cpython/Doc/whatsnew/3.6.rst

****************************
  What's New In Python 3.6
****************************

: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 3.6, compared to 3.5.

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 3.6 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 3.6.
   Brevity is key.

New syntax features:

* PEP 498: :ref:`Formatted string literals <whatsnew-fstrings>`

Standard library improvements:

* PEP 519: :ref:`Adding a file system path protocol <pep-519>`

Windows improvements:

* The ``py.exe`` launcher, when used interactively, no longer prefers
  Python 2 over Python 3 when the user doesn't specify a version (via
  command line arguments or a config file).  Handling of shebang lines
  remains unchanged - "python" refers to Python 2 in that case.

.. 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

.. XXX PEP 520: :ref:`Preserving Class Attribute Definition Order<whatsnew-deforder>`

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

.. _pep-523:

PEP 523: Adding a frame evaluation API to CPython
=================================================

While Python provides extensive support to customize how code
executes, one place it has not done so is in the evaluation of frame
objects. If you wanted some way to intercept frame evaluation in
Python there really wasn't any way without directly manipulating
function pointers for defined functions.

:pep:`523` changes this by providing an API to make frame
evaluation pluggable at the C level. This will allow for tools such
as debuggers and JITs to intercept frame evaluation before the
execution of Python code begins. This enables the use of alternative
evaluation implementations for Python code, tracking frame
evaluation, etc.

This API is not part of the limited C API and is marked as private to
signal that usage of this API is expected to be limited and only
applicable to very select, low-level use-cases.

.. seealso::

    :pep:`523` - Adding a frame evaluation API to CPython
       PEP written by Brett Cannon and Dino Viehland.


.. _pep-519:

PEP 519: Adding a file system path protocol
===========================================

File system paths have historically been represented as :class:`str`
or :class:`bytes` objects. This has led to people who write code which
operate on file system paths to assume that such objects are only one
of those two types (an :class:`int` representing a file descriptor
does not count as that is not a file path). Unfortunately that
assumption prevents alternative object representations of file system
paths like :mod:`pathlib` from working with pre-existing code,
including Python's standard library.

To fix this situation, a new interface represented by
:class:`os.PathLike` has been defined. By implementing the
:meth:`~os.PathLike.__fspath__` method, an object signals that it
represents a path. An object can then provide a low-level
representation of a file system path as a :class:`str` or
:class:`bytes` object. This means an object is considered
:term:`path-like <path-like object>` if it implements
:class:`os.PathLike` or is a :class:`str` or :class:`bytes` object
which represents a file system path. Code can use :func:`os.fspath`,
:func:`os.fsdecode`, or :func:`os.fsencode` to explicitly get a
:class:`str` and/or :class:`bytes` representation of a path-like
object.

The built-in :func:`open` function has been updated to accept
:class:`os.PathLike` objects as have all relevant functions in the
:mod:`os` and :mod:`os.path` modules. The :class:`os.DirEntry` class
and relevant classes in :mod:`pathlib` have also been updated to
implement :class:`os.PathLike`. The hope is that updating the
fundamental functions for operating on file system paths will lead
to third-party code to implicitly support all
:term:`path-like objects <path-like object>` without any code changes
or at least very minimal ones (e.g. calling :func:`os.fspath` at the
beginning of code before operating on a path-like object).

Here are some examples of how the new interface allows for
:class:`pathlib.Path` to be used more easily and transparently with
pre-existing code::

  >>> import pathlib
  >>> with open(pathlib.Path("README")) as f:
  ...     contents = f.read()
  ...
  >>> import os.path
  >>> os.path.splitext(pathlib.Path("some_file.txt"))
  ('some_file', '.txt')
  >>> os.path.join("/a/b", pathlib.Path("c"))
  '/a/b/c'
  >>> import os
  >>> os.fspath(pathlib.Path("some_file.txt"))
  'some_file.txt'

(Implemented by Brett Cannon, Ethan Furman, Dusty Phillips, and Jelle Zijlstra.)

.. seealso::

    :pep:`519` - Adding a file system path protocol
       PEP written by Brett Cannon and Koos Zevenhoven.


.. _whatsnew-fstrings:

PEP 498: Formatted string literals
----------------------------------

Formatted string literals are a new kind of string literal, prefixed
with ``'f'``.  They are similar to the format strings accepted by
:meth:`str.format`.  They contain replacement fields surrounded by
curly braces.  The replacement fields are expressions, which are
evaluated at run time, and then formatted using the :func:`format` protocol.

