Doc/whatsnew/3.4.rst

****************************
  What's New In Python 3.4
****************************

.. :Author: Someone <email>
   (uncomment if there is a principal author)

.. Rules for maintenance:

   * Anyone can add text to this document, but the maintainer reserves the
   right to rewrite any additions. In particular, for obscure or esoteric
   features, the maintainer may reduce any addition to a simple reference to
   the new documentation rather than explaining the feature inline.

   * While the maintainer will periodically go through Misc/NEWS
   and add changes, it's best not to rely on this. We know from experience
   that any changes that aren't in the What's New documentation around the
   time of the original release will remain largely unknown to the community
   for years, even if they're added later. We also know from experience that
   other priorities can arise, and the maintainer will run out of time to do
   updates -- in such cases, end users will be much better served by partial
   notifications that at least give a hint about new features to
   investigate.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS. The What's New should focus on changes that
   are visible to Python *users* and that *require* a feature release (i.e.
   most bug fixes should only be recorded in Misc/NEWS)

   * PEPs should not be marked Final until they have an entry in What's New.
   A placeholder entry that is just a section header and a link to the PEP
   (e.g ":pep:`397` has been implemented") is acceptable. If a PEP has been
   implemented and noted in What's New, don't forget to mark it as Final!

   * 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 add just a very brief note about a change.  For
   example: "The :ref:`~socket.transmogrify()` function was added to the
   :mod:`socket` module."  The maintainer will research the change and
   write the necessary text (if appropriate). The advantage of doing this
   is that even if no more descriptive text is ever added, readers will at
   least have a notification that the new feature exists and a link to the
   relevant documentation.

   * 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:

   The :ref:`~socket.transmogrify()` function was added to the
   :mod:`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.

   * Cross referencing tip: :ref:`mod.attr` will display as ``mod.attr``,
   while :ref:`~mod.attr` will display as ``attr``.

This article explains the new features in Python 3.4, compared to 3.3.

.. Python 3.4 was released on TBD.

For full details, see the
`changelog <http://docs.python.org/3.4/whatsnew/changelog.html>`_.

.. note:: Prerelease users should be aware that this document is currently in
   draft form. It will be updated substantially as Python 3.4 moves towards
   release, so it's worth checking back even after reading earlier versions.


.. seealso::

   :pep:`429` -- Python 3.4 Release Schedule


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

.. This section singles out the most important changes in Python 3.4.
   Brevity is key.

New syntax features:

* No new syntax features were added in Python 3.4.

New expected features for Python implementations:

* :ref:`pip should always be "available" <whatsnew-pep-453>` (:pep:`453`).
* :ref:`Make newly created file descriptors non-inheritable <whatsnew-pep-446>`
  (:pep:`446`).
* command line option for :ref:`isolated mode <using-on-misc-options>`,
  (:issue:`16499`).
* :ref:`improvements in the handling of codecs <codec-handling-improvements>`
  that are not text encodings (multiple issues).
* :ref:`A ModuleSpec Type <whatsnew-pep-451>` for the Import System
  (:pep:`451`).  (Affects importer authors.)

New library modules:

* :mod:`asyncio`: :ref:`New provisional API for asynchronous IO
  <whatsnew-asyncio>` (:pep:`3156`).
* :mod:`ensurepip`: :ref:`Bootstrapping the pip installer <whatsnew-ensurepip>`
  (:pep:`453`).
* :mod:`enum`: :ref:`Support for enumeration types <whatsnew-enum>`
  (:pep:`435`).
* :mod:`pathlib`: :ref:`Object-oriented filesystem paths <whatsnew-pathlib>`
  (:pep:`428`).
* :mod:`selectors`: :ref:`High-level and efficient I/O multiplexing
  <whatsnew-selectors>`, built upon the :mod:`select` module primitives (part
  of :pep:`3156`).
* :mod:`statistics`: A basic :ref:`numerically stable statistics library
  <whatsnew-statistics>` (:pep:`450`).
* :mod:`tracemalloc`: :ref:`Trace Python memory allocations
  <whatsnew-tracemalloc>` (:pep:`454`).

Significantly Improved Library Modules:

* :ref:`Single-dispatch generic functions <whatsnew-singledispatch>` in
  :mod:`functools` (:pep:`443`).
* New :mod:`pickle` :ref:`protocol 4 <whatsnew-protocol-4>` (:pep:`3154`).
* :ref:`SHA-3 (Keccak) support <whatsnew-sha3>` for :mod:`hashlib`
  (:issue:`16113`).
* :ref:`TLSv1.1 and TLSv1.2 support <whatsnew-tls-11-12>` for :mod:`ssl`
  (:issue:`16692`).
* :mod:`multiprocessing` now has :ref:`an option to avoid using os.fork
  on Unix <whatsnew-multiprocessing-no-fork>` (:issue:`8713`).
* :mod:`email` has a new submodule, :mod:`~email.contentmanager`, and
  a new :mod:`~email.message.Message` subclass
  (:class:`~email.contentmanager.EmailMessage`) that :ref:`simplify MIME
  handling <whatsnew_email_contentmanager>` (:issue:`18891`).

