314 lines
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ReStructuredText
314 lines
12 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 ``int`` type now has a new ``as_integer_ratio`` method compatible
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with the existing ``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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* 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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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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Optimizations
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=============
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* :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
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:func:`shutil.copytree` and :func:`shutil.move` use platform-specific
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"fast-copy" syscalls on Linux, macOS and Solaris in order to copy the file
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more efficiently.
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"fast-copy" means that the copying operation occurs within the kernel,
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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.
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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.
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(Contributed by Giampaolo Rodola' in :issue:`25427`.)
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* The default protocol in the :mod:`pickle` module is now Protocol 4,
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first introduced in Python 3.4. It offers better performance and smaller
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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
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objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes.
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(Contributed by Inada Naoki in :issue:`33597`)
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* :class:`uuid.UUID` now uses ``__slots__`` to reduce its memory footprint.
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Note that this means that instances can no longer be weak-referenced and
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that arbitrary attributes can no longer be added to them.
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Build and C API Changes
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=======================
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* The result of :c:func:`PyExceptionClass_Name` is now of type
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``const char *`` rather of ``char *``.
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(Contributed by Serhiy Storchaka in :issue:`33818`.)
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* The duality of ``Modules/Setup.dist`` and ``Modules/Setup`` has been
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removed. Previously, when updating the CPython source tree, one had
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to manually copy ``Modules/Setup.dist`` (inside the source tree) to
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``Modules/Setup`` (inside the build tree) in order to reflect any changes
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upstream. This was of a small benefit to packagers at the expense of
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a frequent annoyance to developers following CPython development, as
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forgetting to copy the file could produce build failures.
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Now the build system always reads from ``Modules/Setup`` inside the source
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tree. People who want to customize that file are encouraged to maintain
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their changes in a git fork of CPython or as patch files, as they would do
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for any other change to the source tree.
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(Contributed by Antoine Pitrou in :issue:`32430`.)
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Deprecated
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==========
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* Deprecated methods ``getchildren()`` and ``getiterator()`` in
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the :mod:`~xml.etree.ElementTree` module emit now a
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:exc:`DeprecationWarning` instead of :exc:`PendingDeprecationWarning`.
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They will be removed in Python 3.9.
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(Contributed by Serhiy Storchaka in :issue:`29209`.)
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* Passing an object that is not an instance of
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:class:`concurrent.futures.ThreadPoolExecutor` to
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:meth:`asyncio.loop.set_default_executor()` is
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deprecated and will be prohibited in Python 3.9.
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(Contributed by Elvis Pranskevichus in :issue:`34075`.)
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* The :meth:`__getitem__` methods of :class:`xml.dom.pulldom.DOMEventStream`,
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:class:`wsgiref.util.FileWrapper` and :class:`fileinput.FileInput` have been
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deprecated.
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Implementations of these methods have been ignoring their *index* parameter,
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and returning the next item instead.
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(Contributed by Berker Peksag in :issue:`9372`.)
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Removed
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=======
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* The ``pyvenv`` script has been removed in favor of ``python3.8 -m venv``
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to help eliminate confusion as to what Python interpreter the ``pyvenv``
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script is tied to. (Contributed by Brett Cannon in :issue:`25427`.)
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* ``parse_qs``, ``parse_qsl``, and ``escape`` are removed from :mod:`cgi`
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module. They are deprecated from Python 3.2 or older.
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* ``filemode`` function is removed from :mod:`tarfile` module.
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It is not documented and deprecated since Python 3.3.
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* The :class:`~xml.etree.ElementTree.XMLParser` constructor no longer accepts
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the *html* argument. It never had effect and was deprecated in Python 3.4.
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All other parameters are now :ref:`keyword-only <keyword-only_parameter>`.
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(Contributed by Serhiy Storchaka in :issue:`29209`.)
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* Removed the ``doctype()`` method of :class:`~xml.etree.ElementTree.XMLParser`.
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(Contributed by Serhiy Storchaka in :issue:`29209`.)
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Porting to Python 3.8
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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 Python behavior
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--------------------------
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* Yield expressions (both ``yield`` and ``yield from`` clauses) are now disallowed
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in comprehensions and generator expressions (aside from the iterable expression
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in the leftmost :keyword:`for` clause).
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(Contributed by Serhiy Storchaka in :issue:`10544`.)
