Doc/whatsnew/3.5.rst

****************************
  What's New In Python 3.5
****************************

:Release: |release|
:Date: |today|
:Editor: Elvis Pranskevichus <elprans@gmail.com>

.. 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 3.5, compared to 3.4.
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 3.5 moves towards release,
   so it's worth checking back even after reading earlier versions.


.. seealso::

    :pep:`478` - Python 3.5 Release Schedule


Summary -- Release highlights
=============================

.. This section singles out the most important changes in Python 3.5.
   Brevity is key.

New syntax features:

* :pep:`465`, a new matrix multiplication operator: ``a @ b``.
* :pep:`492`, coroutines with async and await syntax.
* :pep:`448`, additional unpacking generalizations.

New library modules:

* :mod:`zipapp`: :ref:`Improving Python ZIP Application Support
  <whatsnew-zipapp>` (:pep:`441`).

New built-in features:

* ``bytes % args``, ``bytearray % args``: :pep:`461` - Adding ``%`` formatting
  to bytes and bytearray.

* ``b'\xf0\x9f\x90\x8d'.hex()``, ``bytearray(b'\xf0\x9f\x90\x8d').hex()``,
  ``memoryview(b'\xf0\x9f\x90\x8d').hex()``: :issue:`9951` - A ``hex`` method
  has been added to bytes, bytearray, and memoryview.

* Generators have new ``gi_yieldfrom`` attribute, which returns the
  object being iterated by ``yield from`` expressions. (Contributed
  by Benno Leslie and Yury Selivanov in :issue:`24450`.)

* New :exc:`RecursionError` exception. (Contributed by Georg Brandl
  in :issue:`19235`.)

Implementation improvements:

* When the ``LC_TYPE`` locale is the POSIX locale (``C`` locale),
  :py:data:`sys.stdin` and :py:data:`sys.stdout` are now using the
  ``surrogateescape`` error handler, instead of the ``strict`` error handler
  (:issue:`19977`).

* :pep:`488`, the elimination of ``.pyo`` files.

* :pep:`489`, multi-phase initialization of extension modules.

Significantly Improved Library Modules:

* :class:`collections.OrderedDict` is now implemented in C, which improves
  its performance between 4x to 100x times.  Contributed by Eric Snow in
  :issue:`16991`.

* You may now pass bytes to the :mod:`tempfile` module's APIs and it will
  return the temporary pathname as bytes instead of str.  It also accepts
  a value of ``None`` on parameters where only str was accepted in the past to
  do the right thing based on the types of the other inputs.  Two functions,
  :func:`gettempdirb` and :func:`gettempprefixb`, have been added to go along
  with this.  This behavior matches that of the :mod:`os` APIs.

* :mod:`ssl` module gained support for Memory BIO, which decouples SSL
  protocol handling from network IO.  (Contributed by Geert Jansen in
  :issue:`21965`.)

* :mod:`traceback` has new lightweight and convenient to work with
  classes :class:`~traceback.TracebackException`,
  :class:`~traceback.StackSummary`, and :class:`traceback.FrameSummary`.
  (Contributed by Robert Collins in :issue:`17911`.)

Security improvements:

* SSLv3 is now disabled throughout the standard library.
  It can still be enabled by instantiating a :class:`ssl.SSLContext`
  manually.  (See :issue:`22638` for more details; this change was
  backported to CPython 3.4 and 2.7.)

* HTTP cookie parsing is now stricter, in order to protect
  against potential injection attacks. (Contributed by Antoine Pitrou
  in :issue:`22796`.)

Windows improvements:

* A new installer for Windows has replaced the old MSI. See :ref:`using-on-windows`
  for more information.

* Windows builds now use Microsoft Visual C++ 14.0, and extension modules
  should use the same.

Please read on for a comprehensive list of user-facing changes, including many
other smaller improvements, CPython optimizations, deprecations, and potential
porting issues.


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

.. _whatsnew-pep-492:

PEP 492 - Coroutines with async and await syntax
------------------------------------------------

:pep:`492` greatly improves support for asynchronous programming in Python
by adding :term:`awaitable objects <awaitable>`,
:term:`coroutine functions <coroutine function>`,
:term:`asynchronous iteration <asynchronous iterable>`,
and :term:`asynchronous context managers <asynchronous context manager>`.

Coroutine functions are declared using the new :keyword:`async def` syntax::

    >>> async def coro():
    ...     return 'spam'

Inside a coroutine function, a new :keyword:`await` expression can be used
to suspend coroutine execution until the result is available.  Any object
can be *awaited*, as long as it implements the :term:`awaitable` protocol by
defining the :meth:`__await__` method.

PEP 492 also adds :keyword:`async for` statement for convenient iteration
over asynchronous iterables.

An example of a simple HTTP client written using the new syntax::

    import asyncio

    async def http_get(domain):
        reader, writer = await asyncio.open_connection(domain, 80)

        writer.write(b'\r\n'.join([
            b'GET / HTTP/1.1',
            b'Host: %b' % domain.encode('latin-1'),
            b'Connection: close',
            b'', b''
        ]))

        async for line in reader:
            print('>>>', line)

        writer.close()

    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(http_get('example.com'))
    finally:
        loop.close()


Similarly to asynchronous iteration, there is a new syntax for asynchronous
context managers::

    >>> import asyncio
    >>> async def coro1(lock):
    ...     print('coro1: waiting for lock')
    ...     async with lock:
    ...         print('coro1: holding the lock')
    ...         await asyncio.sleep(1)
    ...         print('coro1: releasing the lock')
    ...
    >>> async def coro2(lock):
    ...     print('coro2: waiting for lock')
    ...     async with lock:
    ...         print('coro2: holding the lock')
    ...         await asyncio.sleep(1)
    ...         print('coro2: releasing the lock')
    ...
    >>> loop = asyncio.get_event_loop()
    >>> lock = asyncio.Lock()
    >>> coros = asyncio.gather(coro1(lock), coro2(lock), loop=loop)
    >>> loop.run_until_complete(coros)
    coro1: waiting for lock
    coro1: holding the lock
    coro2: waiting for lock
    coro1: releasing the lock
    coro2: holding the lock
    coro2: releasing the lock
    >>> loop.close()

Note that both :keyword:`async for` and :keyword:`async with` can only
be used inside a coroutine function declared with :keyword:`async def`.

Coroutine functions are intended to be ran inside a compatible event loop,
such as :class:`asyncio.Loop`.

.. seealso::

   :pep:`492` -- Coroutines with async and await syntax
      PEP written and implemented by Yury Selivanov.


PEP 465 - A dedicated infix operator for matrix multiplication
--------------------------------------------------------------

:pep:`465` adds the ``@`` infix operator for matrix multiplication.
Currently, no builtin Python types implement the new operator, however, it
can be implemented by defining :meth:`__matmul__`, :meth:`__rmatmul__`,
and :meth:`__imatmul__` for regular, reflected, and in-place matrix
multiplication.  The semantics of these methods is similar to that of
methods defining other infix arithmetic operators.

