336 lines
12 KiB
ReStructuredText
336 lines
12 KiB
ReStructuredText
****************************
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What's New In Python 3.9
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****************************
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:Release: |release|
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:Date: |today|
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.. Rules for maintenance:
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* Anyone can add text to this document. Do not spend very much time
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on the wording of your changes, because your text will probably
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get rewritten to some degree.
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* The maintainer will go through Misc/NEWS periodically and add
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changes; it's therefore more important to add your changes to
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Misc/NEWS than to this file.
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* This is not a complete list of every single change; completeness
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is the purpose of Misc/NEWS. Some changes I consider too small
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or esoteric to include. If such a change is added to the text,
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I'll just remove it. (This is another reason you shouldn't spend
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too much time on writing your addition.)
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* If you want to draw your new text to the attention of the
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maintainer, add 'XXX' to the beginning of the paragraph or
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section.
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* It's OK to just add a fragmentary note about a change. For
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example: "XXX Describe the transmogrify() function added to the
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socket module." The maintainer will research the change and
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write the necessary text.
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* You can comment out your additions if you like, but it's not
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necessary (especially when a final release is some months away).
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* Credit the author of a patch or bugfix. Just the name is
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sufficient; the e-mail address isn't necessary.
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* It's helpful to add the bug/patch number as a comment:
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XXX Describe the transmogrify() function added to the socket
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module.
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(Contributed by P.Y. Developer in :issue:`12345`.)
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This saves the maintainer the effort of going through the Mercurial log
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when researching a change.
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This article explains the new features in Python 3.9, compared to 3.8.
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For full details, see the :ref:`changelog <changelog>`.
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.. note::
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Prerelease users should be aware that this document is currently in draft
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form. It will be updated substantially as Python 3.9 moves towards release,
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so it's worth checking back even after reading earlier versions.
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Summary -- Release highlights
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=============================
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.. This section singles out the most important changes in Python 3.9.
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Brevity is key.
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.. PEP-sized items next.
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New Features
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============
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Other Language Changes
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======================
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* :func:`__import__` now raises :exc:`ImportError` instead of
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:exc:`ValueError`, which used to occur when a relative import went past
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its top-level package.
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(Contributed by Ngalim Siregar in :issue:`37444`.)
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* Python now gets the absolute path of the script filename specified on
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the command line (ex: ``python3 script.py``): the ``__file__`` attribute of
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the :mod:`__main__` module, ``sys.argv[0]`` and ``sys.path[0]`` become an
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absolute path, rather than a relative path. These paths now remain valid
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after the current directory is changed by :func:`os.chdir`. As a side effect,
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a traceback also displays the absolute path for :mod:`__main__` module frames
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in this case.
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(Contributed by Victor Stinner in :issue:`20443`.)
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* In development mode and in debug build, *encoding* and *errors* arguments are
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now checked on string encoding and decoding operations. Examples:
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:func:`open`, :meth:`str.encode` and :meth:`bytes.decode`.
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By default, for best performance, the *errors* argument is only checked at
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the first encoding/decoding error and the *encoding* argument is sometimes
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ignored for empty strings.
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(Contributed by Victor Stinner in :issue:`37388`.)
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* ``"".replace("", s, n)`` now returns ``s`` instead of an empty string for
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all non-zero ``n``. It is now consistent with ``"".replace("", s)``.
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There are similar changes for :class:`bytes` and :class:`bytearray` objects.
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(Contributed by Serhiy Storchaka in :issue:`28029`.)
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New Modules
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===========
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* None yet.
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Improved Modules
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================
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ast
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---
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Added the *indent* option to :func:`~ast.dump` which allows it to produce a
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multiline indented output.
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(Contributed by Serhiy Storchaka in :issue:`37995`.)
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asyncio
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-------
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Added a new :term:`coroutine` :meth:`~asyncio.loop.shutdown_default_executor`
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that schedules a shutdown for the default executor that waits on the
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:class:`~concurrent.futures.ThreadPoolExecutor` to finish closing. Also,
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:func:`asyncio.run` has been updated to use the new :term:`coroutine`.
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(Contributed by Kyle Stanley in :issue:`34037`.)