CPython implementation improvements:

* :ref:`Safe object finalization <whatsnew-pep-442>` (:pep:`442`).
* Leveraging :pep:`442`, :ref:`module globals are no longer set to None
  during finalization <whatsnew-pep-442>`, in most cases (:issue:`18214`).
* :ref:`Configurable memory allocators <whatsnew-pep-445>` (:pep:`445`).
* :ref:`Secure and interchangeable hash algorithm <whatsnew-pep-456>`
  (:pep:`456`).
* :ref:`Argument Clinic <whatsnew-pep-436>` (:pep:`436`).
* The :mod:`marshal` format has been made :ref:`more compact and efficient
  <whatsnew-marshal-3>` (:issue:`16475`).

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


New expected features for Python implementations
================================================

.. _whatsnew-pep-453:

PEP 453: Explicit bootstrapping of pip in Python installations
--------------------------------------------------------------

The new :mod:`ensurepip` module (defined in :pep:`453`) provides a standard
cross-platform mechanism to boostrap the pip installer into Python
installations and virtual environments.

The :mod:`venv` module and the :command:`pyvenv` utility make use of this
module to make ``pip`` readily available in virtual environments. When
using the command line interface, ``pip`` is installed by default, while
for the module API installation of ``pip`` must be requested explicitly.

For CPython source builds on POSIX systems, the ``make install`` and
``make altinstall`` commands bootstrap ``pip`` by default. This behaviour
can be controlled through configure options, and overridden through
Makefile options.

On Windows and Mac OS X, the CPython installers now offer the option to
install ``pip`` along with CPython itself.

As `discussed in the PEP`__, platform packagers may choose not to install
``pip`` by default, as long as the command ``pip``, when invoked, provides
clear and simple directions on how to install ``pip`` on the platform.

__ http://www.python.org/dev/peps/pep-0453/#recommendations-for-downstream-distributors

.. note::

   The implementation of PEP 453 is still a work in progress. Refer to
   :issue:`19347` for the progress on additional steps:

   * Having the binary installers install ``pip`` by default
   * Recommending the use of ``pip`` in the "Installing Python Module"
     documentation.

.. seealso::

   :pep:`453` -- Explicit bootstrapping of pip in Python installations
      PEP written by Donald Stufft and Nick Coghlan, implemented by
      Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily.


.. _whatsnew-pep-446:

PEP 446: Make newly created file descriptors non-inheritable
------------------------------------------------------------

:pep:`446` makes newly created file descriptors :ref:`non-inheritable
<fd_inheritance>`.  New functions and methods:

* :func:`os.get_inheritable`, :func:`os.set_inheritable`
* :func:`os.get_handle_inheritable`, :func:`os.set_handle_inheritable`
* :meth:`socket.socket.get_inheritable`, :meth:`socket.socket.set_inheritable`

.. seealso::

   :pep:`446` -- Make newly created file descriptors non-inheritable
      PEP written and implemented by Victor Stinner.


.. _codec-handling-improvements:

Improvements to codec handling
------------------------------

Since it was first introduced, the :mod:`codecs` module has always been
intended to operate as a type-neutral dynamic encoding and decoding
system. However, its close coupling with the Python text model, especially
the type restricted convenience methods on the builtin :class:`str`,
:class:`bytes` and :class:`bytearray` types, has historically obscured that
fact.

As a key step in clarifying the situation, the :meth:`codecs.encode` and
:meth:`codecs.decode` convenience functions are now properly documented in
Python 2.7, 3.3 and 3.4. These functions have existed in the :mod:`codecs`
module (and have been covered by the regression test suite) since Python 2.4,
but were previously only discoverable through runtime introspection.

Unlike the convenience methods on :class:`str`, :class:`bytes` and
:class:`bytearray`, these convenience functions support arbitrary codecs
in both Python 2 and Python 3, rather than being limited to Unicode text
encodings (in Python 3) or ``basestring`` <-> ``basestring`` conversions
(in Python 2).

In Python 3.4, the interpreter is able to identify the known non-text
encodings provided in the standard library and direct users towards these
general purpose convenience functions when appropriate::

    >>> b"abcdef".decode("hex")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs

    >>> "hello".encode("rot13")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    LookupError: 'rot13' is not a text encoding; use codecs.encode() to handle arbitrary codecs

In a related change, whenever it is feasible without breaking backwards
compatibility, exceptions raised during encoding and decoding operations
will be wrapped in a chained exception of the same type that mentions the
name of the codec responsible for producing the error::

    >>> import codecs

    >>> codecs.decode(b"abcdefgh", "hex")
    binascii.Error: Non-hexadecimal digit found

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    binascii.Error: decoding with 'hex' codec failed (Error: Non-hexadecimal digit found)

    >>> codecs.encode("hello", "bz2")
    TypeError: 'str' does not support the buffer interface

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: encoding with 'bz2' codec failed (TypeError: 'str' does not support the buffer interface)

Finally, as the examples above show, these improvements have permitted
the restoration of the convenience aliases for the non-Unicode codecs that
were themselves restored in Python 3.2. This means that encoding binary data
to and from its hexadecimal representation (for example) can now be written
as::

    >>> from codecs import encode, decode
    >>> encode(b"hello", "hex")
    b'68656c6c6f'
    >>> decode(b"68656c6c6f", "hex")
    b'hello'

The binary and text transforms provided in the standard library are detailed
in :ref:`binary-transforms` and :ref:`text-transforms`.