Matrix multiplication is a notably common operation in many fields of
mathematics, science, engineering, and the addition of ``@`` allows writing
cleaner code::

    S = (H @ beta - r).T @ inv(H @ V @ H.T) @ (H @ beta - r)

instead of::

    S = dot((dot(H, beta) - r).T,
            dot(inv(dot(dot(H, V), H.T)), dot(H, beta) - r))

An upcoming release of NumPy 1.10 will add support for the new operator::

    >>> import numpy

    >>> x = numpy.ones(3)
    >>> x
    array([ 1., 1., 1.])

    >>> m = numpy.eye(3)
    >>> m
    array([[ 1., 0., 0.],
           [ 0., 1., 0.],
           [ 0., 0., 1.]])

    >>> x @ m
    array([ 1., 1., 1.])


.. seealso::

   :pep:`465` -- A dedicated infix operator for matrix multiplication
      PEP written by Nathaniel J. Smith; implemented by Benjamin Peterson.


PEP 448 - Additional Unpacking Generalizations
----------------------------------------------

:pep:`448` extends the allowed uses of the ``*`` iterable unpacking
operator and ``**`` dictionary unpacking operator.  It is now possible
to use an arbitrary number of unpackings in function calls::

    >>> print(*[1], *[2], 3, *[4, 5])
    1 2 3 4 5

    >>> def fn(a, b, c, d):
    ...     print(a, b, c, d)
    ...

    >>> fn(**{'a': 1, 'c': 3}, **{'b': 2, 'd': 4})
    1 2 3 4

Similarly, tuple, list, set, and dictionary displays allow multiple
unpackings::

    >>> *range(4), 4
    (0, 1, 2, 3, 4)
    >>> [*range(4), 4]
    [0, 1, 2, 3, 4]
    >>> {*range(4), 4, *(5, 6, 7)}
    {0, 1, 2, 3, 4, 5, 6, 7}
    >>> {'x': 1, **{'y': 2}}
    {'x': 1, 'y': 2}

.. seealso::

   :pep:`448` -- Additional Unpacking Generalizations
      PEP written by Joshua Landau; implemented by Neil Girdhar,
      Thomas Wouters, and Joshua Landau.


PEP 461 - % formatting support for bytes and bytearray
------------------------------------------------------

PEP 461 adds % formatting to :class:`bytes` and :class:`bytearray`, aiding in
handling data that is a mixture of binary and ASCII compatible text.  This
feature also eases porting such code from Python 2.

Examples::

    >>> b'Hello %s!' % b'World'
    b'Hello World!'
    >>> b'x=%i y=%f' % (1, 2.5)
    b'x=1 y=2.500000'

Unicode is not allowed for ``%s``, but it is accepted by ``%a`` (equivalent of
``repr(obj).encode('ascii', 'backslashreplace')``)::

    >>> b'Hello %s!' % 'World'
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires bytes, or an object that implements __bytes__, not 'str'
    >>> b'price: %a' % '10€'
    b"price: '10\\u20ac'"

.. seealso::

   :pep:`461` -- Adding % formatting to bytes and bytearray
      PEP written by Ethan Furman; implemented by Neil Schemenauer and
      Ethan Furman.


PEP 484 - Type Hints
--------------------

This PEP introduces a provisional module to provide these standard
definitions and tools, along with some conventions for situations
where annotations are not available.

For example, here is a simple function whose argument and return type
are declared in the annotations::

    def greeting(name: str) -> str:
        return 'Hello ' + name

The type system supports unions, generic types, and a special type
named ``Any`` which is consistent with (i.e. assignable to and from) all
types.

.. seealso::

   * :mod:`typing` module documentation
   * :pep:`484` -- Type Hints
        PEP written by Guido van Rossum, Jukka Lehtosalo, and Łukasz Langa;
        implemented by Guido van Rossum.


PEP 471 - os.scandir() function -- a better and faster directory iterator
-------------------------------------------------------------------------

:pep:`471` adds a new directory iteration function, :func:`os.scandir`,
to the standard library. Additionally, :func:`os.walk` is now
implemented using :func:`os.scandir`, which speeds it up by 3-5 times
on POSIX systems and by 7-20 times on Windows systems.

.. seealso::

   :pep:`471` -- os.scandir() function -- a better and faster directory iterator
      PEP written and implemented by Ben Hoyt with the help of Victor Stinner.


PEP 475: Retry system calls failing with EINTR
----------------------------------------------

:pep:`475` adds support for automatic retry of system calls failing with
:py:data:`~errno.EINTR`: this means that user code doesn't have to deal with
EINTR or :exc:`InterruptedError` manually, and should make it more robust
against asynchronous signal reception.

Examples of functions which are now retried when interrupted by a signal
instead of raising :exc:`InterruptedError` if the Python signal handler does
not raise an exception:

* :func:`open`, :func:`os.open`, :func:`io.open`;

* functions of the :mod:`faulthandler` module;

* :mod:`os` functions: :func:`~os.fchdir`, :func:`~os.fchmod`,
  :func:`~os.fchown`, :func:`~os.fdatasync`, :func:`~os.fstat`,
  :func:`~os.fstatvfs`, :func:`~os.fsync`, :func:`~os.ftruncate`,
  :func:`~os.mkfifo`, :func:`~os.mknod`, :func:`~os.posix_fadvise`,
  :func:`~os.posix_fallocate`, :func:`~os.pread`, :func:`~os.pwrite`,
  :func:`~os.read`, :func:`~os.readv`, :func:`~os.sendfile`,
  :func:`~os.wait3`, :func:`~os.wait4`, :func:`~os.wait`,
  :func:`~os.waitid`, :func:`~os.waitpid`, :func:`~os.write`,
  :func:`~os.writev`;

* special cases: :func:`os.close` and :func:`os.dup2` now ignore
  :py:data:`~errno.EINTR` error, the syscall is not retried (see the PEP
  for the rationale);

* :mod:`select` functions: :func:`~select.devpoll.poll`,
  :func:`~select.epoll.poll`,  :func:`~select.kqueue.control`,
  :func:`~select.poll.poll`,  :func:`~select.select`;

* :func:`socket.socket` methods: :meth:`~socket.socket.accept`,
  :meth:`~socket.socket.connect` (except for non-blocking sockets),
  :meth:`~socket.socket.recv`,  :meth:`~socket.socket.recvfrom`,
  :meth:`~socket.socket.recvmsg`,  :meth:`~socket.socket.send`,
  :meth:`~socket.socket.sendall`,  :meth:`~socket.socket.sendmsg`,
  :meth:`~socket.socket.sendto`;

* :func:`signal.sigtimedwait`, :func:`signal.sigwaitinfo`;

* :func:`time.sleep`.