    >>> name = "Fred"
    >>> f"He said his name is {name}."
    'He said his name is Fred.'

See :pep:`498` and the main documentation at :ref:`f-strings`.


PEP 487: Simpler customization of class creation
------------------------------------------------

Upon subclassing a class, the ``__init_subclass__`` classmethod (if defined) is
called on the base class. This makes it straightforward to write classes that
customize initialization of future subclasses without introducing the
complexity of a full custom metaclass.

The descriptor protocol has also been expanded to include a new optional method,
``__set_name__``. Whenever a new class is defined, the new method will be called
on all descriptors included in the definition, providing them with a reference
to the class being defined and the name given to the descriptor within the
class namespace.

Also see :pep:`487` and the updated class customization documentation at
:ref:`class-customization` and :ref:`descriptors`.

(Contributed by Martin Teichmann in :issue:`27366`)


PYTHONMALLOC environment variable
---------------------------------

The new :envvar:`PYTHONMALLOC` environment variable allows setting the Python
memory allocators and/or install debug hooks.

It is now possible to install debug hooks on Python memory allocators on Python
compiled in release mode using ``PYTHONMALLOC=debug``. Effects of debug hooks:

* Newly allocated memory is filled with the byte ``0xCB``
* Freed memory is filled with the byte ``0xDB``
* Detect violations of Python memory allocator API. For example,
  :c:func:`PyObject_Free` called on a memory block allocated by
  :c:func:`PyMem_Malloc`.
* Detect write before the start of the buffer (buffer underflow)
* Detect write after the end of the buffer (buffer overflow)
* Check that the :term:`GIL <global interpreter lock>` is held when allocator
  functions of :c:data:`PYMEM_DOMAIN_OBJ` (ex: :c:func:`PyObject_Malloc`) and
  :c:data:`PYMEM_DOMAIN_MEM` (ex: :c:func:`PyMem_Malloc`) domains are called.

Checking if the GIL is held is also a new feature of Python 3.6.

See the :c:func:`PyMem_SetupDebugHooks` function for debug hooks on Python
memory allocators.

It is now also possible to force the usage of the :c:func:`malloc` allocator of
the C library for all Python memory allocations using ``PYTHONMALLOC=malloc``.
It helps to use external memory debuggers like Valgrind on a Python compiled in
release mode.

On error, the debug hooks on Python memory allocators now use the
:mod:`tracemalloc` module to get the traceback where a memory block was
allocated.

Example of fatal error on buffer overflow using
``python3.6 -X tracemalloc=5`` (store 5 frames in traces)::

    Debug memory block at address p=0x7fbcd41666f8: API 'o'
        4 bytes originally requested
        The 7 pad bytes at p-7 are FORBIDDENBYTE, as expected.
        The 8 pad bytes at tail=0x7fbcd41666fc are not all FORBIDDENBYTE (0xfb):
            at tail+0: 0x02 *** OUCH
            at tail+1: 0xfb
            at tail+2: 0xfb
            at tail+3: 0xfb
            at tail+4: 0xfb
            at tail+5: 0xfb
            at tail+6: 0xfb
            at tail+7: 0xfb
        The block was made by call #1233329 to debug malloc/realloc.
        Data at p: 1a 2b 30 00

    Memory block allocated at (most recent call first):
      File "test/test_bytes.py", line 323
      File "unittest/case.py", line 600
      File "unittest/case.py", line 648
      File "unittest/suite.py", line 122
      File "unittest/suite.py", line 84

    Fatal Python error: bad trailing pad byte

    Current thread 0x00007fbcdbd32700 (most recent call first):
      File "test/test_bytes.py", line 323 in test_hex
      File "unittest/case.py", line 600 in run
      File "unittest/case.py", line 648 in __call__
      File "unittest/suite.py", line 122 in run
      File "unittest/suite.py", line 84 in __call__
      File "unittest/suite.py", line 122 in run
      File "unittest/suite.py", line 84 in __call__
      ...

(Contributed by Victor Stinner in :issue:`26516` and :issue:`26564`.)


.. _whatsnew-deforder:

PEP 520: Preserving Class Attribute Definition Order
----------------------------------------------------

Attributes in a class definition body have a natural ordering: the same
order in which the names appear in the source.  This order is now
preserved in the new class's ``__definition_order__`` attribute.  It is
a tuple of the attribute names, in the order in which they appear in
the class definition body.