(Contributed by Nick Coghlan in :issue:`7475`, , :issue:`17827`,
:issue:`17828` and :issue:`19619`)

.. _whatsnew-pep-451:

PEP 451: A ModuleSpec Type for the Import System
------------------------------------------------

:pep:`451` provides an encapsulation of the information about a module that the
import machinery will use to load it (that is, a module specification).  This
helps simplify both the import implementation and several import-related APIs.
The change is also a stepping stone for `several future import-related
improvements`__.

__ https://mail.python.org/pipermail/python-dev/2013-November/130111.html

The public-facing changes from the PEP are entirely backward-compatible.
Furthermore, they should be transparent to everyone but importer authors.  Key
finder and loader methods have been deprecated, but they will continue working.
New importers should use the new methods described in the PEP.  Existing
importers should be updated to implement the new methods.


Other Language Changes
----------------------

Some smaller changes made to the core Python language are:

* Unicode database updated to UCD version 6.3.

* :func:`min` and :func:`max` now accept a *default* argument that can be used
  to specify the value they return if the iterable they are evaluating has no
  elements.  Contributed by Julian Berman in :issue:`18111`.

* Module objects are now :mod:`weakref`'able.

* Module ``__file__`` attributes (and related values) should now always
  contain absolute paths by default, with the sole exception of
  ``__main__.__file__`` when a script has been executed directly using
  a relative path (Contributed by Brett Cannon in :issue:`18416`).

* Now all the UTF-\* codecs (except UTF-7) reject surrogates during both
  encoding and decoding unless the ``surrogatepass`` error handler is used,
  with the exception of the UTF-16 decoder that accepts valid surrogate pairs,
  and the UTF-16 encoder that produces them while encoding non-BMP characters.
  Contributed by Victor Stinner, Kang-Hao (Kenny) Lu and Serhiy Storchaka in
  :issue:`12892`.


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


.. _whatsnew-asyncio:

asyncio
-------

The new :mod:`asyncio` module (defined in :pep:`3156`) provides a standard
pluggable event loop model for Python, providing solid asynchronous IO
support in the standard library, and making it easier for other event loop
implementations to interoperate with the standard library and each other.

For Python 3.4, this module is considered a :term:`provisional API`.

.. seealso::

   :pep:`3156` -- Asynchronous IO Support Rebooted: the "asyncio" Module
      PEP written and implementation led by Guido van Rossum.


.. _whatsnew-ensurepip:

ensurepip
---------

The new :mod:`ensurepip` module is the primary infrastructure for the
:pep:`453` implementation.  In the normal course of events end users will not
need to interact with this module, but it can be used to manually bootstrap
``pip`` if the automated bootstrapping into an installation or virtual
environment was declined.

:mod:`ensurepip` includes a bundled copy of ``pip``, up-to-date as of the first
release candidate of the release of CPython with which it ships (this applies
to both maintenance releases and feature releases).  ``ensurepip`` does not
access the internet.  (If the installation has Internet access, it is of course
possible to upgrade ``pip`` to a release more recent than the bundled ``pip``
by using the bundled ``pip`` command itself once it is installed.)

The module is named *ensure*\ pip because if called when ``pip`` is already
installed, it does nothing.  It also has an ``--upgrade`` option that will
cause it to install the bundled copy of ``pip`` if the existing installed
version of ``pip`` is older than the bundled copy.


.. _whatsnew-enum:

enum
----

The new :mod:`enum` module (defined in :pep:`435`) provides a standard
implementation of enumeration types, allowing other modules (such as
:mod:`socket`) to provide more informative error messages and better
debugging support by replacing opaque integer constants with backwards
compatible enumeration values.

.. seealso::

   :pep:`435` -- Adding an Enum type to the Python standard library
      PEP written by Barry Warsaw, Eli Bendersky and Ethan Furman,
      implemented by Ethan Furman.


.. _whatsnew-pathlib:

pathlib
-------

The new :mod:`pathlib` module offers classes representing filesystem paths
with semantics appropriate for different operating systems.  Path classes are
divided between *pure paths*, which provide purely computational operations
without I/O, and *concrete paths*, which inherit from pure paths but also
provide I/O operations.

For Python 3.4, this module is considered a :term:`provisional API`.

.. seealso::

   :pep:`428` -- The pathlib module -- object-oriented filesystem paths
      PEP written and implemented by Antoine Pitrou.


.. _whatsnew-selectors:

selectors
---------

The new :mod:`selectors` module (created as part of implementing :pep:`3156`)
allows high-level and efficient I/O multiplexing, built upon the
:mod:`select` module primitives.


.. _whatsnew-statistics:

statistics
----------

The new :mod:`statistics` module (defined in :pep:`450`) offers some core
statistics functionality directly in the standard library. This module
supports calculation of the mean, median, mode, variance and standard
deviation of a data series.

