cpython/Doc/whatsnew/3.8.rst

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What's New In Python 3.8
****************************
.. Rules for maintenance:
* Anyone can add text to this document. Do not spend very much time
on the wording of your changes, because your text will probably
get rewritten to some degree.
* The maintainer will go through Misc/NEWS periodically and add
changes; it's therefore more important to add your changes to
Misc/NEWS than to this file.
* This is not a complete list of every single change; completeness
is the purpose of Misc/NEWS. Some changes I consider too small
or esoteric to include. If such a change is added to the text,
I'll just remove it. (This is another reason you shouldn't spend
too much time on writing your addition.)
* If you want to draw your new text to the attention of the
maintainer, add 'XXX' to the beginning of the paragraph or
section.
* It's OK to just add a fragmentary note about a change. For
example: "XXX Describe the transmogrify() function added to the
socket module." The maintainer will research the change and
write the necessary text.
* You can comment out your additions if you like, but it's not
necessary (especially when a final release is some months away).
* Credit the author of a patch or bugfix. Just the name is
sufficient; the e-mail address isn't necessary.
* It's helpful to add the bug/patch number as a comment:
XXX Describe the transmogrify() function added to the socket
module.
(Contributed by P.Y. Developer in :issue:`12345`.)
This saves the maintainer the effort of going through the Mercurial log
when researching a change.
This article explains the new features in Python 3.8, compared to 3.7.
For full details, see the :ref:`changelog <changelog>`.
.. note::
Prerelease users should be aware that this document is currently in draft
form. It will be updated substantially as Python 3.8 moves towards release,
so it's worth checking back even after reading earlier versions.
Summary -- Release highlights
=============================
.. This section singles out the most important changes in Python 3.8.
Brevity is key.
.. PEP-sized items next.
New Features
============
Parallel filesystem cache for compiled bytecode files
-----------------------------------------------------
The new :envvar:`PYTHONPYCACHEPREFIX` setting (also available as
:option:`-X` ``pycache_prefix``) configures the implicit bytecode
cache to use a separate parallel filesystem tree, rather than
the default ``__pycache__`` subdirectories within each source
directory.
The location of the cache is reported in :data:`sys.pycache_prefix`
(:const:`None` indicates the default location in ``__pycache__``
subdirectories).
(Contributed by Carl Meyer in :issue:`33499`.)
Other Language Changes
======================
* A :keyword:`continue` statement was illegal in the :keyword:`finally` clause
due to a problem with the implementation. In Python 3.8 this restriction
was lifted.
(Contributed by Serhiy Storchaka in :issue:`32489`.)
* The ``int`` type now has a new ``as_integer_ratio`` method compatible
with the existing ``float.as_integer_ratio`` method.
(Contributed by Lisa Roach in :issue:`33073`.)
* Added support of ``\N{name}`` escapes in :mod:`regular expressions <re>`.
(Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.)
* The syntax allowed for keyword names in function calls was further
restricted. In particular, ``f((keyword)=arg)`` is no longer allowed. It was
never intended to permit more than a bare name on the left-hand side of a
keyword argument assignment term. See :issue:`34641`.
New Modules
===========
* None yet.
Improved Modules
================
Optimizations
=============
* :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
"``outfd.write(infd.read())``".
On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
instead of 16 KiB) and a :func:`memoryview`-based variant of
:func:`shutil.copyfileobj` is used.
The speedup for copying a 512 MiB file within the same partition is about
+26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles
are consumed.
See :ref:`shutil-platform-dependent-efficient-copy-operations` section.
(Contributed by Giampaolo Rodola' in :issue:`25427`.)
* 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.
* 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`)
* :class:`uuid.UUID` now uses ``__slots__`` to reduce its memory footprint.
Note that this means that instances can no longer be weak-referenced and
that arbitrary attributes can no longer be added to them.
Build and C API Changes
=======================
* 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`.)
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`.)
Removed
=======
* 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`.)
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`.)
Changes in the Python API
-------------------------
* 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`.)
* 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`)
* 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`.)
* :class:`uuid.UUID` now uses ``__slots__``, therefore instances can no longer
be weak-referenced and attributes can no longer be added.
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`.)