For types that don't have a definition (e.g. builtins), or the attribute
order could not be determined, ``__definition_order__`` is ``None``.

Also, the effective default class *execution* namespace (returned from
``type.__prepare__()``) is now an insertion-order-preserving mapping.
For CPython, it is now ``collections.OrderedDict``.  Note that the
class namespace, ``cls.__dict__``, is unchanged.

.. seealso::

   :pep:`520` - Preserving Class Attribute Definition Order
      PEP written and implemented by Eric Snow.


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

Some smaller changes made to the core Python language are:

* Long sequences of repeated traceback lines are now abbreviated as
  ``"[Previous line repeated {count} more times]"`` (see
  :ref:`py36-traceback` for an example).
  (Contributed by Emanuel Barry in :issue:`26823`.)


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

* None yet.


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


asyncio
-------

Since the :mod:`asyncio` module is :term:`provisional <provisional api>`,
all changes introduced in Python 3.6 have also been backported to Python
3.5.x.

Notable changes in the :mod:`asyncio` module since Python 3.5.0:

* The :func:`~asyncio.ensure_future` function and all functions that
  use it, such as :meth:`loop.run_until_complete() <asyncio.BaseEventLoop.run_until_complete>`,
  now accept all kinds of :term:`awaitable objects <awaitable>`.
  (Contributed by Yury Selivanov.)

* New :func:`~asyncio.run_coroutine_threadsafe` function to submit
  coroutines to event loops from other threads.
  (Contributed by Vincent Michel.)

* New :meth:`Transport.is_closing() <asyncio.BaseTransport.is_closing>`
  method to check if the transport is closing or closed.
  (Contributed by Yury Selivanov.)

* The :meth:`loop.create_server() <asyncio.BaseEventLoop.create_server>`
  method can now accept a list of hosts.
  (Contributed by Yann Sionneau.)

* New :meth:`loop.create_future() <asyncio.BaseEventLoop.create_future>`
  method to create Future objects.  This allows alternative event
  loop implementations, such as
  `uvloop <https://github.com/MagicStack/uvloop>`_, to provide a faster
  :class:`asyncio.Future` implementation.
  (Contributed by Yury Selivanov.)

* New :meth:`loop.get_exception_handler() <asyncio.BaseEventLoop.get_exception_handler>`
  method to get the current exception handler.
  (Contributed by Yury Selivanov.)

* New :func:`~asyncio.timeout` context manager to simplify timeouts
  handling code.
  (Contributed by Andrew Svetlov.)

* New :meth:`StreamReader.readuntil() <asyncio.StreamReader.readuntil>`
  method to read data from the stream until a separator bytes
  sequence appears.
  (Contributed by Mark Korenberg.)

* The :meth:`loop.getaddrinfo() <asyncio.BaseEventLoop.getaddrinfo>`
  method is optimized to avoid calling the system ``getaddrinfo``
  function if the address is already resolved.
  (Contributed by A. Jesse Jiryu Davis.)


contextlib
----------

The :class:`contextlib.AbstractContextManager` class has been added to
provide an abstract base class for context managers. It provides a
sensible default implementation for `__enter__()` which returns
``self`` and leaves `__exit__()` an abstract method. A matching
class has been added to the :mod:`typing` module as
:class:`typing.ContextManager`.
(Contributed by Brett Cannon in :issue:`25609`.)


venv
----

:mod:`venv` accepts a new parameter ``--prompt``. This parameter provides an
alternative prefix for the virtual environment. (Proposed by Łukasz.Balcerzak
and ported to 3.6 by Stéphane Wirtel in :issue:`22829`.)


datetime
--------

The :meth:`datetime.strftime() <datetime.datetime.strftime>` and
:meth:`date.strftime() <datetime.date.strftime>` methods now support ISO 8601 date
directives ``%G``, ``%u`` and ``%V``.
(Contributed by Ashley Anderson in :issue:`12006`.)


distutils.command.sdist
-----------------------

The ``default_format`` attribute has been removed from
:class:`distutils.command.sdist.sdist` and the ``formats``
attribute defaults to ``['gztar']``. Although not anticipated,
Any code relying on the presence of ``default_format`` may
need to be adapted. See :issue:`27819` for more details.


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

On Windows, the :mod:`faulthandler` module now installs a handler for Windows
exceptions: see :func:`faulthandler.enable`. (Contributed by Victor Stinner in
:issue:`23848`.)


http.client
-----------

:meth:`HTTPConnection.request() <http.client.HTTPConnection.request>` and
:meth:`~http.client.HTTPConnection.endheaders` both now support
chunked encoding request bodies.
(Contributed by Demian Brecht and Rolf Krahl in :issue:`12319`.)


