path: root/Doc/whatsnew/3.5.rst

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

: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 3.5, 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``.

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

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.

Significantly Improved Library Modules:

* None yet.

Security improvements:

* None yet.

Please read on for a comprehensive list of user-facing changes.


.. PEP-sized items next.

.. _pep-4XX:

.. PEP 4XX: Virtual Environments
.. =============================


.. (Implemented by Foo Bar.)

.. .. seealso::

    :pep:`4XX` - Python Virtual Environments
       PEP written by Carl Meyer


PEP 461 - Adding % formatting to bytes and bytearray
----------------------------------------------------

This PEP proposes adding % formatting operations similar to Python 2's ``str``
type to :class:`bytes` and :class:`bytearray`.

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

This PEP proposes a new binary operator to be used for matrix multiplication,
called ``@``. (Mnemonic: ``@`` is ``*`` for mATrices.)

.. seealso::

   :pep:`465` -- A dedicated infix operator for matrix multiplication


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.

PEP and implementation written by Ben Hoyt with the help of Victor Stinner.

.. seealso::

   :pep:`471` -- os.scandir() function -- a better and faster directory
   iterator


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.

.. seealso::

   :pep:`475` -- Retry system calls failing with EINTR


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 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``, thus allowing unoptimized, ``-O``, and ``-OO`` bytecode files
to all exist simultaneously. :func:`importlib.util.cache_from_source` has an
updated API to help with this change.

.. seealso::

   :pep:`488` -- Elimination of PYO files


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 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.)

cgi
---

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

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`.)

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`.)

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`.)

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`.)

glob
----

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

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`.)

imghdr
------

* :func:`~imghdr.what` now recognizes the `OpenEXR <http://www.openexr.com>`_
  format.  (Contributed by Martin Vignali and Claudiu Popa in :issue:`20295`.)

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.

inspect
-------

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

* 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`.)

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`.)

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`.)

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
  patch.  (Contributed by Rafik Draoui and Serhiy Storchaka in :issue:`10395`.)

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`.)

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`.)

math
----

* :data:`math.inf` and :data:`math.nan` constants added.  (Contributed by Mark
  Dickinson in :issue:`23185`.)

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`.)

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.  (Contributed by Maciej Szulik in :issue:`19662`.)

* 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`.)

* :mod:`~smtpd.SMTPServer` now supports :rfc:`6531` via the *enable_SMTPUTF8*
  constructor argument and a user-provided
  :meth:`~smtpd.SMTPServer.process_smtputf8_message` method.

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`.)

sndhdr
------

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

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`.)

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`.

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 bythe named user and group in the
  tarfile.  (Contributed by Michael Vogt and Eric Smith in :issue:`23193`.)

time
----

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

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`.)

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`.)

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.
  (:issue:`21486`, :issue:`21487`, :issue:`20826`)

* 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`.)


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`


Deprecated
==========

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.


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`: Examples of functions which are now retried when interrupted
  instead of raising :exc:`InterruptedError` if the 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)

  - :func:`select.select`, :func:`select.poll.poll`, :func:`select.epoll.poll`,
    :func:`select.kqueue.control`, :func:`select.devpoll.poll`
  - :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`

* 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.


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`)