913 lines
36 KiB
ReStructuredText
913 lines
36 KiB
ReStructuredText
****************************
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What's New In Python 3.8
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****************************
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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 3.8, compared to 3.7.
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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 3.8 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 3.8.
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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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Parallel filesystem cache for compiled bytecode files
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-----------------------------------------------------
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The new :envvar:`PYTHONPYCACHEPREFIX` setting (also available as
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:option:`-X` ``pycache_prefix``) configures the implicit bytecode
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cache to use a separate parallel filesystem tree, rather than
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the default ``__pycache__`` subdirectories within each source
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directory.
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The location of the cache is reported in :data:`sys.pycache_prefix`
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(:const:`None` indicates the default location in ``__pycache__``
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subdirectories).
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(Contributed by Carl Meyer in :issue:`33499`.)
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Other Language Changes
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======================
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* A :keyword:`continue` statement was illegal in the :keyword:`finally` clause
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due to a problem with the implementation. In Python 3.8 this restriction
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was lifted.
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(Contributed by Serhiy Storchaka in :issue:`32489`.)
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* The :class:`int` type now has a new :meth:`~int.as_integer_ratio` method
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compatible with the existing :meth:`float.as_integer_ratio` method.
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(Contributed by Lisa Roach in :issue:`33073`.)
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* Added support of ``\N{name}`` escapes in :mod:`regular expressions <re>`.
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(Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.)
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* Dict and dictviews are now iterable in reversed insertion order using
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:func:`reversed`. (Contributed by Rémi Lapeyre in :issue:`33462`.)
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* The syntax allowed for keyword names in function calls was further
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restricted. In particular, ``f((keyword)=arg)`` is no longer allowed. It was
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never intended to permit more than a bare name on the left-hand side of a
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keyword argument assignment term. See :issue:`34641`.
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* Iterable unpacking is now allowed without parentheses in :keyword:`yield`
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and :keyword:`return` statements.
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(Contributed by David Cuthbert and Jordan Chapman in :issue:`32117`.)
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* A backslash-character pair that is not a valid escape sequence generates
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a :exc:`DeprecationWarning` since Python 3.6. In Python 3.8 it generates
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a :exc:`SyntaxWarning` instead.
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(Contributed by Serhiy Storchaka in :issue:`32912`.)
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* The compiler now produces a :exc:`SyntaxWarning` in some cases when a comma
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is missed before tuple or list. For example::
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data = [
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(1, 2, 3) # oops, missing comma!
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(4, 5, 6)
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]
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(Contributed by Serhiy Storchaka in :issue:`15248`.)
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* Arithmetic operations between subclasses of :class:`datetime.date` or
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:class:`datetime.datetime` and :class:`datetime.timedelta` objects now return
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an instance of the subclass, rather than the base class. This also affects
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the return type of operations whose implementation (directly or indirectly)
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uses :class:`datetime.timedelta` arithmetic, such as
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:meth:`datetime.datetime.astimezone`.
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(Contributed by Paul Ganssle in :issue:`32417`.)
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* When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the
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resulting :exc:`KeyboardInterrupt` exception is not caught, the Python process
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now exits via a SIGINT signal or with the correct exit code such that the
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calling process can detect that it died due to a Ctrl-C. Shells on POSIX
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and Windows use this to properly terminate scripts in interactive sessions.
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(Contributed by Google via Gregory P. Smith in :issue:`1054041`.)
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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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* The :meth:`_asdict()` method for :func:`collections.namedtuple` now returns
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a :class:`dict` instead of a :class:`collections.OrderedDict`. This works because
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regular dicts have guaranteed ordering in since Python 3.7. If the extra
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features of :class:`OrderedDict` are required, the suggested remediation is
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to cast the result to the desired type: ``OrderedDict(nt._asdict())``.
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(Contributed by Raymond Hettinger in :issue:`35864`.)
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* The :mod:`unicodedata` module has been upgraded to use the `Unicode 12.0.0
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<http://blog.unicode.org/2019/03/announcing-unicode-standard-version-120.html>`_
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release.
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asyncio
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-------
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On Windows, the default event loop is now :class:`~asyncio.ProactorEventLoop`.
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ctypes
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------
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On Windows, :class:`~ctypes.CDLL` and subclasses now accept a *winmode* parameter
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to specify flags for the underlying ``LoadLibraryEx`` call. The default flags are
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set to only load DLL dependencies from trusted locations, including the path
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where the DLL is stored (if a full or partial path is used to load the initial
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DLL) and paths added by :func:`~os.add_dll_directory`.
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gettext
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-------
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Added :func:`~gettext.pgettext` and its variants.
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(Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in :issue:`2504`.)
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inspect
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-------
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The :func:`inspect.getdoc` function can now find docstrings for ``__slots__``
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if that attribute is a :class:`dict` where the values are docstrings.
