path: root/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. While it should be close to complete for the Python 3.4
   release candidates, adjustments and additions to the document may be made
   up until the final release.


.. 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 <whatsnew-isolated-mode>`,
  (: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:`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`).
* The :mod:`ipaddress` module API has been declared stable


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 bootstrap the pip installer into Python
installations and virtual environments.

By default, the scripts ``pipX`` and ``pipX.Y`` will be installed (where
X.Y stands for the version of the Python installation), along with the
``pip`` Python package and its dependencies.

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::

   To avoid conflicts between parallel Python 2 and Python 3 installations,
   only the versioned ``pip3`` and ``pip3.4`` commands are bootstrapped by
   default when ``ensurepip`` is invoked directly (including by the CPython
   installers). ``pyvenv`` ensures that the unqualified ``pip`` command is
   made available in virtual environments, and ``pip`` can always be
   invoked via the ``-m`` switch rather than directly to avoid ambiguity on
   systems with multiple Python installations.

.. 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

    >>> open("foo.txt", encoding="hex")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    LookupError: 'hex' is not a text encoding; use codecs.open() 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")
    Traceback (most recent call last):
      File "/usr/lib/python3.4/encodings/hex_codec.py", line 20, in hex_decode
        return (binascii.a2b_hex(input), len(input))
    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")
    Traceback (most recent call last):
      File "/usr/lib/python3.4/encodings/bz2_codec.py", line 17, in bz2_encode
        return (bz2.compress(input), len(input))
      File "/usr/lib/python3.4/bz2.py", line 498, in compress
        return comp.compress(data) + comp.flush()
    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* keyword-only 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 EBCDIC :ref:`codec <standard-encodings>` ``cp273``.  (Contributed by
  Michael Bierenfeld and Andrew Kuchling in :issue:`1097797`.)

* :class:`bytes`.join() and :class:`bytearray`.join() now accept arbitrary
  buffer objects as arguments.  (Contributed by Antoine Pitrou in
  :issue:`15958`.)

* The :class:`int` constructor now accepts any object that has an ``__index__``
  method for its *base* argument.  (Contributed by Mark Dickinson in
  :issue:`16772`.)

* Frame objects now have a :func:`~frame.clear` method that clears all
  references to local variables from the frame.  (Contributed by Antoine Pitrou
  in :issue:`17934`.)

* :class:`memoryview` is now registered as a :class:`Sequence <collections.abc>`,
  and supports the :func:`reversed` builtin.  (Contributed by Nick Coghlan
  and Claudiu Popa in :issue:`18690` and :issue:`19078`.)


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
================


abc
---

New function :func:`abc.get_cache_token` can be used to know when to invalidate
caches that are affected by changes in the object graph.  (Contributed
by Łukasz Langa in :issue:`16832`.)

New class :class:`~abc.ABC` has :class:`~abc.ABCMeta` as its meta class.
Using ``ABC`` as a base class has essentially the same effect as specifying
``metaclass=abc.ABCMeta``, but is simpler to type and easier to read.
(Contributed by Bruno Dupuis in :issue:`16049`.)


aifc
----

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

:func:`aifc.open` now supports the context manager protocol: when used in a
:keyword:`with` block, the :meth:`~aifc.aifc.close` method of the returned
object will be called automatically at the end of the block.  (Contributed by
Serhiy Storchacha in :issue:`16486`.)


argparse
--------

The :class:`~argparse.FileType` class now accepts *encoding* and
*errors* arguments, which are passed through to :func:`open`.  (Contributed
by Lucas Maystre in :issue:`11175`.)


audioop
-------

The module now supports 24-bit samples.  (Contributed by Serhiy Storchaka
in :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.  (Contributed by Nick Coghlan in
: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.
(Contributed by Brian Landers and Serhiy Storchaka in :issue:`14323`.)


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 write their output to
:data:`sys.stdout` and don't provide any options to redirect it.  Using the
context manager, the :data:`sys.stdout` output can be redirected to any
other stream or, in conjunction with :class:`io.StringIO`, to a string.
The latter can be especially useful, for example, to capture output
from a function that was written to implement a command line interface.
It is recommended only for utility scripts because it affects the
global state of :data:`sys.stdout`.  (Contributed 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
---

Functions :func:`~dis.show_code`, :func:`~dis.dis`, :func:`~dis.distb`, and
:func:`~dis.disassemble` now accept a keyword-only *file* argument that
controls where they write their output.