.. seealso::

   :pep:`475` -- Retry system calls failing with EINTR
      PEP and implementation written by Charles-François Natali and
      Victor Stinner, with the help of Antoine Pitrou (the french connection).


PEP 479: Change StopIteration handling inside generators
--------------------------------------------------------

:pep:`479` changes the behavior of generators: when a :exc:`StopIteration`
exception is raised inside a generator, it is replaced with a
:exc:`RuntimeError`.  To enable the feature a ``__future__`` import should
be used::

    from __future__ import generator_stop

Without a ``__future__`` import, a :exc:`PendingDeprecationWarning` will be
raised.

.. seealso::

   :pep:`479` -- Change StopIteration handling inside generators
      PEP written by Chris Angelico and Guido van Rossum. Implemented by
      Chris Angelico, Yury Selivanov and Nick Coghlan.


PEP 486: Make the Python Launcher aware of virtual environments
---------------------------------------------------------------

:pep:`486` makes the Windows launcher (see :pep:`397`) aware of an active
virtual environment. When the default interpreter would be used and the
``VIRTUAL_ENV`` environment variable is set, the interpreter in the virtual
environment will be used.

.. seealso::

    :pep:`486` -- Make the Python Launcher aware of virtual environments
        PEP written and implemented by Paul Moore.


PEP 488: Elimination of PYO files
---------------------------------

:pep:`488` does away with the concept of ``.pyo`` files. This means that
``.pyc`` files represent both unoptimized and optimized bytecode. To prevent the
need to constantly regenerate bytecode files, ``.pyc`` files now have an
optional ``opt-`` tag in their name when the bytecode is optimized. This has the
side-effect of no more bytecode file name clashes when running under either
``-O`` or ``-OO``. Consequently, bytecode files generated from ``-O``, and
``-OO`` may now exist simultaneously. :func:`importlib.util.cache_from_source`
has an updated API to help with this change.

.. seealso::

   :pep:`488` -- Elimination of PYO files
      PEP written and implemented by Brett Cannon.


PEP 489: Multi-phase extension module initialization
----------------------------------------------------

:pep:`489` updates extension module initialization to take advantage of the
two step module loading mechanism introduced by :pep:`451` in Python 3.4.

This change brings the import semantics of extension modules that opt-in to
using the new mechanism much closer to those of Python source and bytecode
modules, including the ability to use any valid identifier as a module name,
rather than being restricted to ASCII.

.. seealso::

   :pep:`489` -- Multi-phase extension module initialization
      PEP written by Petr Viktorin, Stefan Behnel, and Nick Coghlan;
      implemented by Petr Viktorin.


PEP 485: A function for testing approximate equality
----------------------------------------------------

:pep:`485` adds the :func:`math.isclose` and :func:`cmath.isclose`
functions which tell whether two values are approximately equal or
"close" to each other.  Whether or not two values are considered
close is determined according to given absolute and relative tolerances.

.. seealso::

   :pep:`485` -- A function for testing approximate equality
      PEP written by Christopher Barker; implemented by Chris Barker and
      Tal Einat.


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

Some smaller changes made to the core Python language are:

* Added the ``'namereplace'`` error handlers.  The ``'backslashreplace'``
  error handlers now works with decoding and translating.
  (Contributed by Serhiy Storchaka in :issue:`19676` and :issue:`22286`.)

* The :option:`-b` option now affects comparisons of :class:`bytes` with
  :class:`int`.  (Contributed by Serhiy Storchaka in :issue:`23681`)

* New Kazakh :ref:`codec <standard-encodings>` ``kz1048``.  (Contributed by
  Serhiy Storchaka in :issue:`22682`.)

* Property docstrings are now writable. This is especially useful for
  :func:`collections.namedtuple` docstrings.
  (Contributed by Berker Peksag in :issue:`24064`.)

* New Tajik :ref:`codec <standard-encodings>` ``koi8_t``.  (Contributed by
  Serhiy Storchaka in :issue:`22681`.)


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

.. _whatsnew-zipapp:

zipapp
------

The new :mod:`zipapp` module (specified in :pep:`441`) provides an API and
command line tool for creating executable Python Zip Applications, which
were introduced in Python 2.6 in :issue:`1739468` but which were not well
publicised, either at the time or since.

With the new module, bundling your application is as simple as putting all
the files, including a ``__main__.py`` file, into a directory ``myapp``
and running::

    $ python -m zipapp myapp
    $ python myapp.pyz


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

argparse
--------

* :class:`~argparse.ArgumentParser` now allows to disable
  :ref:`abbreviated usage <prefix-matching>` of long options by setting
  :ref:`allow_abbrev` to ``False``.
  (Contributed by Jonathan Paugh, Steven Bethard, paul j3 and Daniel Eriksson.)

bz2
---

* New option *max_length* for :meth:`~bz2.BZ2Decompressor.decompress`
  to limit the maximum size of decompressed data.
  (Contributed by Nikolaus Rath in :issue:`15955`.)

cgi
---

* :class:`~cgi.FieldStorage` now supports the context management protocol.
  (Contributed by Berker Peksag in :issue:`20289`.)

cmath
-----

* :func:`cmath.isclose` function added.
  (Contributed by Chris Barker and Tal Einat in :issue:`24270`.)


code
----

* The :func:`code.InteractiveInterpreter.showtraceback` method now prints
  the full chained traceback, just like the interactive interpreter.
  (Contributed by Claudiu Popa in :issue:`17442`.)

collections
-----------

* You can now update docstrings produced by :func:`collections.namedtuple`::

    Point = namedtuple('Point', ['x', 'y'])
    Point.__doc__ = 'ordered pair'
    Point.x.__doc__ = 'abscissa'
    Point.y.__doc__ = 'ordinate'

  (Contributed by Berker Peksag in :issue:`24064`.)

compileall
----------

* :func:`compileall.compile_dir` and :mod:`compileall`'s command-line interface
  can now do parallel bytecode compilation.
  (Contributed by Claudiu Popa in :issue:`16104`.)

* *quiet* parameter of :func:`compileall.compile_dir`,
  :func:`compileall.compile_file`, and :func:`compileall.compile_path`
  functions now has a multilevel value.  New ``-qq`` command line option
  is available for suppressing the output.
  (Contributed by Thomas Kluyver in :issue:`21338`.)

concurrent.futures
------------------

* :meth:`~concurrent.futures.Executor.map` now takes a *chunksize*
  argument to allow batching of tasks in child processes and improve
  performance of ProcessPoolExecutor.
  (Contributed by Dan O'Reilly in :issue:`11271`.)

contextlib
----------

* The new :func:`contextlib.redirect_stderr` context manager(similar to
  :func:`contextlib.redirect_stdout`) makes it easier for utility scripts to
  handle inflexible APIs that write their output to :data:`sys.stderr` and
  don't provide any options to redirect it.
  (Contributed by Berker Peksag in :issue:`22389`.)

curses
------
* The new :func:`curses.update_lines_cols` function updates the variables
  :envvar:`curses.LINES` and :envvar:`curses.COLS`.

difflib
-------

* The charset of the HTML document generated by :meth:`difflib.HtmlDiff.make_file`
  can now be customized by using *charset* keyword-only parameter.  The default
  charset of HTML document changed from ``'ISO-8859-1'`` to ``'utf-8'``.
  (Contributed by Berker Peksag in :issue:`2052`.)

