFD_SETLKW`. (Contributed by Dong-hee Na in :issue:`38602`.)

ftplib

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

gc

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

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

hashlib

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

http

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

imaplib

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

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

importlib

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

inspect

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

ipaddress

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

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

math

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

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

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

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

multiprocessing

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

nntplib

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

os

Added :data:`~os.CLD_KILLED` and :data:`~os.CLD_STOPPED` for :attr:`si_code`. (Contributed by Dong-hee Na in :issue:`38493`.)

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

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

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

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

pathlib

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

poplib

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

pprint

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

pydoc

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

random

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

signal

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

smtplib

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

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

socket

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

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

time

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

sys

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

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

tracemalloc

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

typing

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

unicodedata

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

venv

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

xml

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

Optimizations

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

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

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

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

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

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

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

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    4.0
    read_nonlocal                    7.1     8.1     5.8     5.4     4.4    4.8
    read_global                     15.5    19.0    14.3    13.6     7.6    7.7
    read_builtin                    21.1    21.6    18.5    19.0     7.5    7.7
    read_classvar_from_class        25.6    26.5    20.7    19.5    18.4   18.6
    read_classvar_from_instance     22.8    23.5    18.8    17.1    16.4   20.1
    read_instancevar                32.4    33.1    28.0    26.3    25.4   27.7
    read_instancevar_slots          27.8    31.3    20.8    20.8    20.2   24.5
    read_namedtuple                 73.8    57.5    45.0    46.8    18.4   23.2
    read_boundmethod                37.6    37.9    29.6    26.9    27.7   45.9

Variable and attribute write access:
    write_local                      8.7     9.3     5.5     5.3     4.3    4.2
    write_nonlocal                  10.5    11.1     5.6     5.5     4.7    4.9
    write_global                    19.7    21.2    18.0    18.0    15.8   17.2
    write_classvar                  92.9    96.0   104.6   102.1    39.2   43.2
    write_instancevar               44.6    45.8    40.0    38.9    35.5   40.7
    write_instancevar_slots         35.6    36.1    27.3    26.6    25.7   27.7

Data structure read access:
    read_list                       24.2    24.5    20.8    20.8    19.0   21.1
    read_deque                      24.7    25.5    20.2    20.6    19.8   21.6
    read_dict                       24.3    25.7    22.3    23.0    21.0   22.5
    read_strdict                    22.6    24.3    19.5    21.2    18.9   21.6

Data structure write access:
    write_list                      27.1    28.5    22.5    21.6    20.0   21.6
    write_deque                     28.7    30.1    22.7    21.8    23.5   23.2
    write_dict                      31.4    33.3    29.3    29.2    24.7   27.8
    write_strdict                   28.4    29.9    27.5    25.2    23.1   29.8

Stack (or queue) operations:
    list_append_pop                 93.4   112.7    75.4    74.2    50.8   53.9
    deque_append_pop                43.5    57.0    49.4    49.2    42.5   45.5
    deque_append_popleft            43.7    57.3    49.7    49.7    42.8   45.5

Timing loop:
    loop_overhead                    0.5     0.6     0.4     0.3     0.3    0.3

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

Deprecated

Removed

Porting to Python 3.9

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

Changes in the Python API

Changes in the C API

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

    Example:

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

    If your traverse function delegates to tp_traverse of its base class (or another type), ensure that Py_TYPE(self) is visited only once. Note that only 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 (Python issue 35810 and 40217)
    if (base->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        // a heap type's tp_traverse already visited Py_TYPE(self)
    } else {
        Py_VISIT(Py_TYPE(self));
    }
#else

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

CPython bytecode changes

Build Changes

C API Changes

New Features

Porting to Python 3.9

Removed

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

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

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

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

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

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

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

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

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

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