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This provides documentation options similar to what we already have
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for :func:`property`, :func:`classmethod`, and :func:`staticmethod`::
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class AudioClip:
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__slots__ = {'bit_rate': 'expressed in kilohertz to one decimal place',
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'duration': 'in seconds, rounded up to an integer'}
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def __init__(self, bit_rate, duration):
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self.bit_rate = round(bit_rate / 1000.0, 1)
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self.duration = ceil(duration)
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gc
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--
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:func:`~gc.get_objects` can now receive an optional *generation* parameter
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indicating a generation to get objects from. Contributed in
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:issue:`36016` by Pablo Galindo.
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gzip
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----
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Added the *mtime* parameter to :func:`gzip.compress` for reproducible output.
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(Contributed by Guo Ci Teo in :issue:`34898`.)
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idlelib and IDLE
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----------------
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Output over N lines (50 by default) is squeezed down to a button.
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N can be changed in the PyShell section of the General page of the
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Settings dialog. Fewer, but possibly extra long, lines can be squeezed by
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right clicking on the output. Squeezed output can be expanded in place
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by double-clicking the button or into the clipboard or a separate window
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by right-clicking the button. (Contributed by Tal Einat in :issue:`1529353`.)
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The changes above have been backported to 3.7 maintenance releases.
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json.tool
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---------
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Add option ``--json-lines`` to parse every input line as separate JSON object.
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(Contributed by Weipeng Hong in :issue:`31553`.)
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math
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----
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Added new function :func:`math.dist` for computing Euclidean distance
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between two points. (Contributed by Raymond Hettinger in :issue:`33089`.)
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Expanded the :func:`math.hypot` function to handle multiple dimensions.
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Formerly, it only supported the 2-D case.
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(Contributed by Raymond Hettinger in :issue:`33089`.)
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Added new function, :func:`math.prod`, as analogous function to :func:`sum`
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that returns the product of a 'start' value (default: 1) times an iterable of
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numbers. (Contributed by Pablo Galindo in :issue:`35606`)
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os
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--
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Added new function :func:`~os.add_dll_directory` on Windows for providing
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additional search paths for native dependencies when importing extension
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modules or loading DLLs using :mod:`ctypes`.
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os.path
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-------
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:mod:`os.path` functions that return a boolean result like
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:func:`~os.path.exists`, :func:`~os.path.lexists`, :func:`~os.path.isdir`,
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:func:`~os.path.isfile`, :func:`~os.path.islink`, and :func:`~os.path.ismount`
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now return ``False`` instead of raising :exc:`ValueError` or its subclasses
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:exc:`UnicodeEncodeError` and :exc:`UnicodeDecodeError` for paths that contain
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characters or bytes unrepresentable at the OS level.
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(Contributed by Serhiy Storchaka in :issue:`33721`.)
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:func:`~os.path.expanduser` on Windows now prefers the :envvar:`USERPROFILE`
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environment variable and does not use :envvar:`HOME`, which is not normally set
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for regular user accounts.
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ncurses
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-------
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Added a new variable holding structured version information for the
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underlying ncurses library: :data:`~curses.ncurses_version`.
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(Contributed by Serhiy Storchaka in :issue:`31680`.)
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pathlib
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-------
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:mod:`pathlib.Path` methods that return a boolean result like
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:meth:`~pathlib.Path.exists()`, :meth:`~pathlib.Path.is_dir()`,
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:meth:`~pathlib.Path.is_file()`, :meth:`~pathlib.Path.is_mount()`,
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:meth:`~pathlib.Path.is_symlink()`, :meth:`~pathlib.Path.is_block_device()`,
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:meth:`~pathlib.Path.is_char_device()`, :meth:`~pathlib.Path.is_fifo()`,
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:meth:`~pathlib.Path.is_socket()` now return ``False`` instead of raising
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:exc:`ValueError` or its subclass :exc:`UnicodeEncodeError` for paths that
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contain characters unrepresentable at the OS level.
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(Contributed by Serhiy Storchaka in :issue:`33721`.)
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socket
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------
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Added :meth:`~socket.create_server()` and :meth:`~socket.has_dualstack_ipv6()`
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convenience functions to automate the necessary tasks usually involved when
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creating a server socket, including accepting both IPv4 and IPv6 connections
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on the same socket. (Contributed by Giampaolo Rodola in :issue:`17561`.)
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shutil
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------
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:func:`shutil.copytree` now accepts a new ``dirs_exist_ok`` keyword argument.
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(Contributed by Josh Bronson in :issue:`20849`.)
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:func:`shutil.make_archive` now defaults to the modern pax (POSIX.1-2001)
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format for new archives to improve portability and standards conformance,
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inherited from the corresponding change to the :mod:`tarfile` module.
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(Contributed by C.A.M. Gerlach in :issue:`30661`.)
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ssl
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---
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Added :attr:`SSLContext.post_handshake_auth` to enable and
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:meth:`ssl.SSLSocket.verify_client_post_handshake` to initiate TLS 1.3
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post-handshake authentication.
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(Contributed by Christian Heimes in :issue:`34670`.)
