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
============



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.


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`.)


Deprecated
==========



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.



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`.)

* 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.

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`.)