* It's now possible to compare lists of byte strings with
  :func:`difflib.diff_bytes` (fixes a regression from Python 2).

distutils
---------

* The ``build`` and ``build_ext`` commands now accept a ``-j``
  option to enable parallel building of extension modules.
  (Contributed by Antoine Pitrou in :issue:`5309`.)

* Added support for the LZMA compression.
  (Contributed by Serhiy Storchaka in :issue:`16314`.)

doctest
-------

* :func:`doctest.DocTestSuite` returns an empty :class:`unittest.TestSuite` if
  *module* contains no docstrings instead of raising :exc:`ValueError`.
  (Contributed by Glenn Jones in :issue:`15916`.)

email
-----

* A new policy option :attr:`~email.policy.Policy.mangle_from_` controls
  whether or not lines that start with "From " in email bodies are prefixed with
  a '>' character by generators.  The default is ``True`` for
  :attr:`~email.policy.compat32` and ``False`` for all other policies.
  (Contributed by Milan Oberkirch in :issue:`20098`.)

* A new method :meth:`~email.message.Message.get_content_disposition` provides
  easy access to a canonical value for the :mailheader:`Content-Disposition`
  header (``None`` if there is no such header).  (Contributed by Abhilash Raj
  in :issue:`21083`.)

* A new policy option :attr:`~email.policy.EmailPolicy.utf8` can be set
  ``True`` to encode email headers using the utf8 charset instead of using
  encoded words.  This allows ``Messages`` to be formatted according to
  :rfc:`6532` and used with an SMTP server that supports the :rfc:`6531`
  ``SMTPUTF8`` extension.  (Contributed by R. David Murray in :issue:`24211`.)

glob
----

* :func:`~glob.iglob` and :func:`~glob.glob` now support recursive search in
  subdirectories using the "``**``" pattern.
  (Contributed by Serhiy Storchaka in :issue:`13968`.)

heapq
-----

* :func:`~heapq.merge` has two new optional parameters ``reverse`` and
  ``key``.  (Contributed by  Raymond Hettinger in :issue:`13742`.)

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

Since idlelib implements the IDLE shell and editor and is not intended for
import by other programs, it gets improvements with every release. See
:file:`Lib/idlelib/NEWS.txt` for a cumulative list of changes since 3.4.0,
as well as changes made in future 3.5.x releases. This file is also available
from the IDLE Help -> About Idle dialog.

imaplib
-------

* :class:`IMAP4` now supports the context management protocol.  When used in a
  :keyword:`with` statement, the IMAP4 ``LOGOUT`` command will be called
  automatically at the end of the block.  (Contributed by Tarek Ziadé and
  Serhiy Storchaka in :issue:`4972`.)

* :mod:`imaplib` now supports :rfc:`5161`: the :meth:`~imaplib.IMAP4.enable`
  extension), and :rfc:`6855`: utf-8 support (internationalized email, via the
  ``UTF8=ACCEPT`` argument to :meth:`~imaplib.IMAP4.enable`).  A new attribute,
  :attr:`~imaplib.IMAP4.utf8_enabled`, tracks whether or not :rfc:`6855`
  support is enabled.  Milan Oberkirch, R. David Murray, and Maciej Szulik in
  :issue:`21800`.)

* :mod:`imaplib` now automatically encodes non-ASCII string usernames and
  passwords using ``UTF8``, as recommended by the RFCs.  (Contributed by Milan
  Oberkirch in :issue:`21800`.)

imghdr
------

* :func:`~imghdr.what` now recognizes the `OpenEXR <http://www.openexr.com>`_
  format (contributed by Martin Vignali and Claudiu Popa in :issue:`20295`),
  and the `WebP <https://en.wikipedia.org/wiki/WebP>`_ format (contributed
  by Fabrice Aneche and Claudiu Popa in :issue:`20197`.)

importlib
---------

* :class:`importlib.util.LazyLoader` allows for the lazy loading of modules in
  applications where startup time is paramount.
  (Contributed by Brett Cannon in :issue:`17621`.)

* :func:`importlib.abc.InspectLoader.source_to_code` is now a
  static method to make it easier to work with source code in a string.
  With a module object that you want to initialize you can then use
  ``exec(code, module.__dict__)`` to execute the code in the module.

* :func:`importlib.util.module_from_spec` is now the preferred way to create a
  new module. Compared to :class:`types.ModuleType`, this new function will set
  the various import-controlled attributes based on the passed-in spec object.
  (Contributed by Brett Cannon in :issue:`20383`.)

inspect
-------

* :class:`inspect.Signature` and :class:`inspect.Parameter` are now
  picklable and hashable.  (Contributed by Yury Selivanov in :issue:`20726`
  and :issue:`20334`.)

* New method :meth:`inspect.BoundArguments.apply_defaults`.  (Contributed
  by Yury Selivanov in :issue:`24190`.)

* New class method :meth:`inspect.Signature.from_callable`, which makes
  subclassing of :class:`~inspect.Signature` easier.  (Contributed
  by Yury Selivanov and Eric Snow in :issue:`17373`.)

* New argument ``follow_wrapped`` for :func:`inspect.signature`.
  (Contributed by Yury Selivanov in :issue:`20691`.)

* New :func:`~inspect.iscoroutine`, :func:`~inspect.iscoroutinefunction`
  and :func:`~inspect.isawaitable` functions.  (Contributed by
  Yury Selivanov in :issue:`24017`.)

* New :func:`~inspect.getcoroutinelocals` and :func:`~inspect.getcoroutinestate`
  functions.  (Contributed by Yury Selivanov in :issue:`24400`.)

* :func:`~inspect.stack`, :func:`~inspect.trace`, :func:`~inspect.getouterframes`,
  and :func:`~inspect.getinnerframes` now return a list of named tuples.
  (Contributed by Daniel Shahaf in :issue:`16808`.)

ipaddress
---------

* :class:`ipaddress.IPv4Network` and :class:`ipaddress.IPv6Network` now
  accept an ``(address, netmask)`` tuple argument, so as to easily construct
  network objects from existing addresses.  (Contributed by Peter Moody
  and Antoine Pitrou in :issue:`16531`.)

json
----

* The output of :mod:`json.tool` command line interface is now in the same
  order as the input. Use the :option:`--sort-keys` option to sort the output
  of dictionaries alphabetically by key.  (Contributed by Berker Peksag in
  :issue:`21650`.)