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statistics
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----------
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Added :func:`statistics.fmean` as a faster, floating point variant of
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:func:`statistics.mean()`. (Contributed by Raymond Hettinger and
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Steven D'Aprano in :issue:`35904`.)
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Added :func:`statistics.geometric_mean()`
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(Contributed by Raymond Hettinger in :issue:`27181`.)
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Added :func:`statistics.multimode` that returns a list of the most
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common values. (Contributed by Raymond Hettinger in :issue:`35892`.)
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Added :class:`statistics.NormalDist`, a tool for creating
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and manipulating normal distributions of a random variable.
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(Contributed by Raymond Hettinger in :issue:`36018`.)
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::
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>>> temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14])
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>>> temperature_feb
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NormalDist(mu=6.0, sigma=6.356099432828281)
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>>> temperature_feb.cdf(3) # Chance of being under 3 degrees
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0.3184678262814532
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>>> # Relative chance of being 7 degrees versus 10 degrees
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>>> temperature_feb.pdf(7) / temperature_feb.pdf(10)
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1.2039930378537762
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>>> el_nino = NormalDist(4, 2.5)
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>>> temperature_feb += el_nino # Add in a climate effect
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>>> temperature_feb
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NormalDist(mu=10.0, sigma=6.830080526611674)
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>>> temperature_feb * (9/5) + 32 # Convert to Fahrenheit
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NormalDist(mu=50.0, sigma=12.294144947901014)
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>>> temperature_feb.samples(3) # Generate random samples
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[7.672102882379219, 12.000027119750287, 4.647488369766392]
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tarfile
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-------
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The :mod:`tarfile` module now defaults to the modern pax (POSIX.1-2001)
|
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format for new archives, instead of the previous GNU-specific one.
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This improves cross-platform portability with a consistent encoding (UTF-8)
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in a standardized and extensible format, and offers several other benefits.
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(Contributed by C.A.M. Gerlach in :issue:`36268`.)
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tokenize
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--------
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The :mod:`tokenize` module now implicitly emits a ``NEWLINE`` token when
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provided with input that does not have a trailing new line. This behavior
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now matches what the C tokenizer does internally.
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(Contributed by Ammar Askar in :issue:`33899`.)
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tkinter
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-------
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Added methods :meth:`~tkinter.Spinbox.selection_from`,
|
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:meth:`~tkinter.Spinbox.selection_present`,
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:meth:`~tkinter.Spinbox.selection_range` and
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:meth:`~tkinter.Spinbox.selection_to`
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in the :class:`tkinter.Spinbox` class.
|
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(Contributed by Juliette Monsel in :issue:`34829`.)
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Added method :meth:`~tkinter.Canvas.moveto`
|
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in the :class:`tkinter.Canvas` class.
|
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(Contributed by Juliette Monsel in :issue:`23831`.)
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The :class:`tkinter.PhotoImage` class now has
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:meth:`~tkinter.PhotoImage.transparency_get` and
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:meth:`~tkinter.PhotoImage.transparency_set` methods. (Contributed by
|
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Zackery Spytz in :issue:`25451`.)
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time
|
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----
|
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|
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Added new clock :data:`~time.CLOCK_UPTIME_RAW` for macOS 10.12.
|
|
(Contributed by Joannah Nanjekye in :issue:`35702`.)
|
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|
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unicodedata
|
|
-----------
|
|
|
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* New function :func:`~unicodedata.is_normalized` can be used to verify a string
|
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is in a specific normal form. (Contributed by Max Belanger and David Euresti in
|
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:issue:`32285`).
|
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unittest
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|
--------
|
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|
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* Added :func:`~unittest.addModuleCleanup()` and
|
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:meth:`~unittest.TestCase.addClassCleanup()` to unittest to support
|
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cleanups for :func:`~unittest.setUpModule()` and
|
|
:meth:`~unittest.TestCase.setUpClass()`.
|
|
(Contributed by Lisa Roach in :issue:`24412`.)
|
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|
|
venv
|
|
----
|
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|
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* :mod:`venv` now includes an ``Activate.ps1`` script on all platforms for
|
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activating virtual environments under PowerShell Core 6.1.
|
|
(Contributed by Brett Cannon in :issue:`32718`.)
|
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|
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xml
|
|
---
|
|
|
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* As mitigation against DTD and external entity retrieval, the
|
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:mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process
|
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external entities by default.
|
|
(Contributed by Christian Heimes in :issue:`17239`.)
|
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Optimizations
|
|
=============
|
|
|
|
* The :mod:`subprocess` module can now use the :func:`os.posix_spawn` function
|
|
in some cases for better performance. Currently, it is only used on macOS
|
|
and Linux (using glibc 2.24 or newer) if all these conditions are met:
|
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|
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* *close_fds* is false;
|
|
* *preexec_fn*, *pass_fds*, *cwd* and *start_new_session* parameters
|
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are not set;
|
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* the *executable* path contains a directory.
|
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* :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
|
|
:func:`shutil.copytree` and :func:`shutil.move` use platform-specific
|
|
"fast-copy" syscalls on Linux, macOS and Solaris in order to copy the file
|
|
more efficiently.
|
|
"fast-copy" means that the copying operation occurs within the kernel,
|
|
avoiding the use of userspace buffers in Python as in
|
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"``outfd.write(infd.read())``".
|
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On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
|
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instead of 16 KiB) and a :func:`memoryview`-based variant of
|
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:func:`shutil.copyfileobj` is used.
|
|
The speedup for copying a 512 MiB file within the same partition is about
|
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+26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles
|
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are consumed.
