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`.)

* The :class:`int` type now has a new :meth:`~int.as_integer_ratio` method
  compatible with the existing :meth:`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`.)

* Dict and dictviews are now iterable in reversed insertion order using
  :func:`reversed`. (Contributed by Rémi Lapeyre in :issue:`33462`.)

* 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`.

* Iterable unpacking is now allowed without parentheses in :keyword:`yield`
  and :keyword:`return` statements.
  (Contributed by David Cuthbert and Jordan Chapman in :issue:`32117`.)

* A backslash-character pair that is not a valid escape sequence generates
  a :exc:`DeprecationWarning` since Python 3.6. In Python 3.8 it generates
  a :exc:`SyntaxWarning` instead.
  (Contributed by Serhiy Storchaka in :issue:`32912`.)

* The compiler now produces a :exc:`SyntaxWarning` in some cases when a comma
  is missed before tuple or list.  For example::

     data = [
         (1, 2, 3) # oops, missing comma!
         (4, 5, 6)
     ]

  (Contributed by Serhiy Storchaka in :issue:`15248`.)

* Arithmetic operations between subclasses of :class:`datetime.date` or
  :class:`datetime.datetime` and :class:`datetime.timedelta` objects now return
  an instance of the subclass, rather than the base class. This also affects
  the return type of operations whose implementation (directly or indirectly)
  uses :class:`datetime.timedelta` arithmetic, such as
  :meth:`datetime.datetime.astimezone`.
  (Contributed by Paul Ganssle in :issue:`32417`.)


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

* None yet.


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

* The :meth:`_asdict()` method for :func:`collections.namedtuple` now returns
  a :class:`dict` instead of a :class:`collections.OrderedDict`.  This works because
  regular dicts have guaranteed ordering in since Python 3.7.  If the extra
  features of :class:`OrderedDict` are required, the suggested remediation is
  to cast the result to the desired type: ``OrderedDict(nt._asdict())``.
  (Contributed by Raymond Hettinger in :issue:`35864`.)


asyncio
-------

On Windows, the default event loop is now :class:`~asyncio.ProactorEventLoop`.


gettext
-------

Added :func:`~gettext.pgettext` and its variants.
(Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in :issue:`2504`.)


gc
--

:func:`~gc.get_objects` can now receive an optional *generation* parameter
indicating a generation to get objects from. Contributed in
:issue:`36016` by Pablo Galindo.


gzip
----

Added the *mtime* parameter to :func:`gzip.compress` for reproducible output.
(Contributed by Guo Ci Teo in :issue:`34898`.)


idlelib and IDLE
----------------

Output over N lines (50 by default) is squeezed down to a button.
N can be changed in the PyShell section of the General page of the
Settings dialog.  Fewer, but possibly extra long, lines can be squeezed by
right clicking on the output.  Squeezed output can be expanded in place
by double-clicking the button or into the clipboard or a separate window
by right-clicking the button.  (Contributed by Tal Einat in :issue:`1529353`.)

The changes above have been backported to 3.7 maintenance releases.


json.tool
---------

Add option ``--json-lines`` to parse every input line as separate JSON object.
(Contributed by Weipeng Hong in :issue:`31553`.)


math
----

Added new function :func:`math.dist` for computing Euclidean distance
between two points.  (Contributed by Raymond Hettinger in :issue:`33089`.)

Expanded the :func:`math.hypot` function to handle multiple dimensions.
Formerly, it only supported the 2-D case.
(Contributed by Raymond Hettinger in :issue:`33089`.)

Added new function, :func:`math.prod`, as analogous function to :func:`sum`
that returns the product of a 'start' value (default: 1) times an iterable of
numbers. (Contributed by Pablo Galindo in :issue:`35606`)


os.path
-------

:mod:`os.path` functions that return a boolean result like
:func:`~os.path.exists`, :func:`~os.path.lexists`, :func:`~os.path.isdir`,
:func:`~os.path.isfile`, :func:`~os.path.islink`, and :func:`~os.path.ismount`
now return ``False`` instead of raising :exc:`ValueError` or its subclasses
:exc:`UnicodeEncodeError` and :exc:`UnicodeDecodeError` for paths that contain
characters or bytes unrepresentable at the OS level.
(Contributed by Serhiy Storchaka in :issue:`33721`.)