* JSON decoder now raises :exc:`json.JSONDecodeError` instead of
  :exc:`ValueError`.   (Contributed by Serhiy Storchaka in :issue:`19361`.)

locale
------

 * New :func:`~locale.delocalize` function to convert a string into a
   normalized number string, following the ``LC_NUMERIC`` settings.
   (Contributed by Cédric Krier in :issue:`13918`.)

logging
-------

* All logging methods :meth:`~logging.Logger.log`, :meth:`~logging.Logger.exception`,
  :meth:`~logging.Logger.critical`, :meth:`~logging.Logger.debug`, etc,
  now accept exception instances for ``exc_info`` parameter, in addition
  to boolean values and exception tuples.
  (Contributed by Yury Selivanov in :issue:`20537`.)

* :class:`~logging.handlers.HTTPHandler` now accepts optional
  :class:`ssl.SSLContext` instance to configure the SSL settings used
  for HTTP connection.
  (Contributed by Alex Gaynor in :issue:`22788`.)

lzma
----

* New option *max_length* for :meth:`~lzma.LZMADecompressor.decompress`
  to limit the maximum size of decompressed data.
  (Contributed by Martin Panter in :issue:`15955`.)


math
----

* :data:`math.inf` and :data:`math.nan` constants added.  (Contributed by Mark
  Dickinson in :issue:`23185`.)
* :func:`math.isclose` function added.
  (Contributed by Chris Barker and Tal Einat in :issue:`24270`.)

os
--

* New :func:`os.scandir` function that exposes file information from
  the operating system when listing a directory. :func:`os.scandir`
  returns an iterator of :class:`os.DirEntry` objects corresponding to
  the entries in the directory given by *path*. (Contributed by Ben
  Hoyt with the help of Victor Stinner in :issue:`22524`.)

* :class:`os.stat_result` now has a :attr:`~os.stat_result.st_file_attributes`
  attribute on Windows.  (Contributed by Ben Hoyt in :issue:`21719`.)

* :func:`os.urandom`: On Linux 3.17 and newer, the ``getrandom()`` syscall is
  now used when available. On OpenBSD 5.6 and newer, the C ``getentropy()``
  function is now used. These functions avoid the usage of an internal file
  descriptor.

* New :func:`os.get_blocking` and :func:`os.set_blocking` functions to
  get and set the blocking mode of file descriptors.
  (Contributed by Victor Stinner in :issue:`22054`.)

os.path
-------

* New :func:`~os.path.commonpath` function that extracts common path prefix.
  Unlike the :func:`~os.path.commonprefix` function, it always returns a valid
  path.  (Contributed by Rafik Draoui and Serhiy Storchaka in :issue:`10395`.)

pathlib
-------

* New :meth:`~pathlib.Path.samefile` method to check if other path object
  points to the same file.  (Contributed by Vajrasky Kok and Antoine Pitrou
  in :issue:`19775`.)

* :meth:`~pathlib.Path.mkdir` has a new optional parameter ``exist_ok``
  to mimic ``mkdir -p`` and :func:`os.makrdirs` functionality.
  (Contributed by Berker Peksag in :issue:`21539`.)

* New :meth:`~pathlib.Path.expanduser` to expand ``~`` and ``~user``
  constructs.
  (Contributed by Serhiy Storchaka and Claudiu Popa in :issue:`19776`.)

* New class method :meth:`~pathlib.Path.home` to get an instance of
  :class:`~pathlib.Path` object representing the user’s home directory.
  (Contributed by Victor Salgado and Mayank Tripathi in :issue:`19777`.)

pickle
------

* Serializing more "lookupable" objects (such as unbound methods or nested
  classes) now are supported with pickle protocols < 4.
  (Contributed by Serhiy Storchaka in :issue:`23611`.)

poplib
------

* A new command :meth:`~poplib.POP3.utf8` enables :rfc:`6856`
  (internationalized email) support if the POP server supports it.  (Contributed
  by Milan OberKirch in :issue:`21804`.)

re
--

* Number of capturing groups in regular expression is no longer limited by 100.
  (Contributed by Serhiy Storchaka in :issue:`22437`.)

* Now unmatched groups are replaced with empty strings in :func:`re.sub`
  and :func:`re.subn`.  (Contributed by Serhiy Storchaka in :issue:`1519638`.)

readline
--------

* New :func:`~readline.append_history_file` function.
  (Contributed by Bruno Cauet in :issue:`22940`.)

shutil
------

* :func:`~shutil.move` now accepts a *copy_function* argument, allowing,
  for example, :func:`~shutil.copy` to be used instead of the default
  :func:`~shutil.copy2` if there is a need to ignore metadata.  (Contributed by
  Claudiu Popa in :issue:`19840`.)

* :func:`~shutil.make_archive` now supports *xztar* format.
  (Contributed by Serhiy Storchaka in :issue:`5411`.)

signal
------

* On Windows, :func:`signal.set_wakeup_fd` now also supports socket handles.
  (Contributed by Victor Stinner in :issue:`22018`.)

* Different constants of :mod:`signal` module are now enumeration values using
  the :mod:`enum` module. This allows meaningful names to be printed during
  debugging, instead of integer “magic numbers”.  (Contributed by Giampaolo
  Rodola' in :issue:`21076`.)

smtpd
-----

* Both :class:`~smtpd.SMTPServer` and :class:`smtpd.SMTPChannel` now accept a
  *decode_data* keyword to determine if the DATA portion of the SMTP
  transaction is decoded using the ``utf-8`` codec or is instead provided to
  :meth:`~smtpd.SMTPServer.process_message` as a byte string.  The default
  is ``True`` for backward compatibility reasons, but will change to ``False``
  in Python 3.6.  If *decode_data* is set to ``False``, the
  :meth:`~smtpd.SMTPServer.process_message` method must be prepared to accept
  keyword arguments.  (Contributed by Maciej Szulik in :issue:`19662`.)

* :class:`~smtpd.SMTPServer` now advertises the ``8BITMIME`` extension
  (:rfc:`6152`) if if *decode_data* has been set ``True``.  If the client
  specifies ``BODY=8BITMIME`` on the ``MAIL`` command, it is passed to
  :meth:`~smtpd.SMTPServer.process_message` via the ``mail_options`` keyword.
  (Contributed by Milan Oberkirch and R.  David Murray in :issue:`21795`.)

* :class:`~smtpd.SMTPServer` now supports the ``SMTPUTF8`` extension
  (:rfc:`6531`: Internationalized Email).  If the client specified ``SMTPUTF8
  BODY=8BITMIME`` on the ``MAIL`` command, they are passed to
  :meth:`~smtpd.SMTPServer.process_message` via the ``mail_options`` keyword.
  It is the responsibility of the :meth:`~smtpd.SMTPServer.process_message`
  method to correctly handle the ``SMTPUTF8`` data.  (Contributed by Milan
  Oberkirch in :issue:`21725`.)