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See :ref:`shutil-platform-dependent-efficient-copy-operations` section.
|
|
(Contributed by Giampaolo Rodola' in :issue:`33671`.)
|
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|
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* :func:`shutil.copytree` uses :func:`os.scandir` function and all copy
|
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functions depending from it use cached :func:`os.stat` values. The speedup
|
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for copying a directory with 8000 files is around +9% on Linux, +20% on
|
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Windows and +30% on a Windows SMB share. Also the number of :func:`os.stat`
|
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syscalls is reduced by 38% making :func:`shutil.copytree` especially faster
|
|
on network filesystems. (Contributed by Giampaolo Rodola' in :issue:`33695`.)
|
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|
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* The default protocol in the :mod:`pickle` module is now Protocol 4,
|
|
first introduced in Python 3.4. It offers better performance and smaller
|
|
size compared to Protocol 3 available since Python 3.0.
|
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|
|
* Removed one ``Py_ssize_t`` member from ``PyGC_Head``. All GC tracked
|
|
objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes.
|
|
(Contributed by Inada Naoki in :issue:`33597`)
|
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|
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* :class:`uuid.UUID` now uses ``__slots__`` to reduce its memory footprint.
|
|
|
|
* Improved performance of :func:`operator.itemgetter` by 33%. Optimized
|
|
argument handling and added a fast path for the common case of a single
|
|
non-negative integer index into a tuple (which is the typical use case in
|
|
the standard library). (Contributed by Raymond Hettinger in
|
|
:issue:`35664`.)
|
|
|
|
* Sped-up field lookups in :func:`collections.namedtuple`. They are now more
|
|
than two times faster, making them the fastest form of instance variable
|
|
lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and
|
|
Joe Jevnik, Serhiy Storchaka in :issue:`32492`.)
|
|
|
|
* The :class:`list` constructor does not overallocate the internal item buffer
|
|
if the input iterable has a known length (the input implements ``__len__``).
|
|
This makes the created list 12% smaller on average. (Contributed by
|
|
Raymond Hettinger and Pablo Galindo in :issue:`33234`.)
|
|
|
|
* Doubled the speed of class variable writes. When a non-dunder attribute
|
|
was updated, there was an unnecessary call to update slots.
|
|
(Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond Hettinger,
|
|
Neil Schemenauer, and Serhiy Storchaka in :issue:`36012`.)
|
|
|
|
* Reduced an overhead of converting arguments passed to many builtin functions
|
|
and methods. This sped up calling some simple builtin functions and
|
|
methods up to 20--50%. (Contributed by Serhiy Storchaka in :issue:`23867`,
|
|
:issue:`35582` and :issue:`36127`.)
|
|
|
|
|
|
Build and C API Changes
|
|
=======================
|
|
|
|
* The :c:func:`PyByteArray_Init` and :c:func:`PyByteArray_Fini` functions have
|
|
been removed. They did nothing since Python 2.7.4 and Python 3.2.0, were
|
|
excluded from the limited API (stable ABI), and were not documented.
|
|
|
|
* The result of :c:func:`PyExceptionClass_Name` is now of type
|
|
``const char *`` rather of ``char *``.
|
|
(Contributed by Serhiy Storchaka in :issue:`33818`.)
|
|
|
|
* The duality of ``Modules/Setup.dist`` and ``Modules/Setup`` has been
|
|
removed. Previously, when updating the CPython source tree, one had
|
|
to manually copy ``Modules/Setup.dist`` (inside the source tree) to
|
|
``Modules/Setup`` (inside the build tree) in order to reflect any changes
|
|
upstream. This was of a small benefit to packagers at the expense of
|
|
a frequent annoyance to developers following CPython development, as
|
|
forgetting to copy the file could produce build failures.
|
|
|
|
Now the build system always reads from ``Modules/Setup`` inside the source
|
|
tree. People who want to customize that file are encouraged to maintain
|
|
their changes in a git fork of CPython or as patch files, as they would do
|
|
for any other change to the source tree.
|
|
|
|
(Contributed by Antoine Pitrou in :issue:`32430`.)
|
|
|
|
* Functions that convert Python number to C integer like
|
|
:c:func:`PyLong_AsLong` and argument parsing functions like
|
|
:c:func:`PyArg_ParseTuple` with integer converting format units like ``'i'``
|
|
will now use the :meth:`~object.__index__` special method instead of
|
|
:meth:`~object.__int__`, if available. The deprecation warning will be
|
|
emitted for objects with the ``__int__()`` method but without the
|
|
``__index__()`` method (like :class:`~decimal.Decimal` and
|
|
:class:`~fractions.Fraction`). :c:func:`PyNumber_Check` will now return
|
|
``1`` for objects implementing ``__index__()``.