The :mod:`dis` module is now built around an :class:`~dis.Instruction` class
that provides object oriented access to the details of each individual bytecode
operation.

A new method, :func:`~dis.get_instructions`, provides an iterator that emits
the Instruction stream for a given piece of Python code.  Thus it is now
possible to write a program that inspects and manipulates a bytecode
object in ways different from those provided by the :mod:`~dis` module
itself.  For example::

    >>> import dis
    >>> for instr in dis.get_instructions(lambda x: x + 1):
    ...     print(instr.opname)
    LOAD_FAST
    LOAD_CONST
    BINARY_ADD
    RETURN_VALUE

The various display tools in the :mod:`dis` module have been rewritten to use
these new components.

In addition, a new application-friendly class :class:`~dis.Bytecode` provides
an object-oriented API for inspecting bytecode in both in human-readable form
and for iterating over instructions.  The :class:`~dis.Bytecode` constructor
takes the same arguments that :func:`~dis.get_instruction` does (plus an
optional *current_offset*), and the resulting object can be iterated to produce
:class:`~dis.Instruction` objects.  But it also has a :mod:`~dis.Bytecode.dis`
method, equivalent to calling :mod:`~dis.dis` on the constructor argument, but
returned as a multi-line string::

    >>> bytecode = dis.Bytecode(lambda x: x +1, current_offset=3)
    >>> for instr in bytecode:
    ...     print('{} ({})'.format(instr.opname, instr.opcode))
    LOAD_FAST (124)
    LOAD_CONST (100)
    BINARY_ADD (23)
    RETURN_VALUE (83)
    >>> bytecode.dis().splitlines()       # doctest: +NORMALIZE_WHITESPACE
    ['  1           0 LOAD_FAST                0 (x)',
     '      -->     3 LOAD_CONST               1 (1)',
     '              6 BINARY_ADD',
     '              7 RETURN_VALUE']

:class:`~dis.Bytecode` also has a class method,
:meth:`~dis.Bytecode.from_traceback`, that provides the ability to manipulate a
traceback (that is, ``print(Bytecode.from_traceback(tb).dis())`` is equivalent
to ``distb(tb)``).

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


doctest
-------

A new :ref:`option flag <doctest-options>`, :data:`~doctest.FAIL_FAST`, halts
test running as soon as the first failure is detected.  (Contributed by R.
David Murray and Daniel Urban in :issue:`16522`.)

The :mod:`doctest` command line interface now uses :mod:`argparse`, and has two
new options, ``-o`` and ``-f``.  ``-o`` allows :ref:`doctest options
<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`.)


filecmp
-------

A new :func:`~filecmp.clear_cache` function provides the ability to clear the
:mod:`filecmp` comparison cache, which uses :func:`os.stat` information to
determine if the file has changed since the last compare.  This can be used,
for example, if the file might have been changed and re-checked in less time
than the resolution of a particular filesystem's file modification time field.
(Contributed by Mark Levitt in :issue:`18149`.)


functools
---------

The new :func:`~functools.partialmethod` descriptor brings 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.

A pure-python version of the :func:`~functools.partial` function is now in the
stdlib; in CPython it is overridden by the C accelerated version, but it is
available for other implementations to use.  (Contributed by Brian Thorne in
:issue:`12428`.)


gc
--

New function :func:`~gc.get_stats` returns a list of three per-generation
dictionaries containing the collections statistics since interpreter startup.
(Contributed by Antoine Pitrou in :issue:`16351`.)


hashlib
-------

A new :func:`hashlib.pbkdf2_hmac` function provides
the `PKCS#5 password-based key derivation function 2
<http://en.wikipedia.org/wiki/PBKDF2>`_.  (Contributed by Christian
Heimes in :issue:`18582`)

The :attr:`~hashlib.hash.name` attribute of :mod:`hashlib` hash objects is now
a formally supported interface.  It has always existed in CPython's
:mod:`hashlib` (although it did not return lower case names for all supported
hashes), but it was not a public interface and so some other Python
implementations have not previously supported it.  (Contributed by Jason R.
Coombs in :issue:`18532`.)


hmac
----

:mod:`hmac` now accepts ``bytearray`` as well as ``bytes`` for the *key*
argument to the :func:`~hmac.new` function, and the *msg* parameter to both the
:func:`~hmac.new` function and the :meth:`~hmac.HMAC.update` method now
accepts any type supported by the :mod:`hashlib` module.  (Contributed
by Jonas Borgström in :issue:`18240`.)