* It is now possible to provide, directly or via name resolution, IPv6
  addresses in the :class:`~smtpd.SMTPServer` constructor, and have it
  successfully connect.  (Contributed by Milan Oberkirch in :issue:`14758`.)

smtplib
-------

* A new :meth:`~smtplib.SMTP.auth` method provides a convenient way to
  implement custom authentication mechanisms.
  (Contributed by Milan Oberkirch in :issue:`15014`.)

* Additional debuglevel (2) shows timestamps for debug messages in
  :class:`smtplib.SMTP`.  (Contributed by Gavin Chappell and Maciej Szulik in
  :issue:`16914`.)

* :mod:`smtplib` now supports :rfc:`6531` (SMTPUTF8) in both the
  :meth:`~smtplib.SMTP.sendmail` and :meth:`~smtplib.SMTP.send_message`
  commands.  (Contributed by Milan Oberkirch and R. David Murray in
  :issue:`22027`.)

sndhdr
------

* :func:`~sndhdr.what` and :func:`~sndhdr.whathdr` now return
  :func:`~collections.namedtuple`.
  (Contributed by Claudiu Popa in :issue:`18615`.)

ssl
---

* The :meth:`~ssl.SSLSocket.do_handshake`, :meth:`~ssl.SSLSocket.read`,
  :meth:`~ssl.SSLSocket.shutdown`, and :meth:`~ssl.SSLSocket.write` methods of
  :class:`ssl.SSLSocket` don't reset the socket timeout anymore each time bytes
  are received or sent. The socket timeout is now the maximum total duration of
  the method.

* Memory BIO Support: new classes :class:`~ssl.SSLObject`,
  :class:`~ssl.MemoryBIO`, and new
  :meth:`SSLContext.wrap_bio <ssl.SSLContext.wrap_bio>` method.
  (Contributed by Geert Jansen in :issue:`21965`.)

* New :meth:`~ssl.SSLSocket.version` to query the actual protocol version
  in use.  (Contributed by Antoine Pitrou in :issue:`20421`.)

* New :meth:`~ssl.SSLObject.shared_ciphers` and
  :meth:`~ssl.SSLSocket.shared_ciphers` methods to fetch the client's
  list of ciphers sent at handshake.
  (Contributed by Benjamin Peterson in :issue:`23186`.)

socket
------

* New :meth:`socket.socket.sendfile` method allows to send a file over a socket
  by using high-performance :func:`os.sendfile` function on UNIX resulting in
  uploads being from 2x to 3x faster than when using plain
  :meth:`socket.socket.send`.
  (Contributed by Giampaolo Rodola' in :issue:`17552`.)

* The :meth:`socket.socket.sendall` method don't reset the socket timeout
  anymore each time bytes are received or sent. The socket timeout is now the
  maximum total duration to send all data.

* Functions with timeouts now use a monotonic clock, instead of a
  system clock.  (Contributed by Victor Stinner in :issue:`22043`.)

subprocess
----------

* The new :func:`subprocess.run` function runs subprocesses and returns a
  :class:`subprocess.CompletedProcess` object.  It Provides a more consistent
  API than :func:`~subprocess.call`, :func:`~subprocess.check_call` and
  :func:`~subprocess.check_output`.

sys
---

* New :func:`~sys.set_coroutine_wrapper` and :func:`~sys.get_coroutine_wrapper`
  functions.  (Contributed by Yury Selivanov in :issue:`24017`.)

* New :func:`~sys.is_finalizing` to check for :term:`interpreter shutdown`.
  (Contributed by Antoine Pitrou in :issue:`22696`.)

sysconfig
---------

* The user scripts directory on Windows is now versioned.
  (Contributed by Paul Moore in :issue:`23437`.)

tarfile
-------

* The :func:`tarfile.open` function now supports ``'x'`` (exclusive creation)
  mode.  (Contributed by Berker Peksag in :issue:`21717`.)

* The :meth:`~tarfile.TarFile.extractall` and :meth:`~tarfile.TarFile.extract`
  methods now take a keyword parameter *numeric_only*. If set to ``True``,
  the extracted files and directories will be owned by the numeric uid and gid
  from the tarfile. If set to ``False`` (the default, and the behavior in
  versions prior to 3.5), they will be owned by the named user and group in the
  tarfile.  (Contributed by Michael Vogt and Eric Smith in :issue:`23193`.)

threading
---------

* :meth:`~threading.Lock.acquire` and :meth:`~threading.RLock.acquire`
   now use a monotonic clock for managing timeouts.
   (Contributed by Victor Stinner in :issue:`22043`.)

time
----

* The :func:`time.monotonic` function is now always available.  (Contributed by
  Victor Stinner in :issue:`22043`.)

tkinter
-------

* The :mod:`tkinter._fix` module used for setting up the Tcl/Tk environment
  on Windows has been replaced by a private function in the :mod:`_tkinter`
  module which makes no permanent changes to environment variables.
  (Contributed by Zachary Ware in :issue:`20035`.)

traceback
---------

* New :func:`~traceback.walk_stack` and :func:`~traceback.walk_tb`
  functions to conveniently traverse frame and traceback objects.
  (Contributed by Robert Collins in :issue:`17911`.)

* New lightweight classes: :class:`~traceback.TracebackException`,
  :class:`~traceback.StackSummary`, and :class:`traceback.FrameSummary`.
  (Contributed by Robert Collins in :issue:`17911`.)

types
-----

* New :func:`~types.coroutine` function.  (Contributed by Yury Selivanov
  in :issue:`24017`.)

* New :class:`~types.CoroutineType`.  (Contributed by Yury Selivanov
  in :issue:`24400`.)

urllib
------

* A new :class:`~urllib.request.HTTPPasswordMgrWithPriorAuth` allows HTTP Basic
  Authentication credentials to be managed so as to eliminate unnecessary
  ``401`` response handling, or to unconditionally send credentials
  on the first request in order to communicate with servers that return a
  ``404`` response instead of a ``401`` if the ``Authorization`` header is not
  sent.  (Contributed by Matej Cepl in :issue:`19494` and Akshit Khurana in
  :issue:`7159`.)

* A new :func:`~urllib.parse.urlencode` parameter *quote_via* provides a way to
  control the encoding of query parts if needed.  (Contributed by Samwyse and
  Arnon Yaari in :issue:`13866`.)

* :func:`~urllib.request.urlopen` accepts an :class:`ssl.SSLContext`
  object as a *context* argument, which will be used for the HTTPS
  connection.  (Contributed by Alex Gaynor in :issue:`22366`.)

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

* The :mod:`unicodedata` module now uses data from `Unicode 8.0.0
  <http://unicode.org/versions/Unicode8.0.0/>`_.


wsgiref
-------

* *headers* parameter of :class:`wsgiref.headers.Headers` is now optional.
  (Contributed by Pablo Torres Navarrete and SilentGhost in :issue:`5800`.)

xmlrpc
------

* :class:`xmlrpc.client.ServerProxy` is now a :term:`context manager`.
  (Contributed by Claudiu Popa in :issue:`20627`.)

