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.

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

.. 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 Features
============

.. _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`.


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

The new :envvar:`PYTHONMALLOC` environment variable allows to set 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`.)


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

* None yet.


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

* None yet.


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

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`.)


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`.)


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`.)


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`.)


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`).


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`.)


telnetlib
---------

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


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.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.


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`.)


zlib
----

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


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`).


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

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


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`.)


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.


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

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

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

* 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:`csv`,
  :mod:`~xml.etree.ElementTree`, :mod:`enum`,
  :mod:`fileinput`, :mod:`ftplib`, :mod:`logging`,
  :mod:`optparse`, :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`.

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

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