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`)


http
----

:meth:`~http.server.BaseHTTPRequestHandler.send_error` now accepts an
optional additional *explain* parameter which can be used to provide an
extended error description, overriding the hardcoded default if there is one.
This extended error description will be formatted using the
:attr:`~http.server.HTTP.error_message_format` attribute and sent as the body
of the error response.  (Contributed by Karl Cow in :issue:`12921`.)

The :mod:`http.server` :ref:`command line interface <http-server-cli>` now has
a ``-b/--bind`` option that causes the server to listen on a specific address.
(Contributed by Malte Swart in :issue:`17764`.)


importlib
---------

The :class:`~importlib.abc.InspectLoader` ABC defines a new method,
:meth:`~importlib.abc.InspectLoader.source_to_code` that accepts source
data and a path and returns a code object.  The default implementation
is equivalent to ``compile(data, path, 'exec', dont_inherit=True)``.
(Contributed by Eric Snow and Brett Cannon in :issue:`15627`.)

:class:`~importlib.abc.InspectLoader` also now has a default implementation
for the :meth:`~importlib.abc.InspectLoader.get_code` method.  However,
it will normally be desirable to override the default implementation
for performance reasons.  (Contributed by Brett Cannon in :issue:`18072`.)

The :func:`~importlib.reload` function has been moved from :mod:`imp` to
:mod:`importlib` as part of the :mod:`imp` module deprecation.  (Contributed by
Berker Peksag in :issue:`18193`.)

:mod:`importlib.util` now has a :data:`~importlib.util.MAGIC_NUMBER` attribute
providing access to the bytecode version number.  This replaces the
:func:`~imp.get_magic` function in the deprecated :mod:`imp` module.
(Contributed by Brett Cannon in :issue:`18192`.)

New :mod:`importlib.util` functions :func:`~importlib.util.cache_from_source`
and :func:`~importlib.util.source_from_cache` replace the same-named functions
in the deprecated :mod:`imp` module.  (Contributed by Brett Cannon in
:issue:`18194`.)

The :mod:`importlib` bootstrap :class:`.NamespaceLoader` now conforms to
the :class:`.InspectLoader` ABC, which means that ``runpy`` and
``python -m`` can now be used with namespace packages.  (Contributed
by Brett Cannon in :issue:`18058`.)

:mod:`importlib.util` has a new function :func:`~importlib.util.decode_source`
that decodes source from bytes using universal newline processing.  This is
useful for implementing :meth:`.InspectLoader.get_source` methods.

:class:`importlib.machinery.ExtensionFileLoader` now has a
:meth:`~importlib.machinery.ExtensionFileLoader.get_filename` method.  This was
inadvertently omitted in the original implementation.  (Contributed by Eric
Snow in :issue:`19152`.)


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`)

:func:`~inspect.getfullargspec` and :func:`~inspect.getargspec`
now use the :func:`~inspect.signature` API. This allows them to
support much broader range of functions, including some builtins and
callables that follow ``__signature__`` protocol. It is still
recommended to update your code to use :func:`~inspect.signature`
directly. (Contributed by Yury Selivanov in :issue:`17481`)

:func:`~inspect.signature` now supports duck types of CPython functions,
which adds support for functions compiled with Cython. (Contributed
by Stefan Behnel and Yury Selivanov in :issue:`17159`)


ipaddress
---------

:mod:`ipaddress` was added to the standard library in Python 3.3 as a
:term:`provisional API`. With the release of Python 3.4, this qualification
has been removed: :mod:`ipaddress` is now considered a stable API, covered
by the normal standard library requirements to maintain backwards
compatibility.