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

The idlelib package is being modernized and refactored to make IDLE look and work better and to make the code easier to understand, test, and improve. Part of making IDLE look better, especially on Linux and Mac, is using ttk widgets, mostly in the dialogs.  As a result, IDLE no longer runs with tcl/tk 8.4.  It now requires tcl/tk 8.5 or 8.6.  We recommend running the latest release of either.

'Modernizing' includes renaming and consolidation of idlelib modules. The renaming of files with partial uppercase names is similar to the renaming of, for instance, Tkinter and TkFont to tkinter and tkinter.font in 3.0.  As a result, imports of idlelib files that worked in 3.5 will usually not work in 3.6.  At least a module name change will be needed (see idlelib/README.txt), sometimes more.  (Name changes contributed by Al Swiegart and Terry Reedy in :issue:`24225`.  Most idlelib patches since have been and will be part of the process.)

In compensation, the eventual result with be that some idlelib classes will be easier to use, with better APIs and docstrings explaining them.  Additional useful information will be added to idlelib when available.


importlib
---------

:class:`importlib.util.LazyLoader` now calls
:meth:`~importlib.abc.Loader.create_module` on the wrapped loader, removing the
restriction that :class:`importlib.machinery.BuiltinImporter` and
:class:`importlib.machinery.ExtensionFileLoader` couldn't be used with
:class:`importlib.util.LazyLoader`.


os
--

A new :meth:`~os.scandir.close` method allows explicitly closing a
:func:`~os.scandir` iterator.  The :func:`~os.scandir` iterator now
supports the :term:`context manager` protocol.  If a :func:`scandir`
iterator is neither exhausted nor explicitly closed a :exc:`ResourceWarning`
will be emitted in its destructor.
(Contributed by Serhiy Storchaka in :issue:`25994`.)


pickle
------

Objects that need calling ``__new__`` with keyword arguments can now be pickled
using :ref:`pickle protocols <pickle-protocols>` older than protocol version 4.
Protocol version 4 already supports this case.  (Contributed by Serhiy
Storchaka in :issue:`24164`.)


readline
--------

Added :func:`~readline.set_auto_history` to enable or disable
automatic addition of input to the history list.  (Contributed by
Tyler Crompton in :issue:`26870`.)


rlcompleter
-----------

Private and special attribute names now are omitted unless the prefix starts
with underscores.  A space or a colon is added after some completed keywords.
(Contributed by Serhiy Storchaka in :issue:`25011` and :issue:`25209`.)

Names of most attributes listed by :func:`dir` are now completed.
Previously, names of properties and slots which were not yet created on
an instance were excluded.  (Contributed by Martin Panter in :issue:`25590`.)


site
----

When specifying paths to add to :attr:`sys.path` in a `.pth` file,
you may now specify file paths on top of directories (e.g. zip files).
(Contributed by Wolfgang Langner in :issue:`26587`).


sqlite3
-------

* :attr:`sqlite3.Cursor.lastrowid` now supports the ``REPLACE`` statement.
  (Contributed by Alex LordThorsen in :issue:`16864`.)


socket
------

The :func:`~socket.socket.ioctl` function now supports the :data:`~socket.SIO_LOOPBACK_FAST_PATH`
control code.
(Contributed by Daniel Stokes in :issue:`26536`.)

The :meth:`~socket.socket.getsockopt` constants ``SO_DOMAIN``,
``SO_PROTOCOL``, ``SO_PEERSEC``, and ``SO_PASSSEC`` are now supported.
(Contributed by Christian Heimes in :issue:`26907`.)


socketserver
------------

Servers based on the :mod:`socketserver` module, including those
defined in :mod:`http.server`, :mod:`xmlrpc.server` and
:mod:`wsgiref.simple_server`, now support the :term:`context manager`
protocol.
(Contributed by Aviv Palivoda in :issue:`26404`.)