|
|
(Contributed by Serhiy Storchaka in :issue:`36048`.)
|
|
|
|
* Heap-allocated type objects will now increase their reference count
|
|
in :c:func:`PyObject_Init` (and its parallel macro ``PyObject_INIT``)
|
|
instead of in :c:func:`PyType_GenericAlloc`. Types that modify instance
|
|
allocation or deallocation may need to be adjusted.
|
|
(Contributed by Eddie Elizondo in :issue:`35810`.)
|
|
|
|
|
|
Deprecated
|
|
==========
|
|
|
|
* Deprecated methods ``getchildren()`` and ``getiterator()`` in
|
|
the :mod:`~xml.etree.ElementTree` module emit now a
|
|
:exc:`DeprecationWarning` instead of :exc:`PendingDeprecationWarning`.
|
|
They will be removed in Python 3.9.
|
|
(Contributed by Serhiy Storchaka in :issue:`29209`.)
|
|
|
|
* Passing an object that is not an instance of
|
|
:class:`concurrent.futures.ThreadPoolExecutor` to
|
|
:meth:`asyncio.loop.set_default_executor()` is
|
|
deprecated and will be prohibited in Python 3.9.
|
|
(Contributed by Elvis Pranskevichus in :issue:`34075`.)
|
|
|
|
* The :meth:`__getitem__` methods of :class:`xml.dom.pulldom.DOMEventStream`,
|
|
:class:`wsgiref.util.FileWrapper` and :class:`fileinput.FileInput` have been
|
|
deprecated.
|
|
|
|
Implementations of these methods have been ignoring their *index* parameter,
|
|
and returning the next item instead.
|
|
|
|
(Contributed by Berker Peksag in :issue:`9372`.)
|
|
|
|
* The :class:`typing.NamedTuple` class has deprecated the ``_field_types``
|
|
attribute in favor of the ``__annotations__`` attribute which has the same
|
|
information. (Contributed by Raymond Hettinger in :issue:`36320`.)
|
|
|
|
* :mod:`ast` classes ``Num``, ``Str``, ``Bytes``, ``NameConstant`` and
|
|
``Ellipsis`` are considered deprecated and will be removed in future Python
|
|
versions. :class:`~ast.Constant` should be used instead.
|
|
(Contributed by Serhiy Storchaka in :issue:`32892`.)
|
|
|
|
* The following functions and methods are deprecated in the :mod:`gettext`
|
|
module: :func:`~gettext.lgettext`, :func:`~gettext.ldgettext`,
|
|
:func:`~gettext.lngettext` and :func:`~gettext.ldngettext`.
|
|
They return encoded bytes, and it's possible that you will get unexpected
|
|
Unicode-related exceptions if there are encoding problems with the
|
|
translated strings. It's much better to use alternatives which return
|
|
Unicode strings in Python 3. These functions have been broken for a long time.
|
|
|
|
Function :func:`~gettext.bind_textdomain_codeset`, methods
|
|
:meth:`~gettext.NullTranslations.output_charset` and
|
|
:meth:`~gettext.NullTranslations.set_output_charset`, and the *codeset*
|
|
parameter of functions :func:`~gettext.translation` and
|
|
:func:`~gettext.install` are also deprecated, since they are only used for
|
|
for the ``l*gettext()`` functions.
|
|
|
|
(Contributed by Serhiy Storchaka in :issue:`33710`.)
|
|
|
|
* The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread` has been deprecated.
|
|
(Contributed by Dong-hee Na in :issue:`35283`.)
|
|
|
|
* Many builtin and extension functions that take integer arguments will
|
|
now emit a deprecation warning for :class:`~decimal.Decimal`\ s,
|
|
:class:`~fractions.Fraction`\ s and any other objects that can be converted
|
|
to integers only with a loss (e.g. that have the :meth:`~object.__int__`
|
|
method but do not have the :meth:`~object.__index__` method). In future
|
|
version they will be errors.
|
|
(Contributed by Serhiy Storchaka in :issue:`36048`.)
|
|
|
|
* Deprecated passing the following arguments as keyword arguments:
|
|
|
|
- *func* in :func:`functools.partialmethod`, :func:`weakref.finalize`,
|
|
:meth:`profile.Profile.runcall`, :meth:`cProfile.Profile.runcall`,
|
|
:meth:`bdb.Bdb.runcall`, :meth:`trace.Trace.runfunc` and
|
|
:func:`curses.wrapper`.
|
|
- *function* in :func:`unittest.addModuleCleanup` and
|
|
:meth:`unittest.TestCase.addCleanup`.
|
|
- *fn* in the :meth:`~concurrent.futures.Executor.submit` method of
|
|
:class:`concurrent.futures.ThreadPoolExecutor` and
|
|
:class:`concurrent.futures.ProcessPoolExecutor`.