.. seealso::

   :pep:`450` -- Adding A Statistics Module To The Standard Library
      PEP written and implemented by Steven D'Aprano

.. _whatsnew-tracemalloc:

tracemalloc
-----------

The new :mod:`tracemalloc` module (defined in :pep:`454`) is a debug tool to
trace memory blocks allocated by Python. It provides the following information:

* Traceback where an object was allocated
* Statistics on allocated memory blocks per filename and per line number:
  total size, number and average size of allocated memory blocks
* Compute the differences between two snapshots to detect memory leaks

.. seealso::

   :pep:`454` -- Add a new tracemalloc module to trace Python memory allocations
      PEP written and implemented by Victor Stinner


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


aifc
----

The :meth:`~aifc.getparams` method now returns a namedtuple rather than a
plain tuple.  (Contributed by Claudiu Popa in :issue:`17818`.)


audioop
-------

Added support for 24-bit samples (:issue:`12866`).

Added the :func:`~audioop.byteswap` function to convert big-endian samples
to little-endian and vice versa (:issue:`19641`).


base64
------

The encoding and decoding functions in :mod:`base64` now accept any
:term:`bytes-like object` in cases where it previously required a
:class:`bytes` or :class:`bytearray` instance (:issue:`17839`).


colorsys
--------

The number of digits in the coefficients for the RGB --- YIQ conversions have
been expanded so that they match the FCC NTSC versions.  The change in
results should be less than 1% and may better match results found elsewhere.


contextlib
----------

The new :class:`contextlib.suppress` context manager helps to clarify the
intent of code that deliberately suppresses exceptions from a single
statement. (Contributed by Raymond Hettinger in :issue:`15806` and
Zero Piraeus in :issue:`19266`)

The new :func:`contextlib.redirect_stdout` context manager makes it easier
for utility scripts to handle inflexible APIs that don't provide any
options to retrieve their output as a string or direct it to somewhere
other than :data:`sys.stdout`. In conjunction with :class:`io.StringIO`,
this context manager is also useful for checking expected output from
command line utilities. (Contribute by Raymond Hettinger in :issue:`15805`)

The :mod:`contextlib` documentation has also been updated to include a
:ref:`discussion <single-use-reusable-and-reentrant-cms>` of the
differences between single use, reusable and reentrant context managers.


dis
---

The :mod:`dis` module is now built around an :class:`~dis.Instruction` class
that provides details of individual bytecode operations and a
:func:`~dis.get_instructions` iterator that emits the Instruction stream for a
given piece of Python code. The various display tools in the :mod:`dis`
module have been updated to be based on these new components.

The new :class:`dis.Bytecode` class provides an object-oriented API for
inspecting bytecode, both in human-readable form and for iterating over
instructions.

(Contributed by Nick Coghlan, Ryan Kelly and Thomas Kluyver in :issue:`11816`
and Claudiu Popa in :issue:`17916`)


doctest
-------

Added :data:`~doctest.FAIL_FAST` flag to halt test running as soon as the first
failure is detected.  (Contributed by R. David Murray and Daniel Urban in
:issue:`16522`.)

Updated the doctest command line interface to use :mod:`argparse`, and added
``-o`` and ``-f`` options to the interface.  ``-o`` allows doctest options to
be specified on the command line, and ``-f`` is a shorthand for ``-o
FAIL_FAST`` (to parallel the similar option supported by the :mod:`unittest`
CLI).  (Contributed by R. David Murray in :issue:`11390`.)


email
-----

:meth:`~email.message.Message.as_string` now accepts a *policy* argument to
override the default policy of the message when generating a string
representation of it.  This means that ``as_string`` can now be used in more
circumstances, instead of having to create and use a :mod:`~email.generator` in
order to pass formatting parameters to its ``flatten`` method.

New method :meth:`~email.message.Message.as_bytes` added to produce a bytes
representation of the message in a fashion similar to how ``as_string``
produces a string representation.  It does not accept the *maxheaderlen*
argument, but does accept the *unixfrom* and *policy* arguments. The
:class:`~email.message.Message` :meth:`~email.message.Message.__bytes__` method
calls it, meaning that ``bytes(mymsg)`` will now produce the intuitive
result:  a bytes object containing the fully formatted message.

(Contributed by R. David Murray in :issue:`18600`.)

.. _whatsnew_email_contentmanager:

A pair of new subclasses of :class:`~email.message.Message` have been added,
along with a new sub-module, :mod:`~email.contentmanager`.  All documentation
is currently in the new module, which is being added as part of the new
:term:`provisional <provisional package>` email API.  These classes provide a
number of new methods that make extracting content from and inserting content
into email messages much easier.  See the :mod:`~email.contentmanager`
documentation for details.

These API additions complete the bulk of the work that was planned as part of
the email6 project.  The currently provisional API is scheduled to become final
in Python 3.5 (possibly with a few minor additions in the area of error
handling).

(Contributed by R. David Murray in :issue:`18891`.)


functools
---------

The new :func:`~functools.partialmethod` descriptor bring partial argument
application to descriptors, just as :func:`~functools.partial` provides
for normal callables. The new descriptor also makes it easier to get
arbitrary callables (including :func:`~functools.partial` instances)
to behave like normal instance methods when included in a class definition.

(Contributed by Alon Horev and Nick Coghlan in :issue:`4331`)

.. _whatsnew-singledispatch:

The new :func:`~functools.singledispatch` decorator brings support for
single-dispatch generic functions to the Python standard library. Where
object oriented programming focuses on grouping multiple operations on a
common set of data into a class, a generic function focuses on grouping
multiple implementations of an operation that allows it to work with
*different* kinds of data.

