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|
****************************
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 :ref:`changelog <changelog>`.
.. note::
Prerelease users should be aware that this document is currently in draft
form. It will be updated substantially as Python 3.9 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
======================
* :func:`builtins.__import__` now raises :exc:`ImportError` instead of
:exc:`ValueError` as used to occur when a relative import went past
its top-level package.
(Contributed by Ngalim Siregar in :issue:`37444`.)
* 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 Python fatal error if a daemon thread was still
running.
(Contributed by Victor Stinner in :issue:`37266`.)
venv
----
The activation scripts provided by :mod:`venv` now all specify their prompt
customization consistently by always using the value specified by
``__VENV_PROMPT__``. Previously some scripts unconditionally used
``__VENV_PROMPT__``, others only if it happened to be set (which was the default
case), and one used ``__VENV_NAME__`` instead.
(Contributed by Brett Cannon in :issue:`37663`.)
pprint
------
:mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`.
(Contributed by Carl Bordum Hansen in :issue:`37376`.)
importlib
---------
To improve consistency with import statements, :func:`importlib.util.resolve_name`
now raises :exc:`ImportError` instead of :exc:`ValueError` for invalid relative
import attempts.
(Contributed by Ngalim Siregar in :issue:`37444`.)
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`.)
* The :mod:`parser` module is deprecated and will be removed in future versions
of Python. For the majority of use cases users can leverage the Abstract Syntax
Tree (AST) generation and compilation stage, using the :mod:`ast` module.
* The :mod:`random` module currently accepts any hashable type as a
possible seed value. Unfortunately, some of those types are not
guaranteed to have a deterministic hash value. After Python 3.9,
the module will restrict its seeds to *None*, :class:`int`,
:class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`.
Removed
=======
* The undocumented ``sys.callstats()`` function has been removed. Since Python
3.7, it was deprecated and always returned ``None``. It required a special
build option ``CALL_PROFILE`` which was already removed in Python 3.7.
(Contributed by Victor Stinner in :issue:`37414`.)
* 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`.)
* The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread`
has been removed. It was deprecated since Python 3.8.
Use :meth:`~threading.Thread.is_alive()` instead.
(Contributed by Dong-hee Na in :issue:`37804`.)
* Methods ``getchildren()`` and ``getiterator()`` in the
:mod:`~xml.etree.ElementTree` module have been removed. They were
deprecated in Python 3.2. Use functions :func:`list` and :func:`iter`
instead. The ``xml.etree.cElementTree`` module has been removed.
(Contributed by Serhiy Storchaka in :issue:`36543`.)
* The old :mod:`plistlib` API has been removed, it was deprecated since Python
3.4. Use the :func:`load`, :func:`loads`, :func:`dump`, and :func:`dumps`
functions. Additionally, the ``use_builtin_types`` parameter was removed,
standard :class:`bytes` objects are always used.
(Contributed by Jon Janzen in :issue:`36409`.)
Porting to Python 3.9
=====================
This section lists previously described changes and other bugfixes
that may require changes to your code.
Changes in the Python API
-------------------------
* :func:`builtins.__import__` and :func:`importlib.util.resolve_name` now raise
:exc:`ImportError` where it previously raised :exc:`ValueError`. Callers
catching the specific exception type and supporting both Python 3.9 and
earlier versions will need to catch both:
``except (ImportError, ValueError):``
* The :mod:`venv` activation scripts no longer special-case when
``__VENV_PROMPT__`` is set to ``""``.
CPython bytecode changes
------------------------
* The :opcode:`LOAD_ASSERTION_ERROR` opcode was added for handling the
:keyword:`assert` statement. Previously, the assert statement would not work
correctly if the :exc:`AssertionError` exception was being shadowed.
(Contributed by Zackery Spytz in :issue:`34880`.)
|