|
|
- *callback* in :meth:`contextlib.ExitStack.callback`,
|
|
:meth:`contextlib.AsyncExitStack.callback` and
|
|
:meth:`contextlib.AsyncExitStack.push_async_callback`.
|
|
- *c* and *typeid* in the :meth:`~multiprocessing.managers.Server.create`
|
|
method of :class:`multiprocessing.managers.Server` and
|
|
:class:`multiprocessing.managers.SharedMemoryServer`.
|
|
- *obj* in :func:`weakref.finalize`.
|
|
|
|
In future releases of Python they will be :ref:`positional-only
|
|
<positional-only_parameter>`.
|
|
(Contributed by Serhiy Storchaka in :issue:`36492`.)
|
|
|
|
|
|
API and Feature Removals
|
|
========================
|
|
|
|
The following features and APIs have been removed from Python 3.8:
|
|
|
|
* The :mod:`macpath` module, deprecated in Python 3.7, has been removed.
|
|
(Contributed by Victor Stinner in :issue:`35471`.)
|
|
|
|
* The function :func:`platform.popen` has been removed, it was deprecated since
|
|
Python 3.3: use :func:`os.popen` instead.
|
|
|
|
* The ``pyvenv`` script has been removed in favor of ``python3.8 -m venv``
|
|
to help eliminate confusion as to what Python interpreter the ``pyvenv``
|
|
script is tied to. (Contributed by Brett Cannon in :issue:`25427`.)
|
|
|
|
* ``parse_qs``, ``parse_qsl``, and ``escape`` are removed from :mod:`cgi`
|
|
module. They are deprecated from Python 3.2 or older.
|
|
|
|
* ``filemode`` function is removed from :mod:`tarfile` module.
|
|
It is not documented and deprecated since Python 3.3.
|
|
|
|
* The :class:`~xml.etree.ElementTree.XMLParser` constructor no longer accepts
|
|
the *html* argument. It never had effect and was deprecated in Python 3.4.
|
|
All other parameters are now :ref:`keyword-only <keyword-only_parameter>`.
|
|
(Contributed by Serhiy Storchaka in :issue:`29209`.)
|
|
|
|
* Removed the ``doctype()`` method of :class:`~xml.etree.ElementTree.XMLParser`.
|
|
(Contributed by Serhiy Storchaka in :issue:`29209`.)
|
|
|
|
* "unicode_internal" codec is removed.
|
|
(Contributed by Inada Naoki in :issue:`36297`.)
|
|
|
|
|
|
Porting to Python 3.8
|
|
=====================
|
|
|
|
This section lists previously described changes and other bugfixes
|
|
that may require changes to your code.
|
|
|
|
|
|
Changes in Python behavior
|
|
--------------------------
|
|
|
|
* Yield expressions (both ``yield`` and ``yield from`` clauses) are now disallowed
|
|
in comprehensions and generator expressions (aside from the iterable expression
|
|
in the leftmost :keyword:`!for` clause).
|
|
(Contributed by Serhiy Storchaka in :issue:`10544`.)
|
|
|
|
* The compiler now produces a :exc:`SyntaxWarning` when identity checks
|
|
(``is`` and ``is not``) are used with certain types of literals
|
|
(e.g. strings, ints). These can often work by accident in CPython,
|
|
but are not guaranteed by the language spec. The warning advises users
|
|
to use equality tests (``==`` and ``!=``) instead.
|
|
(Contributed by Serhiy Storchaka in :issue:`34850`.)
|
|
|
|
* The CPython interpreter can swallow exceptions in some circumstances.
|
|
In Python 3.8 this happens in less cases. In particular, exceptions
|
|
raised when getting the attribute from the type dictionary are no longer
|
|
ignored. (Contributed by Serhiy Storchaka in :issue:`35459`.)
|
|
|
|
|
|
Changes in the Python API
|
|
-------------------------
|
|
|
|
* The function :func:`platform.popen` has been removed, it was deprecated since
|
|
Python 3.3: use :func:`os.popen` instead.
|
|
|
|
* The :func:`statistics.mode` function no longer raises an exception
|
|
when given multimodal data. Instead, it returns the first mode
|
|
encountered in the input data. (Contributed by Raymond Hettinger
|
|
in :issue:`35892`.)
|
|
|
|
* The :meth:`~tkinter.ttk.Treeview.selection` method of the
|
|
:class:`tkinter.ttk.Treeview` class no longer takes arguments. Using it with
|
|
arguments for changing the selection was deprecated in Python 3.6. Use
|
|
specialized methods like :meth:`~tkinter.ttk.Treeview.selection_set` for
|
|
changing the selection. (Contributed by Serhiy Storchaka in :issue:`31508`.)
|
|
|
|
* The :meth:`writexml`, :meth:`toxml` and :meth:`toprettyxml` methods of the
|
|
:mod:`xml.dom.minidom` module, and :mod:`xml.etree` now preserve the attribute
|
|
order specified by the user.