.. seealso::

   :pep:`443` -- Single-dispatch generic functions
      PEP written and implemented by Łukasz Langa.


hashlib
-------

New :func:`hashlib.pbkdf2_hmac` function.
(Contributed by Christian Heimes in :issue:`18582`)

.. _whatsnew-sha3:

New :ref:`hash algorithms <hash-algorithms>` ``sah3_224()``, ``sha3_256()``,
``sha3_384()``, and ``sha3_512()``.  (Contributed by Christian Heimes in
:issue:`16113`.)


html
----

Added a new :func:`html.unescape` function that converts HTML5 character
references to the corresponding Unicode characters.
(Contributed by Ezio Melotti in :issue:`2927`)

Added a new *convert_charrefs* keyword argument to
:class:`~html.parser.HTMLParser` that, when ``True``, automatically converts
all character references.  For backward-compatibility, its value defaults
to ``False``, but it will change to ``True`` in future versions, so you
are invited to set it explicitly and update your code to use this new feature.
(Contributed by Ezio Melotti in :issue:`13633`)

The *strict* argument of :class:`~html.parser.HTMLParser` is now deprecated.
(Contributed by Ezio Melotti in :issue:`15114`)


inspect
-------


The inspect module now offers a basic :ref:`command line interface
<inspect-module-cli>` to quickly display source code and other
information for modules, classes and functions. (Contributed by Claudiu Popa
and Nick Coghlan in :issue:`18626`)

:func:`~inspect.unwrap` makes it easy to unravel wrapper function chains
created by :func:`functools.wraps` (and any other API that sets the
``__wrapped__`` attribute on a wrapper function). (Contributed by
Daniel Urban, Aaron Iles and Nick Coghlan in :issue:`13266`)

As part of the implementation of the new :mod:`enum` module, the
:mod:`inspect` module now has substantially better support for custom
``__dir__`` methods and dynamic class attributes provided through
metaclasses (Contributed by Ethan Furman in :issue:`18929` and
:issue:`19030`)


.. _whatsnew-marshal-3:

marshal
-------

The default :mod:`marshal` version has been bumped to 3.  The code implementing
the new version restores the Python2 behavior of recording only one copy of
interned strings and preserving the interning on deserialization, and extends
this "one copy" ability to any object type (including handling recursive
references).  This reduces both the size of ``.pyc`` files and the amount of
memory a module occupies in memory when it is loaded from a ``.pyc`` (or
``.pyo``) file.  (Contributed by Kristján Valur Jónsson in :issue:`16475`.)


mmap
----

mmap objects can now be weakref'ed.
(Contributed by Valerie Lambert in :issue:`4885`.)


multiprocessing
---------------

.. _whatsnew-multiprocessing-no-fork:

On Unix, two new :ref:`start methods <multiprocessing-start-methods>`
(``spawn`` and ``forkserver``) have been added for starting processes using
:mod:`multiprocessing`.  These make the mixing of processes with threads more
robust, and the ``spawn`` method matches the semantics that multiprocessing has
always used on Windows. (Contributed by Richard Oudkerk in :issue:`8713`).

Also, except when using the old *fork* start method, child processes
will no longer inherit unneeded handles/file descriptors from their parents
(part of :issue:`8713`).

:mod:`multiprocessing` now relies on :mod:`runpy` (which implements the
``-m`` switch) to initialise ``__main__`` appropriately in child processes
when using the ``spawn`` or ``forkserver`` start methods. This resolves some
edge cases where combining multiprocessing, the ``-m`` command line switch
and explicit relative imports could cause obscure failures in child
processes. (Contributed by Nick Coghlan in :issue:`19946`)


os
--

New functions to get and set the :ref:`inheritable flag <fd_inheritance>` of a file
descriptors or a Windows handle:

* :func:`os.get_inheritable`, :func:`os.set_inheritable`
* :func:`os.get_handle_inheritable`, :func:`os.set_handle_inheritable`


pdb
---

The ``print`` command has been removed from :mod:`pdb`, restoring access to the
``print`` function.

Rationale: Python2's ``pdb`` did not have a ``print`` command; instead,
entering ``print`` executed the ``print`` statement.  In Python3 ``print`` was
mistakenly made an alias for the pdb :pdbcmd:`p` command.  ``p``, however,
prints the ``repr`` of its argument, not the ``str`` like the Python2 ``print``
command did.  Worse, the Python3 ``pdb print`` command shadowed the Python3
``print`` function, making it inaccessible at the ``pdb`` prompt.

(Contributed by Connor Osborn in :issue:`18764`.)


.. _whatsnew-protocol-4:

Pickle
------

protocol 4

:mod:`pickle` now supports (but does not use by default) a new pickle protocol,
protocol 4.  This new protocol addresses a number of issues that were present
in previous protocols, such as the serialization of nested classes, very large
strings and containers, or classes whose :meth:`__new__` method takes
keyword-only arguments.  It also provides some efficiency improvements.

.. seealso::

   :pep:`3154` -- Pickle protocol 4
      PEP written by Antoine Pitrou and implemented by Alexandre Vassalotti.


poplib
------

New :meth:`~poplib.POP3.stls` method to switch a clear-text POP3 session into
an encrypted POP3 session.