The :attr:`~socketserver.StreamRequestHandler.wfile` attribute of
:class:`~socketserver.StreamRequestHandler` classes now implements
the :class:`io.BufferedIOBase` writable interface.  In particular,
calling :meth:`~io.BufferedIOBase.write` is now guaranteed to send the
data in full.  (Contributed by Martin Panter in :issue:`26721`.)


subprocess
----------

:class:`subprocess.Popen` destructor now emits a :exc:`ResourceWarning` warning
if the child process is still running. Use the context manager protocol (``with
proc: ...``) or call explicitly the :meth:`~subprocess.Popen.wait` method to
read the exit status of the child process (Contributed by Victor Stinner in
:issue:`26741`).


telnetlib
---------

:class:`~telnetlib.Telnet` is now a context manager (contributed by
Stéphane Wirtel in :issue:`25485`).


tkinter
-------

Added methods :meth:`~tkinter.Variable.trace_add`,
:meth:`~tkinter.Variable.trace_remove` and :meth:`~tkinter.Variable.trace_info`
in the :class:`tkinter.Variable` class.  They replace old methods
:meth:`~tkinter.Variable.trace_variable`, :meth:`~tkinter.Variable.trace`,
:meth:`~tkinter.Variable.trace_vdelete` and
:meth:`~tkinter.Variable.trace_vinfo` that use obsolete Tcl commands and might
not work in future versions of Tcl.
(Contributed by Serhiy Storchaka in :issue:`22115`).


.. _py36-traceback:

traceback
---------

Both the traceback module and the interpreter's builtin exception display now
abbreviate long sequences of repeated lines in tracebacks as shown in the
following example::

    >>> def f(): f()
    ...
    >>> f()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 1, in f
      File "<stdin>", line 1, in f
      File "<stdin>", line 1, in f
      [Previous line repeated 995 more times]
    RecursionError: maximum recursion depth exceeded

(Contributed by Emanuel Barry in :issue:`26823`.)


typing
------

The :class:`typing.ContextManager` class has been added for
representing :class:`contextlib.AbstractContextManager`.
(Contributed by Brett Cannon in :issue:`25609`.)


unittest.mock
-------------

The :class:`~unittest.mock.Mock` class has the following improvements:

* Two new methods, :meth:`Mock.assert_called()
  <unittest.mock.Mock.assert_called>` and :meth:`Mock.assert_called_once()
  <unittest.mock.Mock.assert_called_once>` to check if the mock object
  was called.
  (Contributed by Amit Saha in :issue:`26323`.)


urllib.request
--------------

If a HTTP request has a file or iterable body (other than a
bytes object) but no Content-Length header, rather than
throwing an error, :class:`~urllib.request.AbstractHTTPHandler` now
falls back to use chunked transfer encoding.
(Contributed by Demian Brecht and Rolf Krahl in :issue:`12319`.)


urllib.robotparser
------------------

:class:`~urllib.robotparser.RobotFileParser` now supports the ``Crawl-delay`` and
``Request-rate`` extensions.
(Contributed by Nikolay Bogoychev in :issue:`16099`.)


warnings
--------

A new optional *source* parameter has been added to the
:func:`warnings.warn_explicit` function: the destroyed object which emitted a
:exc:`ResourceWarning`. A *source* attribute has also been added to
:class:`warnings.WarningMessage` (contributed by Victor Stinner in
:issue:`26568` and :issue:`26567`).

When a :exc:`ResourceWarning` warning is logged, the :mod:`tracemalloc` is now
used to try to retrieve the traceback where the detroyed object was allocated.

Example with the script ``example.py``::

    import warnings

    def func():
        return open(__file__)

    f = func()
    f = None

Output of the command ``python3.6 -Wd -X tracemalloc=5 example.py``::

    example.py:7: ResourceWarning: unclosed file <_io.TextIOWrapper name='example.py' mode='r' encoding='UTF-8'>
      f = None
    Object allocated at (most recent call first):
      File "example.py", lineno 4
        return open(__file__)
      File "example.py", lineno 6
        f = func()

The "Object allocated at" traceback is new and only displayed if
:mod:`tracemalloc` is tracing Python memory allocations and if the
:mod:`warnings` was already imported.


winreg
------

Added the 64-bit integer type :data:`REG_QWORD <winreg.REG_QWORD>`.
(Contributed by Clement Rouault in :issue:`23026`.)


zipfile
-------

A new :meth:`ZipInfo.from_file() <zipfile.ZipInfo.from_file>` class method
allows making a :class:`~zipfile.ZipInfo` instance from a filesystem file.
A new :meth:`ZipInfo.is_dir() <zipfile.ZipInfo.is_dir>` method can be used
to check if the :class:`~zipfile.ZipInfo` instance represents a directory.
(Contributed by Thomas Kluyver in :issue:`26039`.)