A new :attr:`~ipaddress.IPv4Address.is_global` property is ``True`` if
an address is globally routeable.  (Contributed by Peter Moody in
:issue:`17400`.)


logging
-------

The :class:`~logging.handlers.TimedRotatingFileHandler` has a new *atTime*
parameter that can be used to specify the time of day when rollover should
happen.  (Contributed by Ronald Oussoren in :issue:`9556`.)

:class:`~logging.handlers.SocketHandler` and
:class:`~logging.handlers.DatagramHandler` now support Unix domain sockets (by
setting *port* to ``None``).  (Contributed by Vinay Sajip in commit
ce46195b56a9.)


.. _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`,
with additional speedups by Antoine Pitrou in :issue:`19219`.)


mmap
----

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


mock
----

:mod:`~unittest.mock` objects now inspect their specification signatures when
matching calls, which means an argument can now be matched by either position
or name, instead of only by position.  (Contributed by Antoine Pitrou in
:issue:`17015`.)

:func:`~mock.mock_open` objects now have ``readline`` and ``readlines``
methods. (Contributed by Toshio Kuratomi in :issue:`17467`.)


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`)


operator
--------

There is now a pure-python version of the :mod:`operator` module available for
reference and for use by alternate implementations of Python.  (Contributed by
Zachary Ware in :issue:`16694`.)


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`

The :mod:`os` module now provides a :func:`~os.cpu_count` function, analogous to
the :func:`multiprocessing.cpu_count` function (which is now implemented in
terms of the new :mod:`os` function).  (Contributed by Trent Nelson, Yogesh
Chaudhari, Victor Stinner, and Charles-François Natali in :issue:`17914`.)

:func:`os.path.samestat` is now available on the Windows platform (and the
:func:`os.path.samefile` implementation is now shared between Unix and
Windows).  (Contributed by Brian Curtin in :issue:`11939`.)

:func:`os.path.ismount` now recognizes volumes mounted below a drive
root on Windows. (Contributed by Tim Golden in :issue:`9035`.)


pdb
---

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

: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.


plistlib
--------

:mod:`plistlib` now supports binary plist files, and offers the common
``load``/``loads``/``dump``/``dumps`` API pattern for serialization formats
(Contributed by Ronald Oussoren and others in :issue:`14455`).


poplib
------

Two new methods have been added to :mod:`poplib`: :meth:`~poplib.POP3.capa`,
which returns the list of capabilities advertised by the POP server, and
:meth:`~poplib.POP3.stls`, which switches a clear-text POP3 session into an
encrypted POP3 session if the POP server supports it.  (Contributed by Lorenzo
Catucci in :issue:`4473`.)


pprint
------

The :mod:`pprint` module's :class:`~pprint.PrettyPrinter` class and its
:func:`~pprint.pformat`, and :func:`~pprint.pprint` functions have a new
option, *compact*, that controls how the output is formatted.  Currently
setting *compact* to ``True`` means that sequences will be printed with as many
sequence elements as will fit within *width* on each (indented) line.
(Contributed by Serhiy Storchaka in :issue:`19132`.)

Long strings are now wrapped using Python's normal line continuation
syntax.  (Contributed by Antoine Pitrou in :issue:`17150`).


pty
---

:func:`pty.spawn` now returns the status value from :func:`os.waitpid` on
the child process, instead of ``None``.  (Contributed by Gregory P. Smith.)


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, available on Linux platforms with a
kernel version of 2.6.36 or later and glibc of 2.13 or later, provides the
ability to query or set the resource limits for processes other than the one
making the call.  (Contributed by Christian Heimes in :issue:`16595`.)

On Linux kernel version 2.6.36 or later, there are there are also some new
Linux specific constants: :attr:`~resource.RLIMIT_MSGQUEUE`,
:attr:`~resource.RLIMIT_NICE`, :attr:`~resource.RLIMIT_RTPRIO`,
:attr:`~resource.RLIMIT_RTTIME`, and :attr:`~resource.RLIMIT_SIGPENDING`.
(Contributed by Christian Heimes in :issue:`19324`.)

On FreeBSD version 9 and later, there some new FreeBSD specific constants:
:attr:`~resource.RLIMIT_SBSIZE`, :attr:`~resource.RLIMIT_SWAP`, and
:attr:`~resource.RLIMIT_NPTS`.  (Contributed by Claudiu Popa in
:issue:`19343`.)


select
------

:class:`~select.epoll` objects now support the context management protocol.
When used in a :keyword:`with` statement, the :meth:`~select.epoll.close`
method will be called automatically at the end of the block.  (Contributed
by Serhiy Storchaka in :issue:`16488`.)

