cpython/Doc/whatsnew/3.8.rst

****************************
  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`.)

* Added support of ``\N{name}`` escapes in :mod:`regular expressions <re>`.
  (Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.)


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`)


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.AbstractEventLoop.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.AbstractEventLoop.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.


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`.)