|
|
(Contributed by Diego Rojas and Raymond Hettinger in :issue:`34160`.)
|
|
|
|
* A :mod:`dbm.dumb` database opened with flags ``'r'`` is now read-only.
|
|
:func:`dbm.dumb.open` with flags ``'r'`` and ``'w'`` no longer creates
|
|
a database if it does not exist.
|
|
(Contributed by Serhiy Storchaka in :issue:`32749`.)
|
|
|
|
* The ``doctype()`` method defined in a subclass of
|
|
:class:`~xml.etree.ElementTree.XMLParser` will no longer be called and will
|
|
cause emitting a :exc:`RuntimeWarning` instead of a :exc:`DeprecationWarning`.
|
|
Define the :meth:`doctype() <xml.etree.ElementTree.TreeBuilder.doctype>`
|
|
method on a target for handling an XML doctype declaration.
|
|
(Contributed by Serhiy Storchaka in :issue:`29209`.)
|
|
|
|
* A :exc:`RuntimeError` is now raised when the custom metaclass doesn't
|
|
provide the ``__classcell__`` entry in the namespace passed to
|
|
``type.__new__``. A :exc:`DeprecationWarning` was emitted in Python
|
|
3.6--3.7. (Contributed by Serhiy Storchaka in :issue:`23722`.)
|
|
|
|
* The :class:`cProfile.Profile` class can now be used as a context
|
|
manager. (Contributed by Scott Sanderson in :issue:`29235`.)
|
|
|
|
* :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
|
|
:func:`shutil.copytree` and :func:`shutil.move` use platform-specific
|
|
"fast-copy" syscalls (see
|
|
:ref:`shutil-platform-dependent-efficient-copy-operations` section).
|
|
|
|
* :func:`shutil.copyfile` default buffer size on Windows was changed from
|
|
16 KiB to 1 MiB.
|
|
|
|
* ``PyGC_Head`` struct is changed completely. All code touched the
|
|
struct member should be rewritten. (See :issue:`33597`)
|
|
|
|
* The ``PyInterpreterState`` struct has been moved into the "internal"
|
|
header files (specifically Include/internal/pycore_pystate.h). An
|
|
opaque ``PyInterpreterState`` is still available as part of the public
|
|
API (and stable ABI). The docs indicate that none of the struct's
|
|
fields are public, so we hope no one has been using them. However,
|
|
if you do rely on one or more of those private fields and have no
|
|
alternative then please open a BPO issue. We'll work on helping
|
|
you adjust (possibly including adding accessor functions to the
|
|
public API). (See :issue:`35886`.)
|
|
|
|
* Asyncio tasks can now be named, either by passing the ``name`` keyword
|
|
argument to :func:`asyncio.create_task` or
|
|
the :meth:`~asyncio.loop.create_task` event loop method, or by
|
|
calling the :meth:`~asyncio.Task.set_name` method on the task object. The
|
|
task name is visible in the ``repr()`` output of :class:`asyncio.Task` and
|
|
can also be retrieved using the :meth:`~asyncio.Task.get_name` method.
|
|
|
|
* The :meth:`mmap.flush() <mmap.mmap.flush>` method now returns ``None`` on
|
|
success and raises an exception on error under all platforms. Previously,
|
|
its behavior was platform-depended: a nonzero value was returned on success;
|
|
zero was returned on error under Windows. A zero value was returned on
|
|
success; an exception was raised on error under Unix.
|
|
(Contributed by Berker Peksag in :issue:`2122`.)
|
|
|
|
* The function :func:`math.factorial` no longer accepts arguments that are not
|
|
int-like. (Contributed by Pablo Galindo in :issue:`33083`.)
|
|
|
|
* :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process
|
|
external entities by default.
|
|
(Contributed by Christian Heimes in :issue:`17239`.)
|
|
|
|
* Deleting a key from a read-only :mod:`dbm` database (:mod:`dbm.dumb`,
|
|
:mod:`dbm.gnu` or :mod:`dbm.ndbm`) raises :attr:`error` (:exc:`dbm.dumb.error`,
|
|
:exc:`dbm.gnu.error` or :exc:`dbm.ndbm.error`) instead of :exc:`KeyError`.
|
|
(Contributed by Xiang Zhang in :issue:`33106`.)
|
|
|
|
* :func:`~os.path.expanduser` on Windows now prefers the :envvar:`USERPROFILE`
|
|
environment variable and does not use :envvar:`HOME`, which is not normally
|
|
set for regular user accounts.
|
|
|
|
.. _bpo-36085-whatsnew:
|
|
|
|
* DLL dependencies for extension modules and DLLs loaded with :mod:`ctypes` on
|
|
Windows are now resolved more securely. Only the system paths, the directory
|
|
containing the DLL or PYD file, and directories added with
|
|
:func:`~os.add_dll_directory` are searched for load-time dependencies.