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Changes in the Python API
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-------------------------
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* The :meth:`~tkinter.ttk.Treeview.selection` method of the
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:class:`tkinter.ttk.Treeview` class no longer takes arguments. Using it with
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arguments for changing the selection was deprecated in Python 3.6. Use
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specialized methods like :meth:`~tkinter.ttk.Treeview.selection_set` for
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changing the selection. (Contributed by Serhiy Storchaka in :issue:`31508`.)
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* A :mod:`dbm.dumb` database opened with flags ``'r'`` is now read-only.
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:func:`dbm.dumb.open` with flags ``'r'`` and ``'w'`` no longer creates
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a database if it does not exist.
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(Contributed by Serhiy Storchaka in :issue:`32749`.)
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* The ``doctype()`` method defined in a subclass of
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:class:`~xml.etree.ElementTree.XMLParser` will no longer be called and will
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cause emitting a :exc:`RuntimeWarning` instead of a :exc:`DeprecationWarning`.
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Define the :meth:`doctype() <xml.etree.ElementTree.TreeBuilder.doctype>`
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method on a target for handling an XML doctype declaration.
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(Contributed by Serhiy Storchaka in :issue:`29209`.)
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* A :exc:`RuntimeError` is now raised when the custom metaclass doesn't
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provide the ``__classcell__`` entry in the namespace passed to
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``type.__new__``. A :exc:`DeprecationWarning` was emitted in Python
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3.6--3.7. (Contributed by Serhiy Storchaka in :issue:`23722`.)
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* The :class:`cProfile.Profile` class can now be used as a context
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manager. (Contributed by Scott Sanderson in :issue:`29235`.)
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* :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
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:func:`shutil.copytree` and :func:`shutil.move` use platform-specific
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"fast-copy" syscalls (see
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:ref:`shutil-platform-dependent-efficient-copy-operations` section).
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* :func:`shutil.copyfile` default buffer size on Windows was changed from
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16 KiB to 1 MiB.
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* ``PyGC_Head`` struct is changed completely. All code touched the
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struct member should be rewritten. (See :issue:`33597`)
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* Asyncio tasks can now be named, either by passing the ``name`` keyword
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argument to :func:`asyncio.create_task` or
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the :meth:`~asyncio.loop.create_task` event loop method, or by
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calling the :meth:`~asyncio.Task.set_name` method on the task object. The
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task name is visible in the ``repr()`` output of :class:`asyncio.Task` and
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can also be retrieved using the :meth:`~asyncio.Task.get_name` method.
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* The :meth:`mmap.flush() <mmap.mmap.flush>` method now returns ``None`` on
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success and raises an exception on error under all platforms. Previously,
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its behavior was platform-depended: a nonzero value was returned on success;
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zero was returned on error under Windows. A zero value was returned on
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success; an exception was raised on error under Unix.
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(Contributed by Berker Peksag in :issue:`2122`.)
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* The function :func:`math.factorial` no longer accepts arguments that are not
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int-like. (Contributed by Pablo Galindo in :issue:`33083`.)
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* :class:`uuid.UUID` now uses ``__slots__``, therefore instances can no longer
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be weak-referenced and attributes can no longer be added.
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CPython bytecode changes
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------------------------
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* The interpreter loop has been simplified by moving the logic of unrolling
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the stack of blocks into the compiler. The compiler emits now explicit
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instructions for adjusting the stack of values and calling the cleaning
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up code for :keyword:`break`, :keyword:`continue` and :keyword:`return`.
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Removed opcodes :opcode:`BREAK_LOOP`, :opcode:`CONTINUE_LOOP`,
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:opcode:`SETUP_LOOP` and :opcode:`SETUP_EXCEPT`. Added new opcodes
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:opcode:`ROT_FOUR`, :opcode:`BEGIN_FINALLY`, :opcode:`CALL_FINALLY` and
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:opcode:`POP_FINALLY`. Changed the behavior of :opcode:`END_FINALLY`
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and :opcode:`WITH_CLEANUP_START`.
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(Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in
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:issue:`17611`.)
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* Added new opcode :opcode:`END_ASYNC_FOR` for handling exceptions raised
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when awaiting a next item in an :keyword:`async for` loop.
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(Contributed by Serhiy Storchaka in :issue:`33041`.)
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