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Added :class:`asyncio.PidfdChildWatcher`, a Linux-specific child watcher
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implementation that polls process file descriptors. (:issue:`38692`)
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curses
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------
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Add :func:`curses.get_escdelay`, :func:`curses.set_escdelay`,
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:func:`curses.get_tabsize`, and :func:`curses.set_tabsize` functions.
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(Contributed by Anthony Sottile in :issue:`38312`.)
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fcntl
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-----
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Added constants :data:`~fcntl.F_OFD_GETLK`, :data:`~fcntl.F_OFD_SETLK`
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and :data:`~fcntl.F_OFD_SETLKW`.
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(Contributed by Dong-hee Na in :issue:`38602`.)
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os
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--
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Added :data:`~os.CLD_KILLED` and :data:`~os.CLD_STOPPED` for :attr:`si_code`.
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(Contributed by Dong-hee Na in :issue:`38493`.)
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Exposed the Linux-specific :func:`os.pidfd_open` (:issue:`38692`) and
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:data:`os.P_PIDFD` (:issue:`38713`) for process management with file
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descriptors.
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threading
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---------
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In a subinterpreter, spawning a daemon thread now raises a :exc:`RuntimeError`. Daemon
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threads were never supported in subinterpreters. Previously, the subinterpreter
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finalization crashed with a Python fatal error if a daemon thread was still
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running.
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(Contributed by Victor Stinner in :issue:`37266`.)
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venv
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----
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The activation scripts provided by :mod:`venv` now all specify their prompt
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customization consistently by always using the value specified by
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``__VENV_PROMPT__``. Previously some scripts unconditionally used
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``__VENV_PROMPT__``, others only if it happened to be set (which was the default
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case), and one used ``__VENV_NAME__`` instead.
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(Contributed by Brett Cannon in :issue:`37663`.)
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pathlib
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-------
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Added :meth:`~pathlib.Path.readlink()` which acts similar to
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:func:`~os.readlink`.
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(Contributed by Girts Folkmanis in :issue:`30618`)
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pprint
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------
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:mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`.
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(Contributed by Carl Bordum Hansen in :issue:`37376`.)
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importlib
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---------
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To improve consistency with import statements, :func:`importlib.util.resolve_name`
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now raises :exc:`ImportError` instead of :exc:`ValueError` for invalid relative
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import attempts.
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(Contributed by Ngalim Siregar in :issue:`37444`.)
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Optimizations
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=============
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Build and C API Changes
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=======================
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* Provide :c:func:`Py_EnterRecursiveCall` and :c:func:`Py_LeaveRecursiveCall`
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as regular functions for the limited API. Previously, there were defined as
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macros, but these macros didn't work with the limited API which cannot access
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``PyThreadState.recursion_depth`` field. Remove ``_Py_CheckRecursionLimit``
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from the stable ABI.
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(Contributed by Victor Stinner in :issue:`38644`.)
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* Add a new public :c:func:`PyObject_CallNoArgs` function to the C API, which
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calls a callable Python object without any arguments. It is the most efficient
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way to call a callable Python object without any argument.
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(Contributed by Victor Stinner in :issue:`37194`.)
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Deprecated
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==========
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* Currently :func:`math.factorial` accepts :class:`float` instances with
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non-negative integer values (like ``5.0``). It raises a :exc:`ValueError`
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for non-integral and negative floats. It is now deprecated. In future
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Python versions it will raise a :exc:`TypeError` for all floats.
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(Contributed by Serhiy Storchaka in :issue:`37315`.)
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* The :mod:`parser` module is deprecated and will be removed in future versions
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of Python. For the majority of use cases, users can leverage the Abstract Syntax
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Tree (AST) generation and compilation stage, using the :mod:`ast` module.
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* The :mod:`random` module currently accepts any hashable type as a
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possible seed value. Unfortunately, some of those types are not
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guaranteed to have a deterministic hash value. After Python 3.9,
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the module will restrict its seeds to :const:`None`, :class:`int`,
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:class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`.
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* Deprecated the ``split()`` method of :class:`_tkinter.TkappType` in
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favour of the ``splitlist()`` method which has more consistent and
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predicable behavior.
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(Contributed by Serhiy Storchaka in :issue:`38371`.)