|
|
Specifically, :envvar:`PATH` and the current working directory are no longer
|
|
used, and modifications to these will no longer have any effect on normal DLL
|
|
resolution. If your application relies on these mechanisms, you should check
|
|
for :func:`~os.add_dll_directory` and if it exists, use it to add your DLLs
|
|
directory while loading your library. Note that Windows 7 users will need to
|
|
ensure that Windows Update KB2533625 has been installed (this is also verified
|
|
by the installer).
|
|
(See :issue:`36085`.)
|
|
|
|
|
|
Changes in the C API
|
|
--------------------
|
|
|
|
* Use of ``#`` variants of formats in parsing or building value (e.g.
|
|
:c:func:`PyArg_ParseTuple`, :c:func:`Py_BuildValue`, :c:func:`PyObject_CallFunction`,
|
|
etc.) without ``PY_SSIZE_T_CLEAN`` defined raises ``DeprecationWarning`` now.
|
|
It will be removed in 3.10 or 4.0. Read :ref:`arg-parsing` for detail.
|
|
(Contributed by Inada Naoki in :issue:`36381`.)
|
|
|
|
|
|
Changes in the C API
|
|
--------------------------
|
|
|
|
* Instances of heap-allocated types (such as those created with
|
|
:c:func:`PyType_FromSpec`) hold a reference to their type object.
|
|
Increasing the reference count of these type objects has been moved from
|
|
:c:func:`PyType_GenericAlloc` to the more low-level functions,
|
|
:c:func:`PyObject_Init` and :c:func:`PyObject_INIT`.
|
|
This makes types created through :c:func:`PyType_FromSpec` behave like
|
|
other classes in managed code.
|
|
|
|
Statically allocated types are not affected.
|
|
|
|
For the vast majority of cases, there should be no side effect.
|
|
However, types that manually increase the reference count after allocating
|
|
an instance (perhaps to work around the bug) may now become immortal.
|
|
To avoid this, these classes need to call Py_DECREF on the type object
|
|
during instance deallocation.
|
|
|
|
To correctly port these types into 3.8, please apply the following
|
|
changes:
|
|
|
|
* Remove :c:macro:`Py_INCREF` on the type object after allocating an
|
|
instance - if any.
|
|
This may happen after calling :c:func:`PyObject_New`,
|
|
:c:func:`PyObject_NewVar`, :c:func:`PyObject_GC_New`,
|
|
:c:func:`PyObject_GC_NewVar`, or any other custom allocator that uses
|
|
:c:func:`PyObject_Init` or :c:func:`PyObject_INIT`.
|
|
|
|
Example::
|
|
|
|
static foo_struct *
|
|
foo_new(PyObject *type) {
|
|
foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type);
|
|
if (foo == NULL)
|
|
return NULL;
|
|
#if PY_VERSION_HEX < 0x03080000
|
|
// Workaround for Python issue 35810; no longer necessary in Python 3.8
|
|
PY_INCREF(type)
|
|
#endif
|
|
return foo;
|
|
}
|
|
|
|
* Ensure that all custom ``tp_dealloc`` functions of heap-allocated types
|
|
decrease the type's reference count.
|
|
|
|
Example::
|
|
|
|
static void
|
|
foo_dealloc(foo_struct *instance) {
|
|
PyObject *type = Py_TYPE(instance);
|
|
PyObject_GC_Del(instance);
|
|
#if PY_VERSION_HEX >= 0x03080000
|
|
// This was not needed before Python 3.8 (Python issue 35810)
|
|
Py_DECREF(type);
|
|
#endif
|
|
}
|
|
|
|
(Contributed by Eddie Elizondo in :issue:`35810`.)
|
|
|
|
|
|
CPython bytecode changes
|
|
------------------------
|
|
|
|
* The interpreter loop has been simplified by moving the logic of unrolling
|
|
the stack of blocks into the compiler. The compiler emits now explicit
|
|
instructions for adjusting the stack of values and calling the
|
|
cleaning-up code for :keyword:`break`, :keyword:`continue` and
|
|
:keyword:`return`.
|
|
|
|
Removed opcodes :opcode:`BREAK_LOOP`, :opcode:`CONTINUE_LOOP`,
|
|
:opcode:`SETUP_LOOP` and :opcode:`SETUP_EXCEPT`. Added new opcodes
|
|
:opcode:`ROT_FOUR`, :opcode:`BEGIN_FINALLY`, :opcode:`CALL_FINALLY` and
|
|
:opcode:`POP_FINALLY`. Changed the behavior of :opcode:`END_FINALLY`
|
|
and :opcode:`WITH_CLEANUP_START`.
|
|
|
|
(Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in
|
|
:issue:`17611`.)
|
|
|
|
* Added new opcode :opcode:`END_ASYNC_FOR` for handling exceptions raised
|
|
when awaiting a next item in an :keyword:`async for` loop.
|
|
(Contributed by Serhiy Storchaka in :issue:`33041`.)
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Demos and Tools
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---------------
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* Added a benchmark script for timing various ways to access variables:
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``Tools/scripts/var_access_benchmark.py``.
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(Contributed by Raymond Hettinger in :issue:`35884`.)
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