New :meth:`~poplib.POP3.capa` method to query the capabilities advertised by the
POP3 server.

(Contributed by Lorenzo Catucci in :issue:`4473`.)


pprint
------

The :mod:`pprint` module now supports *compact* mode for formatting long
sequences (:issue:`19132`).


pydoc
-----

While significant changes have not been made to :mod:`pydoc` directly,
its handling of custom ``__dir__`` methods and various descriptor
behaviours has been improved substantially by the underlying changes in
the :mod:`inspect` module.


re
--

Added :func:`re.fullmatch` function and :meth:`regex.fullmatch` method,
which anchor the pattern at both ends of the string to match.
(Contributed by Matthew Barnett in :issue:`16203`.)

The repr of :ref:`regex objects <re-objects>` now includes the pattern
and the flags; the repr of :ref:`match objects <match-objects>` now
includes the start, end, and the part of the string that matched.
(Contributed by Serhiy Storchaka in :issue:`13592` and :issue:`17087`.)


resource
--------

New :func:`resource.prlimit` function and Linux specific constants.
(Contributed by Christian Heimes in :issue:`16595` and :issue:`19324`.)

smtplib
-------

:exc:`~smtplib.SMTPException` is now a subclass of :exc:`OSError`, which allows
both socket level errors and SMTP protocol level errors to be caught in one
try/except statement by code that only cares whether or not an error occurred.
(:issue:`2118`).


socket
------

Socket objects have new methods to get or set their :ref:`inheritable flag
<fd_inheritance>`:

* :meth:`socket.socket.get_inheritable`, :meth:`socket.socket.set_inheritable`

The ``socket.AF_*`` and ``socket.SOCK_*`` constants are enumeration values,
using the new :mod:`enum` module. This allows descriptive reporting during
debugging, instead of seeing integer "magic numbers".

ssl
---

.. _whatsnew-tls-11-12:

:data:`~ssl.PROTOCOL_TLSv1_1` and :data:`~ssl.PROTOCOL_TLSv1_2` (TLSv1.1 and
TLSv1.2 support) have been added; support for these protocols is only available if
Python is linked with OpenSSL 1.0.1 or later.  (Contributed by Michele Orrù and
Antoine Pitrou in :issue:`16692`)

New diagnostic functions :func:`~ssl.get_default_verify_paths`,
:meth:`~ssl.SSLContext.cert_store_stats` and
:meth:`~ssl.SSLContext.get_ca_certs` (Contributed by Christian Heimes
in :issue:`18143` and :issue:`18147`)

Add :func:`ssl.enum_cert_store` to retrieve certificates and CRL from Windows'
cert store.  (Contributed by Christian Heimes in :issue:`17134`.)

Support for server-side SNI using the new
:meth:`ssl.SSLContext.set_servername_callback` method.
(Contributed by Daniel Black in :issue:`8109`.)


stat
----

The :mod:`stat` module is now backed by a C implementation in :mod:`_stat`. A C
implementation is required as most of the values aren't standardized and
platform-dependent.  (Contributed by Christian Heimes in :issue:`11016`.)

The module supports new file types: door, event port and whiteout.


struct
------

Streaming struct unpacking using :func:`struct.iter_unpack`.
(Contributed by Antoine Pitrou in :issue:`17804`.)


sunau
-----

The :meth:`~sunau.getparams` method now returns a namedtuple rather than a
plain tuple.  (Contributed by Claudiu Popa in :issue:`18901`.)

:meth:`sunau.open` now supports the context manager protocol (:issue:`18878`).


traceback
---------

A new :func:`traceback.clear_frames` function takes a traceback object
and clears the local variables in all of the frames it references,
reducing the amount of memory consumed (:issue:`1565525`).


urllib
------

Add support.for ``data:`` URLs in :mod:`urllib.request`.
(Contributed by Mathias Panzenböck in :issue:`16423`.)


unittest
--------

Support for easy dynamically-generated subtests using the
:meth:`~unittest.TestCase.subTest` context manager.
(Contributed by Antoine Pitrou in :issue:`16997`.)


wave
----

The :meth:`~wave.getparams` method now returns a namedtuple rather than a
plain tuple.  (Contributed by Claudiu Popa in :issue:`17487`.)

:meth:`wave.open` now supports the context manager protocol.  (Contributed
by Claudiu Popa in :issue:`17616`.)


weakref
-------

New :class:`~weakref.WeakMethod` class simulates weak references to bound
methods. (Contributed by Antoine Pitrou in :issue:`14631`.)

New :class:`~weakref.finalize` class makes it possible to register a callback
to be invoked when an object is garbage collected, without needing to
carefully manage the lifecycle of the weak reference itself. (Contributed by
Richard Oudkerk in :issue:`15528`)


xml.etree
---------

Add an event-driven parser for non-blocking applications,
:class:`~xml.etree.ElementTree.XMLPullParser`.
(Contributed by Antoine Pitrou in :issue:`17741`.)


zipfile.PyZipfile
-----------------

Add a filter function to ignore some packages (tests for instance),
:meth:`~zipfile.PyZipFile.writepy`.
(Contributed by Christian Tismer in :issue:`19274`.)


