Doc/whatsnew/3.9.rst

****************************
  What's New In Python 3.9
****************************

:Release: |release|
:Date: |today|

.. 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 3.9, compared to 3.8.

For full details, see the :source:`Misc/NEWS` file.

.. note::

   Prerelease users should be aware that this document is currently in draft
   form. It will be updated substantially as Python 3.9 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 3.9.
   Brevity is key.


.. PEP-sized items next.



New Features
============



Other Language Changes
======================

* Python now gets the absolute path of the script filename specified on
  the command line (ex: ``python3 script.py``): the ``__file__`` attribute of
  the ``__main__`` module, ``sys.argv[0]`` and ``sys.path[0]`` become an
  absolute path, rather than a relative path. These paths now remain valid
  after the current directory is changed by :func:`os.chdir`. As a side effect,
  a traceback also displays the absolute path for ``__main__`` module frames in
  this case.
  (Contributed by Victor Stinner in :issue:`20443`.)

* In development mode and in debug build, *encoding* and *errors* arguments are
  now checked on string encoding and decoding operations. Examples:
  :func:`open`, :meth:`str.encode` and :meth:`bytes.decode`.

  By default, for best performances, the *errors* argument is only checked at
  the first encoding/decoding error, and the *encoding* argument is sometimes
  ignored for empty strings.
  (Contributed by Victor Stinner in :issue:`37388`.)


New Modules
===========

* None yet.


Improved Modules
================

threading
---------

In a subinterpreter, spawning a daemon thread now raises an exception. Daemon
threads were never supported in subinterpreters. Previously, the subinterpreter
finalization crashed with a Pyton fatal error if a daemon thread was still
running.


Optimizations
=============


Build and C API Changes
=======================

* Add a new public :c:func:`PyObject_CallNoArgs` function to the C API:
  call a callable Python object without any arguments. It is the most efficient
  way to call a callable Python object without any argument.
  (Contributed by Victor Stinner in :issue:`37194`.)



Deprecated
==========

* Currently :func:`math.factorial` accepts :class:`float` instances with
  non-negative integer values (like ``5.0``).  It raises a :exc:`ValueError`
  for non-integral and negative floats.  It is deprecated now.  In future
  Python versions it will raise a :exc:`TypeError` for all floats.
  (Contributed by Serhiy Storchaka in :issue:`37315`.)


Removed
=======

* The ``sys.getcheckinterval()`` and ``sys.setcheckinterval()`` functions have
  been removed. They were deprecated since Python 3.2. Use
  :func:`sys.getswitchinterval` and :func:`sys.setswitchinterval` instead.
  (Contributed by Victor Stinner in :issue:`37392`.)

* The C function ``PyImport_Cleanup()`` has been removed. It was documented as:
  "Empty the module table.  For internal use only."
  (Contributed by Victor Stinner in :issue:`36710`.)

* ``_dummy_thread`` and ``dummy_threading`` modules have been removed. These
  modules were deprecated since Python 3.7 which requires threading support.
  (Contributed by Victor Stinner in :issue:`37312`.)

* ``aifc.openfp()`` alias to ``aifc.open()``, ``sunau.openfp()`` alias to
  ``sunau.open()``, and ``wave.openfp()`` alias to ``wave.open()`` have been
  removed. They were deprecated since Python 3.7.
  (Contributed by Victor Stinner in :issue:`37320`.)


Porting to Python 3.9
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code.