****************************
  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 :ref:`changelog <changelog>`.

.. note::

   Prerelease users should be aware that this document is currently in draft
   form. It will be updated substantially as Python 3.6 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:

* A ``global`` or ``nonlocal`` statement must now textually appear
  before the first use of the affected name in the same scope.
  Previously this was a SyntaxWarning.

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

* PEP 515: Underscores in Numeric Literals

* PEP 526: :ref:`Syntax for Variable Annotations <variable-annotations>`

* PEP 525: Asynchronous Generators

* PEP 530: Asynchronous Comprehensions

Standard library improvements:

Security improvements:

* On Linux, :func:`os.urandom` now blocks until the system urandom entropy pool
  is initialized to increase the security. See the :pep:`524` for the
  rationale.

* :mod:`hashlib` and :mod:`ssl` now support OpenSSL 1.1.0.

* The default settings and feature set of the :mod:`ssl` have been improved.

* The :mod:`hashlib` module has got support for BLAKE2, SHA-3 and SHAKE hash
  algorithms and :func:`~hashlib.scrypt` key derivation function.

Windows improvements:

* PEP 529: :ref:`Change Windows filesystem encoding to UTF-8 <pep-529>`

* PEP 528: :ref:`Change Windows console encoding to UTF-8 <pep-528>`

* 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.

* ``python.exe`` and ``pythonw.exe`` have been marked as long-path aware,
  which means that when the 260 character path limit may no longer apply.
  See :ref:`removing the MAX_PATH limitation <max-path>` for details.

* A ``._pth`` file can be added to force isolated mode and fully specify
  all search paths to avoid registry and environment lookup. See
  :ref:`the documentation <finding_modules>` for more information.

* A ``python36.zip`` file now works as a landmark to infer
  :envvar:`PYTHONHOME`. See :ref:`the documentation <finding_modules>` for
  more information.

.. 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

New built-in features:

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

* PEP 468: :ref:`Preserving Keyword Argument Order<whatsnew-kwargs>`

A complete list of PEP's implemented in Python 3.6:

* :pep:`468`, :ref:`Preserving Keyword Argument Order<whatsnew-kwargs>`
* :pep:`487`, :ref:`Simpler customization of class creation<whatsnew-pep487>`
* :pep:`495`, Local Time Disambiguation
* :pep:`498`, :ref:`Formatted string literals <whatsnew-fstrings>`
* :pep:`506`, Adding A Secrets Module To The Standard Library
* :pep:`509`, :ref:`Add a private version to dict<whatsnew-pep509>`
* :pep:`515`, :ref:`Underscores in Numeric Literals<pep-515>`
* :pep:`519`, :ref:`Adding a file system path protocol<pep-519>`
* :pep:`520`, :ref:`Preserving Class Attribute Definition Order<whatsnew-deforder>`
* :pep:`523`, :ref:`Adding a frame evaluation API to CPython<pep-523>`
* :pep:`524`, Make os.urandom() blocking on Linux (during system startup)
* :pep:`525`, Asynchronous Generators (provisional)
* :pep:`526`, :ref:`Syntax for Variable Annotations (provisional)<variable-annotations>`
* :pep:`528`, :ref:`Change Windows console encoding to UTF-8 (provisional)<pep-528>`
* :pep:`529`, :ref:`Change Windows filesystem encoding to UTF-8 (provisional)<pep-529>`
* :pep:`530`, Asynchronous Comprehensions


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

.. _pep-515:

PEP 515: Underscores in Numeric Literals
----------------------------------------

Prior to PEP 515, there was no support for writing long numeric
literals with some form of separator to improve readability. For
instance, how big is ``1000000000000000``? With :pep:`515`, though,
you can use underscores to separate digits as desired to make numeric
literals easier to read: ``1_000_000_000_000_000``. Underscores can be
used with other numeric literals beyond integers, e.g.
``0x_FF_FF_FF_FF``.

Single underscores are allowed between digits and after any base
specifier. More than a single underscore in a row, leading, or
trailing underscores are not allowed.

.. seealso::

   :pep:`515` -- Underscores in Numeric Literals
      PEP written by Georg Brandl and Serhiy Storchaka.