CPython Implementation Changes
==============================


.. _whatsnew-pep-445:

PEP 445: Customization of CPython memory allocators
---------------------------------------------------

:pep:`445` adds new C level interfaces to customize memory allocation in
the CPython interpreter.

.. seealso::

   :pep:`445` -- Add new APIs to customize Python memory allocators
      PEP written and implemented by Victor Stinner.


.. _whatsnew-pep-442:

PEP 442: Safe object finalization
---------------------------------

:pep:`442` removes the current limitations and quirks of object finalization
in CPython. With it, objects with :meth:`__del__` methods, as well as
generators with :keyword:`finally` clauses, can be finalized when they are
part of a reference cycle.

As part of this change, module globals are no longer forcibly set to
:const:`None` during interpreter shutdown in most cases, instead relying
on the normal operation of the cyclic garbage collector.  This avoids a
whole class of interpreter-shutdown-time errors, usually involving
``__del__`` methods, that have plagued Python since the cyclic GC
was first introduced.

.. seealso::

   :pep:`442` -- Safe object finalization
      PEP written and implemented by Antoine Pitrou.


.. _whatsnew-pep-456:

PEP 456: Secure and Interchangeable Hash Algorithm
--------------------------------------------------

:pep:`456` follows up on earlier security fix work done on Python's hash
algorithm to address certain DOS attacks to which public facing APIs backed by
dictionary lookups may be subject.  (See :issue:`14621` for the start of the
current round of improvements.)  The PEP unifies CPython's hash code to make it
easier for a packager to substitute a different hash algorithm, and switches
Python's default implementation to a SipHash implementation on platforms that
have a 64 bit data type.  Any performance differences in comparison with the
older FNV algorithm are trivial.

The PEP adds additional fields to the :func:`sys.hash_info` struct sequence to
describe the hash algorithm in use by the currently executing binary.  Otherwise,
the PEP does not alter any existing CPython APIs.


.. _whatsnew-pep-436:

PEP 436: Argument Clinic
------------------------

"Argument Clinic" (:pep:`436`) is now part of the CPython build process
and can be used to simplify the process of defining and maintaining
accurate signatures for builtins and standard library extension modules
implemented in C.

.. note::
   The Argument Clinic PEP is not fully up to date with the state of the
   implementation. This has been deemed acceptable by the release manager
   and core development team in this case, as Argument Clinic will not
   be made available as a public API for third party use in Python 3.4.

.. seealso::

   :pep:`436` -- The Argument Clinic DSL
       PEP written and implemented by Larry Hastings.


Other build and C API changes
-----------------------------

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

* The new :c:func:`Py_SetStandardStreamEncoding` pre-initialization API
  allows applications embedding the CPython interpreter to reliably force
  a particular encoding and error handler for the standard streams
  (Contributed by Bastien Montagne and Nick Coghlan in :issue:`16129`)

* Most Python C APIs that don't mutate string arguments are now correctly
  marked as accepting ``const char *`` rather than ``char *`` (Contributed
  by Serhiy Storchaka in :issue:`1772673`).

* New shell version of ``python-config``; can be used even when a python
  interpreter is not available (for example, in cross compilation scenarios).


Other improvements
==================

* Tab-completion is now enabled by default in the interactive interpreter.
  (Contributed by Antoine Pitrou and Éric Araujo in :issue:`5845`.)

* Invoking the Python interpreter with ``--version`` now outputs the version to
  standard output instead of standard error (:issue:`18338`). Similar changes
  were made to :mod:`argparse` (:issue:`18920`) and other modules that have
  script-like invocation capabilities (:issue:`18922`).

* The CPython Windows installer now adds ``.py`` to the :envvar:`PATHEXT`
  variable when extensions are registered, allowing users to run a python
  script at the windows command prompt by just typing its name without the
  ``.py`` extension.  (Contributed by Paul Moore in :issue:`18569`.)


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

Major performance enhancements have been added:

* The UTF-32 decoder is now 3x to 4x faster.

* The cost of hash collisions for sets is now reduced.  Each hash table
  probe now checks a series of consecutive, adjacent key/hash pairs before
  continuing to make random probes through the hash table.  This exploits
  cache locality to make collision resolution less expensive.

  The collision resolution scheme can be described as a hybrid of linear
  probing and open addressing.  The number of additional linear probes
  defaults to nine.  This can be changed at compile-time by defining
  LINEAR_PROBES to be any value.  Set LINEAR_PROBES=0 to turn-off
  linear probing entirely.

  (Contributed by Raymond Hettinger in :issue:`18771`.)

* The interpreter starts about 30% faster. A couple of measures lead to the
  speedup. The interpreter loads fewer modules on startup, e.g. the :mod:`re`,
  :mod:`collections` and :mod:`locale` modules and their dependencies are no
  longer imported by default. The marshal module has been improved to load
  compiled Python code faster.

  (Contributed by Antoine Pitrou, Christian Heimes and Victor Stinner in
  :issue:`19219`, :issue:`19218`, :issue:`19209`, :issue:`19205` and
  :issue:`9548`)


Deprecated
==========

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

* OS/2
* Windows 2000
* VMS


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

* :meth:`difflib.SequenceMatcher.isbjunk` and
  :meth:`difflib.SequenceMatcher.isbpopular` were removed: use ``x in sm.bjunk`` and
  ``x in sm.bpopular``, where *sm* is a :class:`~difflib.SequenceMatcher` object.

