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

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

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.

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`.)

: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:`18939`.)

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`.)

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 Nikolau in :issue:`40334`.)

• :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`.)

• None yet.

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`.)

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 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`.)

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`.)

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`.)

: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`.)

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

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`.)

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`.)

: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`.)

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`.)

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

: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`.)

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`.)

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`.)

: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`.)

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`.)

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

: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`.)

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

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`.)

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

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`)

: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`.)

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`.)

Add a new :attr:`sys.platlibdir` attribute: name of the platform-specific library directory. It is used to build the path of platform-specific dynamic libraries and the path of the standard library. 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`.)

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.)

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

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`.)

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`.)

• 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 to 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`.)

• The distutils bdist_msi command is now deprecated, use bdist_wheel (wheel packages) instead. (Contributed by Hugo van Kemenade in :issue:`39586`.)

• Currently :func:`math.factorial` accepts :class:`float` instances with non-negative integer values (like 5.0). It raises a :exc:`ValueError` for non-integral and negative floats. It is now deprecated. In future Python versions it will raise a :exc:`TypeError` for all floats. (Contributed by Serhiy Storchaka in :issue:`37315`.)

• The :mod:`parser` module is 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 :mod:`ast` module.

• Using :data:`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 :exc:`TypeError` in a future version of Python. (Contributed by Josh Rosenberg in :issue:`35712`.)

• The :mod:`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 :const:`None`, :class:`int`, :class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`.

• Opening the :class:`~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 :issue:`28286`.)

• Deprecated the split() method of :class:`_tkinter.TkappType` in favour of the splitlist() method which has more consistent and predicable behavior. (Contributed by Serhiy Storchaka in :issue:`38371`.)

• The explicit passing of coroutine objects to :func:`asyncio.wait` has been deprecated and will be removed in version 3.11. (Contributed by Yury Selivanov and Kyle Stanley in :issue:`34790`.)

• binhex4 and hexbin4 standards are now deprecated. The :mod:`binhex` module and the following :mod:`binascii` functions are now deprecated:

  • :func:`~binascii.b2a_hqx`, :func:`~binascii.a2b_hqx`
  • :func:`~binascii.rlecode_hqx`, :func:`~binascii.rledecode_hqx`

  (Contributed by Victor Stinner in :issue:`39353`.)

• :mod:`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 :issue:`32892`.)

• :mod:`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 :issue:`39639` and :issue:`39969` and Serhiy Storchaka in :issue:`39988`.)

• The :c:func:`PyEval_InitThreads` and :c:func:`PyEval_ThreadsInitialized` functions are now deprecated and will be removed in Python 3.11. Calling :c:func:`PyEval_InitThreads` now does nothing. The :term:`GIL` is initialized by :c:func:`Py_Initialize()` since Python 3.7. (Contributed by Victor Stinner in :issue:`39877`.)

• Passing None as the first argument to the :func:`shlex.split` function has been deprecated. (Contributed by Zackery Spytz in :issue:`33262`.)

• The :mod:`lib2to3` module now emits a :exc:`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 :issue:`40360`.)

• The erroneous version at :data:`unittest.mock.__version__` has been removed.
• :class:`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 :meth:`nntplib.NNTP.descriptions` or :meth:`nntplib.NNTP.description` instead. (Contributed by Dong-hee Na in :issue:`39366`.)
• :class:`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 :issue:`38916`.)
• The undocumented sys.callstats() function has been removed. Since Python 3.7, it was deprecated and always returned :const:`None`. It required a special build option CALL_PROFILE which was already removed in Python 3.7. (Contributed by Victor Stinner in :issue:`37414`.)
• The sys.getcheckinterval() and sys.setcheckinterval() functions have been removed. They were deprecated since Python 3.2. Use :func:`sys.getswitchinterval` and :func:`sys.setswitchinterval` instead. (Contributed by Victor Stinner in :issue:`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 :issue:`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 :issue:`37312`.)
• aifc.openfp() alias to aifc.open(), sunau.openfp() alias to sunau.open(), and wave.openfp() alias to :func:`wave.open()` have been removed. They were deprecated since Python 3.7. (Contributed by Victor Stinner in :issue:`37320`.)
• The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread` has been removed. It was deprecated since Python 3.8. Use :meth:`~threading.Thread.is_alive()` instead. (Contributed by Dong-hee Na in :issue:`37804`.)
• Methods getchildren() and getiterator() of classes :class:`~xml.etree.ElementTree.ElementTree` and :class:`~xml.etree.ElementTree.Element` in the :mod:`~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(). The xml.etree.cElementTree module has been removed, use the :mod:`xml.etree.ElementTree` module instead. Since Python 3.3 the xml.etree.cElementTree module has been deprecated, the xml.etree.ElementTree module uses a fast implementation whenever available. (Contributed by Serhiy Storchaka in :issue:`36543`.)
• The old :mod:`plistlib` API has been removed, it was deprecated since Python 3.4. Use the :func:`~plistlib.load`, :func:`~plistlib.loads`, :func:`~plistlib.dump`, and :func:`~plistlib.dumps` functions. Additionally, the use_builtin_types parameter was removed, standard :class:`bytes` objects are always used instead. (Contributed by Jon Janzen in :issue:`36409`.)
• The C function PyThreadState_DeleteCurrent() has been removed. It was not documented. (Contributed by Joannah Nanjekye in :issue:`37878`.)
• 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 :issue:`15088`)
• base64.encodestring() and base64.decodestring(), aliases deprecated since Python 3.1, have been removed: use :func:`base64.encodebytes` and :func:`base64.decodebytes` instead. (Contributed by Victor Stinner in :issue:`39351`.)
• fractions.gcd() function has been removed, it was deprecated since Python 3.5 (:issue:`22486`): use :func:`math.gcd` instead. (Contributed by Victor Stinner in :issue:`39350`.)
• The buffering parameter of :class:`bz2.BZ2File` has been removed. Since Python 3.0, it was ignored and using it emitted a :exc:`DeprecationWarning`. Pass an open file object to control how the file is opened. (Contributed by Victor Stinner in :issue:`39357`.)
• The encoding parameter of :func:`json.loads` has been removed. As of Python 3.1, it was deprecated and ignored; using it has emitted a :exc:`DeprecationWarning` since Python 3.8. (Contributed by Inada Naoki in :issue:`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 :issue:`34793`.)
• The :func:`sys.getcounts` function, the -X showalloccount command line option and the show_alloc_count field of the C structure :c:type:`PyConfig` have been removed. They required a special Python build by defining COUNT_ALLOCS macro. (Contributed by Victor Stinner in :issue:`39489`.)
• The _field_types attribute of the :class:`typing.NamedTuple` class has been removed. It was deprecated deprecated since Python 3.8. Use the __annotations__ attribute instead. (Contributed by Serhiy Storchaka in :issue:`40182`.)
• The :meth:`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 :issue:`40208`)