.. _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. Semantics of the
API will change with Python as necessary.

.. 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. :c:func:`PyUnicode_FSConverter`
and :c:func:`PyUnicode_FSConverter` have been changed to accept
path-like objects. The :class:`os.DirEntry` class
and relevant classes in :mod:`pathlib` have also been updated to
implement :class:`os.PathLike`.

The hope in 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`.


.. _variable-annotations:

PEP 526: Syntax for variable annotations
----------------------------------------

:pep:`484` introduced standard for type annotations of function parameters,
a.k.a. type hints. This PEP adds syntax to Python for annotating the
types of variables including class variables and instance variables::

    primes: List[int] = []

    captain: str  # Note: no initial value!

    class Starship:
        stats: Dict[str, int] = {}

Just as for function annotations, the Python interpreter does not attach any
particular meaning to variable annotations and only stores them in a special
attribute ``__annotations__`` of a class or module.
In contrast to variable declarations in statically typed languages,
the goal of annotation syntax is to provide an easy way to specify structured
type metadata for third party tools and libraries via the abstract syntax tree
and the ``__annotations__`` attribute.

.. seealso::

   :pep:`526` -- Syntax for variable annotations.
      PEP written by Ryan Gonzalez, Philip House, Ivan Levkivskyi, Lisa Roach,
      and Guido van Rossum. Implemented by Ivan Levkivskyi.

   Tools that use or will use the new syntax:
   `mypy <http://github.com/python/mypy>`_,
   `pytype <http://github.com/google/pytype>`_, PyCharm, etc.


.. _pep-529:

PEP 529: Change Windows filesystem encoding to UTF-8
----------------------------------------------------

Representing filesystem paths is best performed with str (Unicode) rather than
bytes. However, there are some situations where using bytes is sufficient and
correct.

Prior to Python 3.6, data loss could result when using bytes paths on Windows.
With this change, using bytes to represent paths is now supported on Windows,
provided those bytes are encoded with the encoding returned by
:func:`sys.getfilesystemencoding()`, which now defaults to ``'utf-8'``.

Applications that do not use str to represent paths should use
:func:`os.fsencode()` and :func:`os.fsdecode()` to ensure their bytes are
correctly encoded. To revert to the previous behaviour, set
:envvar:`PYTHONLEGACYWINDOWSFSENCODING` or call
:func:`sys._enablelegacywindowsfsencoding`.

See :pep:`529` for more information and discussion of code modifications that
may be required.

.. note::

   This change is considered experimental for 3.6.0 beta releases. The default
   encoding may change before the final release.

.. _whatsnew-pep487:

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`)

.. _pep-528:

PEP 528: Change Windows console encoding to UTF-8
-------------------------------------------------

The default console on Windows will now accept all Unicode characters and
provide correctly read str objects to Python code. ``sys.stdin``,
``sys.stdout`` and ``sys.stderr`` now default to utf-8 encoding.

This change only applies when using an interactive console, and not when
redirecting files or pipes. To revert to the previous behaviour for interactive
console use, set :envvar:`PYTHONLEGACYWINDOWSIOENCODING`.

.. seealso::

   :pep:`528` -- Change Windows console encoding to UTF-8
      PEP written and implemented by Steve Dower.

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`.)


DTrace and SystemTap probing support
------------------------------------

Python can now be built ``--with-dtrace`` which enables static markers
for the following events in the interpreter:

* function call/return

* garbage collection started/finished

* line of code executed.

This can be used to instrument running interpreters in production,
without the need to recompile specific debug builds or providing
application-specific profiling/debugging code.

More details in :ref:`instrumentation`.

The current implementation is tested on Linux and macOS.  Additional
markers may be added in the future.

(Contributed by Łukasz Langa in :issue:`21590`, based on patches by
Jesús Cea Avión, David Malcolm, and Nikhil Benesch.)


.. _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 ``__dict__`` attribute.

Also, the effective default class *execution* namespace (returned from
``type.__prepare__()``) is now an insertion-order-preserving mapping.

.. seealso::

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


.. _whatsnew-kwargs:

PEP 468: Preserving Keyword Argument Order
------------------------------------------

``**kwargs`` in a function signature is now guaranteed to be an
insertion-order-preserving mapping.