* :func:`importlib.util.module_for_loader` is pending deprecation. Using
  :func:`importlib.util.module_to_load` and
  :meth:`importlib.abc.Loader.init_module_attrs` allows subclasses of a loader
  to more easily customize module loading.

* The :mod:`imp` module is pending deprecation. To keep compatibility with
  Python 2/3 code bases, the module's removal is currently not scheduled.

* The :mod:`formatter` module is pending deprecation and is slated for removal
  in Python 3.6.

* MD5 as default digestmod for :mod:`hmac` is deprecated. Python 3.6 will
  require an explicit digest name or constructor as *digestmod* argument.


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

* The ``PyThreadState.tick_counter`` field has been removed: its value was
  meaningless since Python 3.2 ("new GIL").


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

* The site module adding a "site-python" directory to sys.path, if it
  exists, is deprecated (:issue:`19375`).


Removed
=======

* The unmaintained ``Misc/TextMate`` and ``Misc/vim`` directories have been
  removed (see the `devguide <http://docs.python.org/devguide>`_
  for what to use instead).


* OS/2 support code has been removed from the source tree and build tools
  (:issue:`16135`).

* Windows 2000 support code has been removed from the source tree and build
  tools (changeset e52df05b496a).


Porting to Python 3.4
=====================

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

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

* The ABCs defined in :mod:`importlib.abc` now either raise the appropriate
  exception or return a default value instead of raising
  :exc:`NotImplementedError` blindly. This will only affect code calling
  :func:`super` and falling through all the way to the ABCs. For compatibility,
  catch both :exc:`NotImplementedError` or the appropriate exception as needed.

* The module type now initializes the :attr:`__package__` and :attr:`__loader__`
  attributes to ``None`` by default. To determine if these attributes were set
  in a backwards-compatible fashion, use e.g.
  ``getattr(module, '__loader__', None) is not None``.

* :meth:`importlib.util.module_for_loader` now sets ``__loader__`` and
  ``__package__`` unconditionally to properly support reloading. If this is not
  desired then you will need to set these attributes manually. You can use
  :func:`importlib.util.module_to_load` for module management.

* Import now resets relevant attributes (e.g. ``__name__``, ``__loader__``,
  ``__package__``, ``__file__``, ``__cached__``) unconditionally when reloading.

* Frozen packages no longer set ``__path__`` to a list containing the package
  name but an empty list instead. Determing if a module is a package should be
  done using ``hasattr(module, '__path__')``.

* :func:`py_compile.compile` now raises :exc:`FileExistsError` if the file path
  it would write to is a symlink or a non-regular file. This is to act as a
  warning that import will overwrite those files with a regular file regardless
  of what type of file path they were originally.

* :meth:`importlib.abc.SourceLoader.get_source` no longer raises
  :exc:`ImportError` when the source code being loaded triggers a
  :exc:`SyntaxError` or :exc:`UnicodeDecodeError`. As :exc:`ImportError` is
  meant to be raised only when source code cannot be found but it should, it was
  felt to be over-reaching/overloading of that meaning when the source code is
  found but improperly structured. If you were catching ImportError before and
  wish to continue to ignore syntax or decoding issues, catch all three
  exceptions now.

* :func:`functools.update_wrapper` and :func:`functools.wraps` now correctly
  set the ``__wrapped__`` attribute to the function being wrapper, even if
  that function also had its ``__wrapped__`` attribute set. This means
  ``__wrapped__`` attributes now correctly link a stack of decorated
  functions rather than every ``__wrapped__`` attribute in the chain
  referring to the innermost function. Introspection libraries that
  assumed the previous behaviour was intentional can use
  :func:`inspect.unwrap` to access the first function in the chain that has
  no ``__wrapped__`` attribute.

* :class:`importlib.machinery.PathFinder` now passes on the current working
  directory to objects in :data:`sys.path_hooks` for the empty string. This
  results in :data:`sys.path_importer_cache` never containing ``''``, thus
  iterating through :data:`sys.path_importer_cache` based on :data:`sys.path`
  will not find all keys. A module's ``__file__`` when imported in the current
  working directory will also now have an absolute path, including when using
  ``-m`` with the interpreter (this does not influence when the path to a file
  is specified on the command-line).

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

* :c:func:`PyErr_SetImportError` now sets :exc:`TypeError` when its **msg**
  argument is not set. Previously only ``NULL`` was returned with no exception
  set.

* The result of the :c:data:`PyOS_ReadlineFunctionPointer` callback must
  now be a string allocated by :c:func:`PyMem_RawMalloc` or
  :c:func:`PyMem_RawRealloc`, or *NULL* if an error occurred, instead of a
  string allocated by :c:func:`PyMem_Malloc` or :c:func:`PyMem_Realloc`.

* :c:func:`PyThread_set_key_value` now always set the value. In Python
  3.3, the function did nothing if the key already exists (if the current
  value is a non-NULL pointer).

* The ``f_tstate`` (thread state) field of the :c:type:`PyFrameObject`
  structure has been removed to fix a bug: see :issue:`14432` for the
  rationale.