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

• :func:`__import__` and :func:`importlib.util.resolve_name` now raise :exc:`ImportError` where it previously raised :exc:`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 :mod:`venv` activation scripts no longer special-case when __VENV_PROMPT__ is set to "".
• The :meth:`select.epoll.unregister` method no longer ignores the :data:`~errno.EBADF` error. (Contributed by Victor Stinner in :issue:`39239`.)
• The compresslevel parameter of :class:`bz2.BZ2File` became keyword-only, since the buffering parameter has been removed. (Contributed by Victor Stinner in :issue:`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 :issue:`34822`.)
• The :mod:`importlib` module now ignores the :envvar:`PYTHONCASEOK` environment variable when the :option:`-E` or :option:`-I` command line options are being used.
• The encoding parameter has been added to the classes :class:`ftplib.FTP` and :class:`ftplib.FTP_TLS` as a keyword-only parameter, and the default encoding is changed from Latin-1 to UTF-8 to follow RFC 2640.
• :meth:`asyncio.loop.shutdown_default_executor` has been added to :class:`~asyncio.AbstractEventLoop`, meaning alternative event loops that inherit from it should have this method defined. (Contributed by Kyle Stanley in :issue:`34037`.)
• The constant values of future flags in the :mod:`__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 :issue:`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 :issue:`34538`.)

• The :opcode:`LOAD_ASSERTION_ERROR` opcode was added for handling the :keyword:`assert` statement. Previously, the assert statement would not work correctly if the :exc:`AssertionError` exception was being shadowed. (Contributed by Zackery Spytz in :issue:`34880`.)

• Add --with-platlibdir option to the configure script: name of the platform-specific library directory, stored in the new :attr:`sys.platlibdir` attribute. See :attr:`sys.platlibdir` attribute for more information. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)
• The COUNT_ALLOCS special build macro has been removed. (Contributed by Victor Stinner in :issue:`39489`.)
• On non-Windows platforms, the :c:func:`setenv` and :c:func:`unsetenv` functions are now required to build Python. (Contributed by Victor Stinner in :issue:`39395`.)

• Add :c:func:`PyFrame_GetCode` function: get a frame code. Add :c:func:`PyFrame_GetBack` function: get the frame next outer frame. (Contributed by Victor Stinner in :issue:`40421`.)

• Add :c:func:`PyFrame_GetLineNumber` to the limited C API. (Contributed by Victor Stinner in :issue:`40421`.)

• Add :c:func:`PyThreadState_GetInterpreter` and :c:func:`PyInterpreterState_Get` functions to get the interpreter. Add :c:func:`PyThreadState_GetFrame` function to get the current frame of a Python thread state. Add :c:func:`PyThreadState_GetID` function: get the unique identifier of a Python thread state. (Contributed by Victor Stinner in :issue:`39947`.)

• Add a new public :c:func:`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 :issue:`37194`.)

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

  • Provide :c:func:`Py_EnterRecursiveCall` and :c:func:`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 :issue:`38644` and :issue:`39542`.)

• The :c:func:`PyModule_AddType` function is added to help adding a type to a module. (Contributed by Dong-hee Na in :issue:`40024`.)

• Add the functions :c:func:`PyObject_GC_IsTracked` and :c:func:`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 in :issue:`40241`.)

• PyInterpreterState.eval_frame (PEP 523) now requires a new mandatory tstate parameter (PyThreadState*). (Contributed by Victor Stinner in :issue:`38500`.)

• Extension modules: :c:member:`~PyModuleDef.m_traverse`, :c:member:`~PyModuleDef.m_clear` and :c:member:`~PyModuleDef.m_free` functions of :c:type:`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 (:c:data:`Py_mod_exec` function). More precisely, these functions are not called if :c:member:`~PyModuleDef.m_size` is greater than 0 and the module state (as returned by :c:func:`PyModule_GetState`) is NULL.

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

• If :c:func:`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 :issue:`39984`.)

• The Windows registry is no longer used to initialize :data:`sys.path` when the -E option is used (if :c:member:`PyConfig.use_environment` is set to 0). This is significant when embedding Python on Windows. (Contributed by Zackery Spytz in :issue:`8901`.)

• The global variable :c:data:`PyStructSequence_UnnamedField` is now a constant and refers to a constant string. (Contributed by Serhiy Storchaka in :issue:`38650`.)

• 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:
    • _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.