.. seealso::

   :pep:`468` -- Preserving Keyword Argument Order
      PEP written and implemented by Eric Snow.

.. _whatsnew-pep509:

PEP 509: Add a private version to dict
--------------------------------------

Add a new private version to the builtin ``dict`` type, incremented at
each dictionary creation and at each dictionary change, to implement
fast guards on namespaces.

(Contributed by Victor Stinner in :issue:`26058`.)


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

Some smaller changes made to the core Python language are:

* :func:`dict` now uses a "compact" representation `pioneered by PyPy
  <https://morepypy.blogspot.com/2015/01/faster-more-memory-efficient-and-more.html>`_.
  The memory usage of the new :func:`dict` is between 20% and 25% smaller
  compared to Python 3.5.
  :pep:`468` (Preserving the order of ``**kwargs`` in a function.) is
  implemented by this. The order-preserving aspect of this new
  implementation is considered an implementation detail and should
  not be relied upon (this may change in the future, but it is desired
  to have this new dict implementation in the language for a few
  releases before changing the language spec to mandate
  order-preserving semantics for all current and future Python
  implementations; this also helps preserve backwards-compatibility
  with older versions of the language where random iteration order is
  still in effect, e.g. Python 3.5).
  (Contributed by INADA Naoki in :issue:`27350`. Idea
  `originally suggested by Raymond Hettinger
  <https://mail.python.org/pipermail/python-dev/2012-December/123028.html>`_.)

* 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`.)

* Import now raises the new exception :exc:`ModuleNotFoundError`
  (subclass of :exc:`ImportError`) when it cannot find a module.  Code
  that current checks for ImportError (in try-except) will still work.


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

* None yet.


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

On Linux, :func:`os.urandom` now blocks until the system urandom entropy pool
is initialized to increase the security. See the :pep:`524` for the rationale.


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 :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.


email
-----

The new email API, enabled via the *policy* keyword to various constructors, is
no longer provisional.  The :mod:`email` documentation has been reorganized and
rewritten to focus on the new API, while retaining the old documentation for
the legacy API.  (Contributed by R. David Murray in :issue:`24277`.)

The :mod:`email.mime` classes now all accept an optional *policy* keyword.
(Contributed by Berker Peksag in :issue:`27331`.)

The :class:`~email.generator.DecodedGenerator` now supports the *policy*
keyword.

There is a new :mod:`~email.policy` attribute,
:attr:`~email.policy.Policy.message_factory`, that controls what class is used
by default when the parser creates new message objects.  For the
:attr:`email.policy.compat32` policy this is :class:`~email.message.Message`,
for the new policies it is :class:`~email.message.EmailMessage`.
(Contributed by R. David Murray in :issue:`20476`.)


encodings
---------

On Windows, added the ``'oem'`` encoding to use ``CP_OEMCP`` and the ``'ansi'``
alias for the existing ``'mbcs'`` encoding, which uses the ``CP_ACP`` code page.


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`.)


hashlib
-------

:mod:`hashlib` supports OpenSSL 1.1.0. The minimum recommend version is 1.0.2.
It has been tested with 0.9.8zc, 0.9.8zh and 1.0.1t as well as LibreSSL 2.3
and 2.4.
(Contributed by Christian Heimes in :issue:`26470`.)

BLAKE2 hash functions were added to the module. :func:`~hashlib.blake2b`
and :func:`~hashlib.blake2s` are always available and support the full
feature set of BLAKE2.
(Contributed by Christian Heimes in :issue:`26798` based on code by
Dmitry Chestnykh and Samuel Neves. Documentation written by Dmitry Chestnykh.)

The SHA-3 hash functions :func:`~hashlib.sha3_224`, :func:`~hashlib.sha3_256`,
:func:`~hashlib.sha3_384`, :func:`~hashlib.sha3_512`, and SHAKE hash functions
:func:`~hashlib.shake_128` and :func:`~hashlib.shake_256` were added.
(Contributed by Christian Heimes in :issue:`16113`. Keccak Code Package
by Guido Bertoni, Joan Daemen, Michaël Peeters, Gilles Van Assche, and
Ronny Van Keer.)