The :meth:`ZipFile.open() <zipfile.ZipFile.open>` method can now be used to
write data into a ZIP file, as well as for extracting data.
(Contributed by Thomas Kluyver in :issue:`26039`.)


zlib
----

The :func:`~zlib.compress` function now accepts keyword arguments.
(Contributed by Aviv Palivoda in :issue:`26243`.)


fileinput
---------

:func:`~fileinput.hook_encoded` now supports the *errors* argument.
(Contributed by Joseph Hackman in :issue:`25788`.)


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

* The ASCII decoder is now up to 60 times as fast for error handlers
  ``surrogateescape``, ``ignore`` and ``replace`` (Contributed
  by Victor Stinner in :issue:`24870`).

* The ASCII and the Latin1 encoders are now up to 3 times as fast for the
  error handler ``surrogateescape`` (Contributed by Victor Stinner in :issue:`25227`).

* The UTF-8 encoder is now up to 75 times as fast for error handlers
  ``ignore``, ``replace``, ``surrogateescape``, ``surrogatepass`` (Contributed
  by Victor Stinner in :issue:`25267`).

* The UTF-8 decoder is now up to 15 times as fast for error handlers
  ``ignore``, ``replace`` and ``surrogateescape`` (Contributed
  by Victor Stinner in :issue:`25301`).

* ``bytes % args`` is now up to 2 times faster. (Contributed by Victor Stinner
  in :issue:`25349`).

* ``bytearray % args`` is now between 2.5 and 5 times faster. (Contributed by
  Victor Stinner in :issue:`25399`).

* Optimize :meth:`bytes.fromhex` and :meth:`bytearray.fromhex`: they are now
  between 2x and 3.5x faster. (Contributed by Victor Stinner in :issue:`25401`).

* Optimize ``bytes.replace(b'', b'.')`` and ``bytearray.replace(b'', b'.')``:
  up to 80% faster. (Contributed by Josh Snider in :issue:`26574`).

* Allocator functions of the :c:func:`PyMem_Malloc` domain
  (:c:data:`PYMEM_DOMAIN_MEM`) now use the :ref:`pymalloc memory allocator
  <pymalloc>` instead of :c:func:`malloc` function of the C library. The
  pymalloc allocator is optimized for objects smaller or equal to 512 bytes
  with a short lifetime, and use :c:func:`malloc` for larger memory blocks.
  (Contributed by Victor Stinner in :issue:`26249`).

* :func:`pickle.load` and :func:`pickle.loads` are now up to 10% faster when
  deserializing many small objects (Contributed by Victor Stinner in
  :issue:`27056`).

- Passing :term:`keyword arguments <keyword argument>` to a function has an
  overhead in comparison with passing :term:`positional arguments
  <positional argument>`.  Now in extension functions implemented with using
  Argument Clinic this overhead is significantly decreased.
  (Contributed by Serhiy Storchaka in :issue:`27574`).


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

* New :c:func:`Py_FinalizeEx` API which indicates if flushing buffered data
  failed (:issue:`5319`).

* :c:func:`PyArg_ParseTupleAndKeywords` now supports :ref:`positional-only
  parameters <positional-only_parameter>`.  Positional-only parameters are
  defined by empty names.
  (Contributed by Serhiy Storchaka in :issue:`26282`).

* ``PyTraceback_Print`` method now abbreviates long sequences of repeated lines
  as ``"[Previous line repeated {count} more times]"``.
  (Contributed by Emanuel Barry in :issue:`26823`.)


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.


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

* :meth:`importlib.machinery.SourceFileLoader.load_module` and
  :meth:`importlib.machinery.SourcelessFileLoader.load_module` are now
  deprecated. They were the only remaining implementations of
  :meth:`importlib.abc.Loader.load_module` in :mod:`importlib` that had not
  been deprecated in previous versions of Python in favour of
  :meth:`importlib.abc.Loader.exec_module`.


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

* None yet.


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

* The ``pyvenv`` script has been deprecated in favour of ``python3 -m venv``.
  This prevents confusion as to what Python interpreter ``pyvenv`` is
  connected to and thus what Python interpreter will be used by the virtual
  environment. (Contributed by Brett Cannon in :issue:`25154`.)

* When performing a relative import, falling back on ``__name__`` and
  ``__path__`` from the calling module when ``__spec__`` or
  ``__package__`` are not defined now raises an :exc:`ImportWarning`.
  (Contributed by Rose Ames in :issue:`25791`.)