* :class:`~xmlrpc.client.ServerProxy` constructor now accepts an optional
  :class:`ssl.SSLContext` instance.
  (Contributed by Alex Gaynor in :issue:`22960`.)

xml.sax
-------

* SAX parsers now support a character stream of
  :class:`~xml.sax.xmlreader.InputSource` object.
  (Contributed by Serhiy Storchaka in :issue:`2175`.)

faulthandler
------------

* :func:`~faulthandler.enable`, :func:`~faulthandler.register`,
  :func:`~faulthandler.dump_traceback` and
  :func:`~faulthandler.dump_traceback_later` functions now accept file
  descriptors.  (Contributed by Wei Wu in :issue:`23566`.)

zipfile
-------

* Added support for writing ZIP files to unseekable streams.
  (Contributed by Serhiy Storchaka in :issue:`23252`.)

* The :func:`zipfile.ZipFile.open` function now supports ``'x'`` (exclusive
  creation) mode.  (Contributed by Serhiy Storchaka in :issue:`21717`.)


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

The following performance enhancements have been added:

* :func:`os.walk` has been sped up by 3-5x on POSIX systems and 7-20x
  on Windows. This was done using the new :func:`os.scandir` function,
  which exposes file information from the underlying ``readdir`` and
  ``FindFirstFile``/``FindNextFile`` system calls. (Contributed by
  Ben Hoyt with help from Victor Stinner in :issue:`23605`.)

* Construction of ``bytes(int)`` (filled by zero bytes) is faster and uses less
  memory for large objects. ``calloc()`` is used instead of ``malloc()`` to
  allocate memory for these objects.

* Some operations on :class:`~ipaddress.IPv4Network` and
  :class:`~ipaddress.IPv6Network` have been massively sped up, such as
  :meth:`~ipaddress.IPv4Network.subnets`, :meth:`~ipaddress.IPv4Network.supernet`,
  :func:`~ipaddress.summarize_address_range`, :func:`~ipaddress.collapse_addresses`.
  The speed up can range from 3x to 15x.
  (See :issue:`21486`, :issue:`21487`, :issue:`20826`, :issue:`23266`.)

* Pickling of :mod:`ipaddress` classes was optimized to produce significantly
  smaller output.  (Contributed by Serhiy Storchaka in :issue:`23133`.)

* Many operations on :class:`io.BytesIO` are now 50% to 100% faster.
  (Contributed by Serhiy Storchaka in :issue:`15381` and David Wilson in
  :issue:`22003`.)

* :func:`marshal.dumps` is now faster (65%-85% with versions 3--4, 20-25% with
  versions 0--2 on typical data, and up to 5x in best cases).
  (Contributed by Serhiy Storchaka in :issue:`20416` and :issue:`23344`.)

* The UTF-32 encoder is now 3x to 7x faster.  (Contributed by Serhiy Storchaka
  in :issue:`15027`.)

* Regular expressions are now parsed up to 10% faster.
  (Contributed by Serhiy Storchaka in :issue:`19380`.)

* :func:`json.dumps` was optimized to run with ``ensure_ascii=False``
  as fast as with ``ensure_ascii=True``.
  (Contributed by Naoki Inada in :issue:`23206`.)


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

Changes to Python's build process and to the C API include:

* New ``calloc`` functions:

  * :c:func:`PyMem_RawCalloc`
  * :c:func:`PyMem_Calloc`
  * :c:func:`PyObject_Calloc`
  * :c:func:`_PyObject_GC_Calloc`

* Windows builds now require Microsoft Visual C++ 14.0, which
  is available as part of `Visual Studio 2015 <http://www.visualstudio.com>`_.

Deprecated
==========

New Keywords
------------

``async`` and ``await`` are not recommended to be used as variable, class,
function or module names.  Introduced by :pep:`492` in Python 3.5, they will
become proper keywords in Python 3.7.


Unsupported Operating Systems
-----------------------------

* Windows XP - Per :PEP:`11`, Microsoft support of Windows XP has ended.


Deprecated Python modules, functions and methods
------------------------------------------------

* The :mod:`formatter` module has now graduated to full deprecation and is still
  slated for removal in Python 3.6.

* :mod:`smtpd` has in the past always decoded the DATA portion of email
  messages using the ``utf-8`` codec.  This can now be controlled by the new
  *decode_data* keyword to :class:`~smtpd.SMTPServer`.  The default value is
  ``True``, but this default is deprecated.  Specify the *decode_data* keyword
  with an appropriate value to avoid the deprecation warning.

* Directly assigning values to the :attr:`~http.cookies.Morsel.key`,
  :attr:`~http.cookies.Morsel.value` and
  :attr:`~http.cookies.Morsel.coded_value` of :class:`~http.cookies.Morsel`
  objects is deprecated.  Use the :func:`~http.cookies.Morsel.set` method
  instead.  In addition, the undocumented *LegalChars* parameter of
  :func:`~http.cookies.Morsel.set` is deprecated, and is now ignored.

* Passing a format string as keyword argument *format_string* to the
  :meth:`~string.Formatter.format` method of the :class:`string.Formatter`
  class has been deprecated.

* :func:`platform.dist` and :func:`platform.linux_distribution` functions are
  now deprecated and will be removed in Python 3.7.  Linux distributions use
  too many different ways of describing themselves, so the functionality is
  left to a package.
  (Contributed by Vajrasky Kok and Berker Peksag in :issue:`1322`.)

* The previously undocumented ``from_function`` and ``from_builtin`` methods of
  :class:`inspect.Signature` are deprecated.  Use new
  :meth:`inspect.Signature.from_callable` instead. (Contributed by Yury
  Selivanov in :issue:`24248`.)

* :func:`inspect.getargspec` is deprecated and scheduled to be removed in
  Python 3.6.  (See :issue:`20438` for details.)

* :func:`~inspect.getfullargspec`, :func:`~inspect.getargvalues`,
  :func:`~inspect.getcallargs`, :func:`~inspect.getargvalues`,
  :func:`~inspect.formatargspec`, and :func:`~inspect.formatargvalues` are
  deprecated in favor of :func:`inspect.signature` API.  (See :issue:`20438`
  for details.)

* Use of ``re.LOCALE`` flag with str patterns or ``re.ASCII`` is now
  deprecated.  (Contributed by Serhiy Storchaka in :issue:`22407`.)


Deprecated functions and types of the C API
-------------------------------------------

* None yet.


Deprecated features
-------------------

* None yet.


Removed
=======

API and Feature Removals
------------------------

The following obsolete and previously deprecated APIs and features have been
removed:

* The ``__version__`` attribute has been dropped from the email package.  The
  email code hasn't been shipped separately from the stdlib for a long time,
  and the ``__version__`` string was not updated in the last few releases.

* The internal ``Netrc`` class in the :mod:`ftplib` module was deprecated in
  3.4, and has now been removed.
  (Contributed by Matt Chaput in :issue:`6623`.)