The password-based key derivation function :func:`~hashlib.scrypt` is now
available with OpenSSL 1.1.0 and newer.
(Contributed by Christian Heimes in :issue:`27928`.)

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`.

:func:`importlib.util.cache_from_source`,
:func:`importlib.util.source_from_cache`, and
:func:`importlib.util.spec_from_file_location` now accept a
:term:`path-like object`.


json
----

:func:`json.load` and :func:`json.loads` now support binary input.  Encoded
JSON should be represented using either UTF-8, UTF-16, or UTF-32.
(Contributed by Serhiy Storchaka in :issue:`17909`.)


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`.)

The Linux ``getrandom()`` syscall (get random bytes) is now exposed as the new
:func:`os.getrandom` function.
(Contributed by Victor Stinner, part of the :pep:`524`)

See the summary for :ref:`PEP 519 <pep-519>` for details on how the
:mod:`os` and :mod:`os.path` modules now support
:term:`path-like objects <path-like object>`.


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`.)


re
--

Added support of modifier spans in regular expressions.  Examples:
``'(?i:p)ython'`` matches ``'python'`` and ``'Python'``, but not ``'PYTHON'``;
``'(?i)g(?-i:v)r'`` matches ``'GvR'`` and ``'gvr'``, but not ``'GVR'``.
(Contributed by Serhiy Storchaka in :issue:`433028`.)

Match object groups can be accessed by ``__getitem__``, which is
equivalent to ``group()``.  So ``mo['name']`` is now equivalent to
``mo.group('name')``.  (Contributed by Eric Smith in :issue:`24454`.)


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`.)

The socket module now supports the address family
:data:`~socket.AF_ALG` to interface with Linux Kernel crypto API. ``ALG_*``,
``SOL_ALG`` and :meth:`~socket.socket.sendmsg_afalg` were added.
(Contributed by Christian Heimes in :issue:`27744` with support from
Victor Stinner.)

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`.)

ssl
---

:mod:`ssl` supports OpenSSL 1.1.0. The minimum recommend version is 1.0.2.
It has been tested with 0.9.8zc, 0.9.8zh and 1.0.1t as well as LibreSSL 2.3
and 2.4.
(Contributed by Christian Heimes in :issue:`26470`.)

3DES has been removed from the default cipher suites and ChaCha20 Poly1305
cipher suites are now in the right position.
(Contributed by Christian Heimes in :issue:`27850` and :issue:`27766`.)

:class:`~ssl.SSLContext` has better default configuration for options
and ciphers.
(Contributed by Christian Heimes in :issue:`28043`.)

SSL session can be copied from one client-side connection to another
with :class:`~ssl.SSLSession`. TLS session resumption can speed up
the initial handshake, reduce latency and improve performance
(Contributed by Christian Heimes in :issue:`19500` based on a draft by
Alex Warhawk.)

All constants and flags have been converted to :class:`~enum.IntEnum` and
:class:`~enum.IntFlags`.
(Contributed by Christian Heimes in :issue:`28025`.)

Server and client-side specific TLS protocols for :class:`~ssl.SSLContext`
were added.
(Contributed by Christian Heimes in :issue:`28085`.)

General resource ids (``GEN_RID``) in subject alternative name extensions
no longer case a SystemError.
(Contributed by Christian Heimes in :issue:`27691`.)

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`).

The :class:`subprocess.Popen` constructor and all functions that pass arguments
through to it now accept *encoding* and *errors* arguments. Specifying either
of these will enable text mode for the *stdin*, *stdout* and *stderr* streams.

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`.)


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

The internal database has been upgraded to use Unicode 9.0.0. (Contributed by
Benjamin Peterson.)


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`.)


winsound
--------

Allowed keyword arguments to be passed to :func:`Beep <winsound.Beep>`,
:func:`MessageBeep <winsound.MessageBeep>`, and :func:`PlaySound
<winsound.PlaySound>` (:issue:`27982`).


xmlrpc.client
-------------

The module now supports unmarshalling additional data types used by
Apache XML-RPC implementation for numerics and ``None``.
(Contributed by Serhiy Storchaka in :issue:`26885`.)