:class:`~select.devpoll` objects now have :meth:`~select.devpoll.fileno` and
:meth:`~select.devpoll.close` methods, as well as a new attribute
:attr:`~select.devpoll.closed`.  (Contributed by Victor Stinner in
:issue:`18794`.)


shelve
------

:class:`~shelve.Shelf` instances may now be used in :keyword:`with` statements,
and will be automatically closed at the end of the :keyword:`with` block.
(Contributed by Filip Gruszczyński in :issue:`13896`.)


smtpd
-----

The :class:`~smtpd.SMTPServer` and :class:`~smtpd.SMTPChannel` classes now
accept a *map* keyword argument, which if specified is passed in to
:class:`asynchat.async_chat` as its *map* argument.  This allows an application
to avoid affecting the global socket map.  (Contributed by Vinay Sajip in
:issue:`11959`.)


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
------

The socket module now supports the :data:`~socket.CAN_BCM` protocol on
platforms that support it.  (Contributed by Brian Thorne in :issue:`15359`.)

Socket objects have new methods to get or set their :ref:`inheritable flag
<fd_inheritance>`, :meth:`~socket.socket.get_inheritable` and
:meth:`~socket.socket.set_inheritable`.

The ``socket.AF_*`` and ``socket.SOCK_*`` constants are now enumeration values
using the new :mod:`enum` module.  This allows meaningful names to be printed
during debugging, instead of integer "magic numbers".

The :data:`~socket.AF_LINK` constant is now available on BSD and OSX.


sqlite3
-------

A new boolean parameter, *uri*, to the :func:`~sqlite3.connect` function can
be used to indicate that the *database* parameter is a ``uri`` (see
the `SQLite URI documentation <http://www.sqlite.org/uri.html>`_).
(Contributed by poq in :issue:`13773`.)


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 function :func:`~ssl.get_default_verify_paths` returns
a named tuple of the paths and environment variables that the
:meth:`~ssl.SSLContext.set_default_verify_paths` method uses to set
OpenSSL's default ``cafile`` and ``capath``.  This can be an aid in
debugging default verification issues.  (Contributed by Christian Heimes
in :issue:`18143`.)

:class:`~ssl.SSLContext` has a new method,
:meth:`~ssl.SSLContext.cert_store_stats`, that reports the number of loaded
``X.509`` certs, ``X.509 CA`` certs, and certificate revocation lists (``crl``\
s), as well as a :meth:`~ssl.SSLContext.get_ca_certs` method that returns a
list of the loaded ``CA`` certificates.  (Contributed by Christian Heimes in
and :issue:`18147`.)

Two new windows-only functions, :func:`~ssl.enum_certificates` and
:func:`~ssl.enum_crls` provide the ability to retrieve certificates,
certificate information, and CRLs from the 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
------

:mod:`struct` now supports the streamed unpacking of a buffer containing
repeated instances of a given format of data.  Both a module level
:mod:`~struct.iter_unpack` function and a :meth:`struct.Struct.iter_unpack`
method on compiled formats have been added.  (Contributed by Antoine Pitrou in
:issue:`17804`.)


subprocess
----------

:func:`~subprocess.check_output` now accepts an *input* argument that can
be used to provide the contents of ``stdin`` for the command that is run.
(Contributed by Zack Weinberg in :issue:`16624`.)

:func:`~subprocess.getstatus` and :func:`~subprocess.getstatusoutput` now
work on Windows.  This change was actually made in 3.3.4.  (Contributed
by Tim Golden in :issue:`10197`.)