* The concept of ``.pyo`` files has been removed.

* The JoinableQueue class in the provisional asyncio module was deprecated
  in 3.4.4 and is now removed (:issue:`23464`).


Porting to Python 3.5
=====================

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

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

* :pep:`475`: System calls are now retried when interrupted by a signal instead
  of raising :exc:`InterruptedError` if the Python signal handler does not
  raise an exception.

* Before Python 3.5, a :class:`datetime.time` object was considered to be false
  if it represented midnight in UTC.  This behavior was considered obscure and
  error-prone and has been removed in Python 3.5.  See :issue:`13936` for full
  details.

* :meth:`ssl.SSLSocket.send()` now raises either :exc:`ssl.SSLWantReadError`
  or :exc:`ssl.SSLWantWriteError` on a non-blocking socket if the operation
  would block. Previously, it would return 0.  See :issue:`20951`.

* The ``__name__`` attribute of generator is now set from the function name,
  instead of being set from the code name. Use ``gen.gi_code.co_name`` to
  retrieve the code name. Generators also have a new ``__qualname__``
  attribute, the qualified name, which is now used for the representation
  of a generator (``repr(gen)``). See :issue:`21205`.

* The deprecated "strict" mode and argument of :class:`~html.parser.HTMLParser`,
  :meth:`HTMLParser.error`, and the :exc:`HTMLParserError` exception have been
  removed.  (Contributed by Ezio Melotti in :issue:`15114`.)
  The *convert_charrefs* argument of :class:`~html.parser.HTMLParser` is
  now ``True`` by default.  (Contributed by Berker Peksag in :issue:`21047`.)

* Although it is not formally part of the API, it is worth noting for porting
  purposes (ie: fixing tests) that error messages that were previously of the
  form "'sometype' does not support the buffer protocol" are now of the form "a
  bytes-like object is required, not 'sometype'".  (Contributed by Ezio Melotti
  in :issue:`16518`.)

* If the current directory is set to a directory that no longer exists then
  :exc:`FileNotFoundError` will no longer be raised and instead
  :meth:`~importlib.machinery.FileFinder.find_spec` will return ``None``
  **without** caching ``None`` in :data:`sys.path_importer_cache` which is
  different than the typical case (:issue:`22834`).

* HTTP status code and messages from :mod:`http.client` and :mod:`http.server`
  were refactored into a common :class:`~http.HTTPStatus` enum.  The values in
  :mod:`http.client` and :mod:`http.server` remain available for backwards
  compatibility.  (Contributed by Demian Brecht in :issue:`21793`.)

* When an import loader defines :meth:`~importlib.machinery.Loader.exec_module`
  it is now expected to also define
  :meth:`~importlib.machinery.Loader.create_module` (raises a
  :exc:`DeprecationWarning` now, will be an error in Python 3.6). If the loader
  inherits from :class:`importlib.abc.Loader` then there is nothing to do, else
  simply define :meth:`~importlib.machinery.Loader.create_module` to return
  ``None`` (:issue:`23014`).

* :func:`re.split` always ignored empty pattern matches, so the ``'x*'``
  pattern worked the same as ``'x+'``, and the ``'\b'`` pattern never worked.
  Now :func:`re.split` raises a warning if the pattern could match
  an empty string.  For compatibility use patterns that never match an empty
  string (e.g. ``'x+'`` instead of ``'x*'``).  Patterns that could only match
  an empty string (such as ``'\b'``) now raise an error.

* The :class:`~http.cookies.Morsel` dict-like interface has been made self
  consistent:  morsel comparison now takes the :attr:`~http.cookies.Morsel.key`
  and :attr:`~http.cookies.Morsel.value` into account,
  :meth:`~http.cookies.Morsel.copy` now results in a
  :class:`~http.cookies.Morsel` instance rather than a :class:`dict`, and
  :meth:`~http.cookies.Morsel.update` will now raise an exception if any of the
  keys in the update dictionary are invalid.  In addition, the undocumented
  *LegalChars* parameter of :func:`~http.cookies.Morsel.set` is deprecated and
  is now ignored.  (:issue:`2211`)

* :pep:`488` has removed ``.pyo`` files from Python and introduced the optional
  ``opt-`` tag in ``.pyc`` file names. The
  :func:`importlib.util.cache_from_source` has gained an *optimization*
  parameter to help control the ``opt-`` tag. Because of this, the
  *debug_override* parameter of the function is now deprecated. `.pyo` files
  are also no longer supported as a file argument to the Python interpreter and
  thus serve no purpose when distributed on their own (i.e. sourcless code
  distribution). Due to the fact that the magic number for bytecode has changed
  in Python 3.5, all old `.pyo` files from previous versions of Python are
  invalid regardless of this PEP.

* The :mod:`socket` module now exports the CAN_RAW_FD_FRAMES constant on linux
  3.6 and greater.

* The ``pygettext.py`` Tool now uses the standard +NNNN format for timezones in
  the POT-Creation-Date header.

* The :mod:`smtplib` module now uses :data:`sys.stderr` instead of previous
  module level :data:`stderr` variable for debug output.  If your (test)
  program depends on patching the module level variable to capture the debug
  output, you will need to update it to capture sys.stderr instead.

* The :meth:`str.startswith` and :meth:`str.endswith` methods no longer return
  ``True`` when finding the empty string and the indexes are completely out of
  range.  See :issue:`24284`.

* The :func:`inspect.getdoc` function now returns documentation strings
  inherited from base classes.  Documentation strings no longer need to be
  duplicated if the inherited documentation is appropriate.  To suppress an
  inherited string, an empty string must be specified (or the documentation
  may be filled in).  This change affects the output of the :mod:`pydoc`
  module and the :func:`help` function.  See :issue:`15582`.

Changes in the C API
--------------------

* The undocumented :c:member:`~PyMemoryViewObject.format` member of the
  (non-public) :c:type:`PyMemoryViewObject` structure has been removed.

  All extensions relying on the relevant parts in ``memoryobject.h``
  must be rebuilt.

* The :c:type:`PyMemAllocator` structure was renamed to
  :c:type:`PyMemAllocatorEx` and a new ``calloc`` field was added.

* Removed non-documented macro :c:macro:`PyObject_REPR` which leaked references.
  Use format character ``%R`` in :c:func:`PyUnicode_FromFormat`-like functions
  to format the :func:`repr` of the object.

* Because the lack of the :attr:`__module__` attribute breaks pickling and
  introspection, a deprecation warning now is raised for builtin type without
  the :attr:`__module__` attribute.  Would be an AttributeError in future.
  (:issue:`20204`)

* As part of :pep:`492` implementation, ``tp_reserved`` slot of
  :c:type:`PyTypeObject` was replaced with a
  :c:member:`tp_as_async` slot.  Refer to :ref:`coro-objects` for
  new types, structures and functions.