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`).

* Optimized :func:`~glob.glob` and :func:`~glob.iglob` functions in the
  :mod:`glob` module; they are now about 3--6 times faster.
  (Contributed by Serhiy Storchaka in :issue:`25596`).

* Optimized globbing in :mod:`pathlib` by using :func:`os.scandir`;
  it is now about 1.5--4 times faster.
  (Contributed by Serhiy Storchaka in :issue:`26032`).

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

* Python now requires some C99 support in the toolchain to build. For more
  information, see :pep:`7`.

* Cross-compiling CPython with the Android NDK and the Android API level set to
  21 (Android 5.0 Lollilop) or greater, runs successfully. While Android is not
  yet a supported platform, the Python test suite runs on the Android emulator
  with only about 16 tests failures. See the Android meta-issue :issue:`26865`.

* The ``--with-optimizations`` configure flag has been added. Turning it on
  will activate LTO and PGO build support (when available).
  (Original patch by Alecsandru Patrascu of Intel in :issue:`26539`.)

* 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
==========

Deprecated Build Options
------------------------

The ``--with-system-ffi`` configure flag is now on by default on non-OSX UNIX
platforms.  It may be disabled by using ``--without-system-ffi``, but using the
flag is deprecated and will not be accepted in Python 3.7.  OSX is unaffected
by this change.  Note that many OS distributors already use the
``--with-system-ffi`` flag when building their system Python.


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`.

* The :mod:`tkinter.tix` module is now deprecated.  :mod:`tkinter` users should
  use :mod:`tkinter.ttk` instead.


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

* Undocumented functions :c:func:`PyUnicode_AsEncodedObject`,
  :c:func:`PyUnicode_AsDecodedObject`, :c:func:`PyUnicode_AsEncodedUnicode`
  and :c:func:`PyUnicode_AsDecodedUnicode` are deprecated now.
  Use :ref:`generic codec based API <codec-registry>` instead.


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.

* A backslash-character pair that is not a valid escape sequence now generates
  a DeprecationWarning.  Although this will eventually become a SyntaxError,
  that will not be for several Python releases.  (Contributed by Emanuel Barry
  in :issue:`27364`.)

* Inline flags ``(?letters)`` now should be used only at the start of the
  regular expression.  Inline flags in the middle of the regular expression
  affects global flags in Python :mod:`re` module.  This is an exception to
  other regular expression engines that either apply flags to only part of
  the regular expression or treat them as an error.  To avoid distinguishing
  inline flags in the middle of the regular expression now emit a deprecation
  warning.  It will be an error in future Python releases.
  (Contributed by Serhiy Storchaka in :issue:`22493`.)

* SSL-related arguments like ``certfile``, ``keyfile`` and ``check_hostname``
  in :mod:`ftplib`, :mod:`http.client`, :mod:`imaplib`, :mod:`poplib`,
  and :mod:`smtplib` have been deprecated in favor of ``context``.
  (Contributed by Christian Heimes in :issue:`28022`.)

* A couple of protocols and functions of the :mod:`ssl` module are now
  deprecated. Some features will no longer be available in future versions
  of OpenSSL. Other features are deprecated in favor of a different API.
  (Contributed by Christian Heimes in :issue:`28022` and :issue:`26470`.)


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.

* The undocumented ``IN``, ``CDROM``, ``DLFCN``, ``TYPES``, ``CDIO``, and
  ``STROPTS`` modules have been removed.  They had been available in the
  platform specific ``Lib/plat-*/`` directories, but were chronically out of
  date, inconsistently available across platforms, and unmaintained.  The
  script that created these modules is still available in the source
  distribution at :source:`Tools/scripts/h2py.py`.


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
-------------------------

* :mod:`sqlite3` no longer implicitly commit an open transaction before DDL
  statements.

* On Linux, :func:`os.urandom` now blocks until the system urandom entropy pool
  is initialized to increase the security.

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

* :c:func:`PyErr_SetImportError` now sets :exc:`TypeError` when its **msg**
  argument is not set. Previously only ``NULL`` was returned.

* 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`.