* Unlike to other :mod:`dbm` implementations, the :mod:`dbm.dumb` module
  creates database in ``'r'`` and ``'w'`` modes if it doesn't exist and
  allows modifying database in ``'r'`` mode.  This behavior is now deprecated
  and will be removed in 3.8.
  (Contributed by Serhiy Storchaka in :issue:`21708`.)

* Undocumented support of general :term:`bytes-like objects <bytes-like object>`
  as paths in :mod:`os` functions, :func:`compile` and similar functions is
  now deprecated.
  (Contributed by Serhiy Storchaka in :issue:`25791` and :issue:`26754`.)

* The undocumented ``extra_path`` argument to a distutils Distribution
  is now considered
  deprecated, will raise a warning during install if set. Support for this
  parameter will be dropped in a future Python release and likely earlier
  through third party tools. See :issue:`27919` for details.


Deprecated Python behavior
--------------------------

* Raising the :exc:`StopIteration` exception inside a generator will now generate a
  :exc:`DeprecationWarning`, and will trigger a :exc:`RuntimeError` in Python 3.7.
  See :ref:`whatsnew-pep-479` for details.


Removed
=======

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

* ``inspect.getmoduleinfo()`` was removed (was deprecated since CPython 3.3).
  :func:`inspect.getmodulename` should be used for obtaining the module
  name for a given path.

* ``traceback.Ignore`` class and ``traceback.usage``, ``traceback.modname``,
  ``traceback.fullmodname``, ``traceback.find_lines_from_code``,
  ``traceback.find_lines``, ``traceback.find_strings``,
  ``traceback.find_executable_lines`` methods were removed from the
  :mod:`traceback` module. They were undocumented methods deprecated since
  Python 3.2 and equivalent functionality is available from private methods.

* The ``tk_menuBar()`` and ``tk_bindForTraversal()`` dummy methods in
  :mod:`tkinter` widget classes were removed (corresponding Tk commands
  were obsolete since Tk 4.0).

* The :meth:`~zipfile.ZipFile.open` method of the :class:`zipfile.ZipFile`
  class no longer supports the ``'U'`` mode (was deprecated since Python 3.4).
  Use :class:`io.TextIOWrapper` for reading compressed text files in
  :term:`universal newlines` mode.


Porting to Python 3.6
=====================

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

Changes in 'python' Command Behavior
------------------------------------

* The output of a special Python build with defined ``COUNT_ALLOCS``,
  ``SHOW_ALLOC_COUNT`` or ``SHOW_TRACK_COUNT`` macros is now off by
  default.  It can be re-enabled using the ``-X showalloccount`` option.
  It now outputs to ``stderr`` instead of ``stdout``.
  (Contributed by Serhiy Storchaka in :issue:`23034`.)


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

* When :meth:`importlib.abc.Loader.exec_module` is defined,
  :meth:`importlib.abc.Loader.create_module` must also be defined.

* The format of the ``co_lnotab`` attribute of code objects changed to support
  negative line number delta. By default, Python does not emit bytecode with
  negative line number delta. Functions using ``frame.f_lineno``,
  ``PyFrame_GetLineNumber()`` or ``PyCode_Addr2Line()`` are not affected.
  Functions decoding directly ``co_lnotab`` should be updated to use a signed
  8-bit integer type for the line number delta, but it's only required to
  support applications using negative line number delta. See
  ``Objects/lnotab_notes.txt`` for the ``co_lnotab`` format and how to decode
  it, and see the :pep:`511` for the rationale.

* The functions in the :mod:`compileall` module now return booleans instead
  of ``1`` or ``0`` to represent success or failure, respectively. Thanks to
  booleans being a subclass of integers, this should only be an issue if you
  were doing identity checks for ``1`` or ``0``. See :issue:`25768`.

* Reading the :attr:`~urllib.parse.SplitResult.port` attribute of
  :func:`urllib.parse.urlsplit` and :func:`~urllib.parse.urlparse` results
  now raises :exc:`ValueError` for out-of-range values, rather than
  returning :const:`None`.  See :issue:`20059`.

* The :mod:`imp` module now raises a :exc:`DeprecationWarning` instead of
  :exc:`PendingDeprecationWarning`.