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Removed
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=======
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* The abstract base classes in :mod:`collections.abc` no longer are
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exposed in the regular :mod:`collections` module. This will help
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create a clearer distinction between the concrete classes and the abstract
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base classes.
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* The undocumented ``sys.callstats()`` function has been removed. Since Python
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3.7, it was deprecated and always returned :const:`None`. It required a special
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build option ``CALL_PROFILE`` which was already removed in Python 3.7.
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(Contributed by Victor Stinner in :issue:`37414`.)
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* The ``sys.getcheckinterval()`` and ``sys.setcheckinterval()`` functions have
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been removed. They were deprecated since Python 3.2. Use
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:func:`sys.getswitchinterval` and :func:`sys.setswitchinterval` instead.
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(Contributed by Victor Stinner in :issue:`37392`.)
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* The C function ``PyImport_Cleanup()`` has been removed. It was documented as:
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"Empty the module table. For internal use only."
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(Contributed by Victor Stinner in :issue:`36710`.)
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* ``_dummy_thread`` and ``dummy_threading`` modules have been removed. These
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modules were deprecated since Python 3.7 which requires threading support.
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(Contributed by Victor Stinner in :issue:`37312`.)
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* ``aifc.openfp()`` alias to ``aifc.open()``, ``sunau.openfp()`` alias to
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``sunau.open()``, and ``wave.openfp()`` alias to :func:`wave.open()` have been
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removed. They were deprecated since Python 3.7.
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(Contributed by Victor Stinner in :issue:`37320`.)
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* The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread`
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has been removed. It was deprecated since Python 3.8.
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Use :meth:`~threading.Thread.is_alive()` instead.
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(Contributed by Dong-hee Na in :issue:`37804`.)
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* Methods ``getchildren()`` and ``getiterator()`` in the
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:mod:`~xml.etree.ElementTree` module have been removed. They were
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deprecated in Python 3.2. Use functions :func:`list` and :func:`iter`
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instead. The ``xml.etree.cElementTree`` module has been removed.
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(Contributed by Serhiy Storchaka in :issue:`36543`.)
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* The old :mod:`plistlib` API has been removed, it was deprecated since Python
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3.4. Use the :func:`~plistlib.load`, :func:`~plistlib.loads`, :func:`~plistlib.dump`, and
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:func:`~plistlib.dumps` functions. Additionally, the *use_builtin_types* parameter was
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removed, standard :class:`bytes` objects are always used instead.
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(Contributed by Jon Janzen in :issue:`36409`.)
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* The C function ``PyThreadState_DeleteCurrent()`` has been removed. It was not documented.
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(Contributed by Joannah Nanjekye in :issue:`37878`.)
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* The C function ``PyGen_NeedsFinalizing`` has been removed. It was not
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documented, tested, or used anywhere within CPython after the implementation
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of :pep:`442`. Patch by Joannah Nanjekye.
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(Contributed by Joannah Nanjekye in :issue:`15088`)
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Porting to Python 3.9
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=====================
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This section lists previously described changes and other bugfixes
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that may require changes to your code.
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Changes in the Python API
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-------------------------
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* :func:`open`, :func:`io.open`, :func:`codecs.open` and
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:class:`fileinput.FileInput` no longer accept ``'U'`` ("universal newline")
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in the file mode. This flag was deprecated since Python 3.3. In Python 3, the
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"universal newline" is used by default when a file is open in text mode. The
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:ref:`newline parameter <open-newline-parameter>` of :func:`open` controls
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how universal newlines works.
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(Contributed by Victor Stinner in :issue:`37330`.)
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* :func:`__import__` and :func:`importlib.util.resolve_name` now raise
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:exc:`ImportError` where it previously raised :exc:`ValueError`. Callers
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catching the specific exception type and supporting both Python 3.9 and
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earlier versions will need to catch both using ``except (ImportError, ValueError):``.
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* The :mod:`venv` activation scripts no longer special-case when
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``__VENV_PROMPT__`` is set to ``""``.
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CPython bytecode changes
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------------------------
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* The :opcode:`LOAD_ASSERTION_ERROR` opcode was added for handling the
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:keyword:`assert` statement. Previously, the assert statement would not work
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correctly if the :exc:`AssertionError` exception was being shadowed.
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(Contributed by Zackery Spytz in :issue:`34880`.)
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