ncurses
-------

Added a new variable holding structured version information for the
underlying ncurses library: :data:`~curses.ncurses_version`.
(Contributed by Serhiy Storchaka in :issue:`31680`.)


pathlib
-------

:mod:`pathlib.Path` methods that return a boolean result like
:meth:`~pathlib.Path.exists()`, :meth:`~pathlib.Path.is_dir()`,
:meth:`~pathlib.Path.is_file()`, :meth:`~pathlib.Path.is_mount()`,
:meth:`~pathlib.Path.is_symlink()`, :meth:`~pathlib.Path.is_block_device()`,
:meth:`~pathlib.Path.is_char_device()`, :meth:`~pathlib.Path.is_fifo()`,
:meth:`~pathlib.Path.is_socket()` now return ``False`` instead of raising
:exc:`ValueError` or its subclass :exc:`UnicodeEncodeError` for paths that
contain characters unrepresentable at the OS level.
(Contributed by Serhiy Storchaka in :issue:`33721`.)


shutil
------

:func:`shutil.copytree` now accepts a new ``dirs_exist_ok`` keyword argument.
(Contributed by Josh Bronson in :issue:`20849`.)


ssl
---

Added :attr:`SSLContext.post_handshake_auth` to enable and
:meth:`ssl.SSLSocket.verify_client_post_handshake` to initiate TLS 1.3
post-handshake authentication.
(Contributed by Christian Heimes in :issue:`34670`.)


statistics
----------

Added :func:`statistics.fmean` as a faster, floating point variant of
:func:`statistics.mean()`.  (Contributed by Raymond Hettinger and
Steven D'Aprano in :issue:`35904`.)


tokenize
--------

The :mod:`tokenize` module now implicitly emits a ``NEWLINE`` token when
provided with input that does not have a trailing new line.  This behavior
now matches what the C tokenizer does internally.
(Contributed by Ammar Askar in :issue:`33899`.)

tkinter
-------

Added methods :meth:`~tkinter.Spinbox.selection_from`,
:meth:`~tkinter.Spinbox.selection_present`,
:meth:`~tkinter.Spinbox.selection_range` and
:meth:`~tkinter.Spinbox.selection_to`
in the :class:`tkinter.Spinbox` class.
(Contributed by Juliette Monsel in :issue:`34829`.)

Added method :meth:`~tkinter.Canvas.moveto`
in the :class:`tkinter.Canvas` class.
(Contributed by Juliette Monsel in :issue:`23831`.)

time
----

Added new clock :data:`~time.CLOCK_UPTIME_RAW` for macOS 10.12.
(Contributed by Joannah Nanjekye in :issue:`35702`.)

unicodedata
-----------

* New function :func:`~unicodedata.is_normalized` can be used to verify a string
  is in a specific normal form. (Contributed by Max Belanger and David Euresti in
  :issue:`32285`).

unittest
--------

* Added :func:`~unittest.addModuleCleanup()` and
  :meth:`~unittest.TestCase.addClassCleanup()` to unittest to support
  cleanups for :func:`~unittest.setUpModule()` and
  :meth:`~unittest.TestCase.setUpClass()`.
  (Contributed by Lisa Roach in :issue:`24412`.)

venv
----

* :mod:`venv` now includes an ``Activate.ps1`` script on all platforms for
  activating virtual environments under PowerShell Core 6.1.
  (Contributed by Brett Cannon in :issue:`32718`.)

xml
---

* As mitigation against DTD and external entity retrieval, the
  :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process
  external entities by default.
  (Contributed by Christian Heimes in :issue:`17239`.)


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

* The :mod:`subprocess` module can now use the :func:`os.posix_spawn` function
  in some cases for better performance. Currently, it is only used on macOS
  and Linux (using glibc 2.24 or newer) if all these conditions are met:

  * *close_fds* is false;
  * *preexec_fn*, *pass_fds*, *cwd* and *start_new_session* parameters
    are not set;
  * the *executable* path contains a directory.

* :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:`33671`.)

* :func:`shutil.copytree` uses :func:`os.scandir` function and all copy
  functions depending from it use cached :func:`os.stat` values. The speedup
  for copying a directory with 8000 files is around +9% on Linux, +20% on
  Windows and +30% on a Windows SMB share. Also the number of :func:`os.stat`
  syscalls is reduced by 38% making :func:`shutil.copytree` especially faster
  on network filesystems. (Contributed by Giampaolo Rodola' in :issue:`33695`.)