* The following modules have had missing APIs added to their :attr:`__all__`
  attributes to match the documented APIs:
  :mod:`calendar`, :mod:`cgi`, :mod:`csv`,
  :mod:`~xml.etree.ElementTree`, :mod:`enum`,
  :mod:`fileinput`, :mod:`ftplib`, :mod:`logging`, :mod:`mailbox`,
  :mod:`mimetypes`, :mod:`optparse`, :mod:`plistlib`, :mod:`smtpd`,
  :mod:`subprocess`, :mod:`tarfile`, :mod:`threading` and
  :mod:`wave`.  This means they will export new symbols when ``import *``
  is used.  See :issue:`23883`.

* When performing a relative import, if ``__package__`` does not compare equal
  to ``__spec__.parent`` then :exc:`ImportWarning` is raised.
  (Contributed by Brett Cannon in :issue:`25791`.)

* When a relative import is performed and no parent package is known, then
  :exc:`ImportError` will be raised. Previously, :exc:`SystemError` could be
  raised. (Contributed by Brett Cannon in :issue:`18018`.)

* Servers based on the :mod:`socketserver` module, including those
  defined in :mod:`http.server`, :mod:`xmlrpc.server` and
  :mod:`wsgiref.simple_server`, now only catch exceptions derived
  from :exc:`Exception`. Therefore if a request handler raises
  an exception like :exc:`SystemExit` or :exc:`KeyboardInterrupt`,
  :meth:`~socketserver.BaseServer.handle_error` is no longer called, and
  the exception will stop a single-threaded server. (Contributed by
  Martin Panter in :issue:`23430`.)

* :func:`spwd.getspnam` now raises a :exc:`PermissionError` instead of
  :exc:`KeyError` if the user doesn't have privileges.

* The :meth:`socket.socket.close` method now raises an exception if
  an error (e.g. EBADF) was reported by the underlying system call.
  See :issue:`26685`.

* The *decode_data* argument for :class:`smtpd.SMTPChannel` and
  :class:`smtpd.SMTPServer` constructors is now ``False`` by default.
  This means that the argument passed to
  :meth:`~smtpd.SMTPServer.process_message` is now a bytes object by
  default, and ``process_message()`` will be passed keyword arguments.
  Code that has already been updated in accordance with the deprecation
  warning generated by 3.5 will not be affected.

* All optional parameters of the :func:`~json.dump`, :func:`~json.dumps`,
  :func:`~json.load` and :func:`~json.loads` functions and
  :class:`~json.JSONEncoder` and :class:`~json.JSONDecoder` class
  constructors in the :mod:`json` module are now :ref:`keyword-only
  <keyword-only_parameter>`.
  (Contributed by Serhiy Storchaka in :issue:`18726`.)

* As part of :pep:`487`, the handling of keyword arguments passed to
  :class:`type` (other than the metaclass hint, ``metaclass``) is now
  consistently delegated to :meth:`object.__init_subclass__`. This means that
  :meth:`type.__new__` and :meth:`type.__init__` both now accept arbitrary
  keyword arguments, but :meth:`object.__init_subclass__` (which is called from
  :meth:`type.__new__`) will reject them by default. Custom metaclasses
  accepting additional keyword arguments will need to adjust their calls to
  :meth:`type.__new__` (whether direct or via :class:`super`) accordingly.

* In :class:`distutils.command.sdist.sdist`, the ``default_format``
  attribute has been removed and is no longer honored. Instead, the
  gzipped tarfile format is the default on all platforms and no
  platform-specific selection is made.
  In environments where distributions are
  built on Windows and zip distributions are required, configure
  the project with a ``setup.cfg`` file containing the following::

    [sdist]
    formats=zip

  This behavior has also been backported to earlier Python versions
  by Setuptools 26.0.0.

* In the :mod:`urllib.request` module and the
  :meth:`http.client.HTTPConnection.request` method, if no Content-Length
  header field has been specified and the request body is a file object,
  it is now sent with HTTP 1.1 chunked encoding. If a file object has to
  be sent to a HTTP 1.0 server, the Content-Length value now has to be
  specified by the caller. See :issue:`12319`.

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

* :c:func:`PyMem_Malloc` allocator family now uses the :ref:`pymalloc allocator
  <pymalloc>` rather than system :c:func:`malloc`. Applications calling
  :c:func:`PyMem_Malloc` without holding the GIL can now crash. Set the
  :envvar:`PYTHONMALLOC` environment variable to ``debug`` to validate the
  usage of memory allocators in your application. See :issue:`26249`.

* :c:func:`Py_Exit` (and the main interpreter) now override the exit status
  with 120 if flushing buffered data failed.  See :issue:`5319`.