* 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.

* Improved performance of :func:`operator.itemgetter` by 33%.  Optimized
  argument handling and added a fast path for the common case of a single
  non-negative integer index into a tuple (which is the typical use case in
  the standard library).  (Contributed by Raymond Hettinger in
  :issue:`35664`.)

* Sped-up field lookups in :func:`collections.namedtuple`.  They are now more
  than two times faster, making them the fastest form of instance variable
  lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and
  Joe Jevnik, Serhiy Storchaka in :issue:`32492`.)

* The :class:`list` constructor does not overallocate the internal item buffer
  if the input iterable has a known length (the input implements ``__len__``).
  This makes the created list 12% smaller on average. (Contributed by
  Raymond Hettinger and Pablo Galindo in :issue:`33234`.)

* Doubled the speed of class variable writes.  When a non-dunder attribute
  was updated, there was an unnecessary call to update slots.
  (Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond Hettinger,
  Neil Schemenauer, and Serhiy Storchaka in :issue:`36012`.)


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

* The :c:func:`PyByteArray_Init` and :c:func:`PyByteArray_Fini` functions have
  been removed. They did nothing since Python 2.7.4 and Python 3.2.0, were
  excluded from the limited API (stable ABI), and were not documented.

* 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`.)

* :mod:`ast` classes ``Num``, ``Str``, ``Bytes``, ``NameConstant`` and
  ``Ellipsis`` are considered deprecated and will be removed in future Python
  versions. :class:`~ast.Constant` should be used instead.
  (Contributed by Serhiy Storchaka in :issue:`32892`.)

* The following functions and methods are deprecated in the :mod:`gettext`
  module: :func:`~gettext.lgettext`, :func:`~gettext.ldgettext`,
  :func:`~gettext.lngettext` and :func:`~gettext.ldngettext`.
  They return encoded bytes, and it's possible that you will get unexpected
  Unicode-related exceptions if there are encoding problems with the
  translated strings. It's much better to use alternatives which return
  Unicode strings in Python 3. These functions have been broken for a long time.

  Function :func:`~gettext.bind_textdomain_codeset`, methods
  :meth:`~gettext.NullTranslations.output_charset` and
  :meth:`~gettext.NullTranslations.set_output_charset`, and the *codeset*
  parameter of functions :func:`~gettext.translation` and
  :func:`~gettext.install` are also deprecated, since they are only used for
  for the ``l*gettext()`` functions.

  (Contributed by Serhiy Storchaka in :issue:`33710`.)

* The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread` has been deprecated.
  (Contributed by Dong-hee Na in :issue:`35283`.)

API and Feature Removals
========================

The following features and APIs have been removed from Python 3.8:

* The :mod:`macpath` module, deprecated in Python 3.7, has been removed.
  (Contributed by Victor Stinner in :issue:`35471`.)

* The function :func:`platform.popen` has been removed, it was deprecated since
  Python 3.3: use :func:`os.popen` instead.

* 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`.)

* The compiler now produces a :exc:`SyntaxWarning` when identity checks
  (``is`` and ``is not``) are used with certain types of literals
  (e.g. strings, ints).  These can often work by accident in CPython,
  but are not guaranteed by the language spec.  The warning advises users
  to use equality tests (``==`` and ``!=``) instead.
  (Contributed by Serhiy Storchaka in :issue:`34850`.)


Changes in the Python API
-------------------------

* The function :func:`platform.popen` has been removed, it was deprecated since
  Python 3.3: use :func:`os.popen` instead.

* 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`.)

* :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process
  external entities by default.
  (Contributed by Christian Heimes in :issue:`17239`.)

* Deleting a key from a read-only :mod:`dbm` database (:mod:`dbm.dumb`,
  :mod:`dbm.gnu` or :mod:`dbm.ndbm`) raises :attr:`error` (:exc:`dbm.dumb.error`,
  :exc:`dbm.gnu.error` or :exc:`dbm.ndbm.error`) instead of :exc:`KeyError`.
  (Contributed by Xiang Zhang in :issue:`33106`.)


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`.)


Demos and Tools
---------------

* Added a benchmark script for timing various ways to access variables:
  ``Tools/scripts/var_access_benchmark.py``.
  (Contributed by Raymond Hettinger in :issue:`35884`.)