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: when used in a
:keyword:`with` block, the ``close`` method of the returned object will be
called automatically at the end of the block.  (Contributed by Serhiy Storchaka
in :issue:`18878`.)


sys
---

New function :func:`sys.getallocatedblocks` returns the current number of
blocks allocated by the interpreter (in CPython with the default
``--with-pymalloc`` setting, this is allocations made through the
:c:func:`PyObject_Malloc` API).  This can be useful for tracking memory leaks,
especially if automated via a test suite.  (Contributed by Antoine Pitrou
in :issue:`13390`.)


textwrap
--------

The :class:`~textwrap.TextWrapper` class has two new attributes/constructor
arguments: :attr:`~textwrap.TextWrapper.max_lines`, which limits the number of
lines in the output, and :attr:`~textwrap.TextWrapper.placeholder`, which is a
string that will appear at the end of the output if it has been truncated
because of *max_lines*.  Building on these capabilities, a new convenience
function :func:`~textwrap.shorten` collapses all of the whitespace in the input
to single spaces and produces a single line of a given *width* that ends with
the *placeholder* (by default, ``[...]``).  (Contributed by Antoine Pitrou and
Serhiy Storchaka in :issue:`18585` and :issue:`18725`.)


threading
---------

The :class:`~threading.Thread` object representing the main thread can be
obtained from the new :func:`~threading.main_thread` function.  In normal
conditions this will be the thread from which the Python interpreter was
started.  (Contributed by Andrew Svetlov in :issue:`18882`.)


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.  (Contributed by Andrew Kuchling in
:issue:`1565525`).


types
-----

A new :func:`~types.DynamicClassAttribute` descriptor provides a way to define
an attribute that acts normally when looked up through an instance object, but
which is routed to the *class* ``__getattr__`` when looked up through the
class.  This allows one to have properties active on a class, and have virtual
attributes on the class with the same name (see :mod:`Enum` for an example).
(Contributed by Ethan Furman in :issue:`19030`.)


urllib
------

:mod:`urllib.request` now supports ``data:`` URLs via the
:class:`~urllib.request.DataHandler` class.  (Contributed by Mathias Panzenböck
in :issue:`16423`.)

The http method that will be used by a :class:`~urllib.request.Request` class
can now be specified by setting a :class:`~urllib.request.Request.method`
class attribute on the subclass. (Contributed by Jason R Coombs in
:issue:`18978`.)


unittest
--------

The :class:`~unittest.TestCase` class has a new method,
:meth:`~unittest.TestCase.subTest`, that produces a context manager whose
:keyword:`with` block becomes a "sub-test".  This context manager allows a test
method to dynamically generate subtests  by, say, calling the ``subTest``
context manager inside a loop.  A single test method can thereby produce an
indefinite number of separately-identified and separately-counted tests, all of
which will run even if one or more of them fail.  For example::

    class NumbersTest(unittest.TestCase):
        def test_even(self):
            for i in range(6):
                with self.subTest(i=1):
                    self.assertEqual(i % 2, 0)

will result in six subtests, each identified in the unittest verbose output
with a label consisting of the variable name ``i`` and a particular value for
that variable (``i=0``, ``i=1``, etc).  See :ref:`subtests` for the full
version of this example.  (Contributed by Antoine Pitrou in :issue:`16997`.)

:func:`unittest.main` now accepts an iterable of test names for
*defaultTest*, where previously it only accepted a single test name as a
string.  (Contributed by Jyrki Pulliainen in :issue:`15132`.)

If :class:`~unittest.SkipTest` is raised during test discovery (that is, at the
module level in the test file), it is now reported as a skip instead of an
error.  (Contributed by Zach Ware in :issue:`16935`.)

:meth:`~unittest.TestLoader.discover` now sorts the discovered files to provide
consistent test ordering.  (Contributed by Martin Melin and Jeff Ramnani in
:issue:`16709`.)

:class:`~unittest.TestSuite` now drops references to tests as soon as the test
has been run, if the test is successful.  On Python interpreters that do
garbage collection, this allows the tests to be garbage collected if nothing
else is holding a reference to the test.  It is possible to override this
behavior by creating a :class:`~unittest.TestSuite` subclass that defines a
custom ``_removeTestAtIndex`` method.  (Contributed by Tom Wardill, Matt
McClure, and Andrew Svetlov in :issue:`11798`.)

A new test assertion context-manager, :meth:`~unittest.TestCase.assertLogs`,
will ensure that a given block of code emits a log message using the
:mod:`logging` module.  By default the message can come from any logger and
have a priority of ``INFO`` or higher, but both the logger name and an
alternative minimum logging level may be specified.  The object returned by the
context manager can be queried for the :class:`~logging.LogRecord`\ s and/or
formatted messages that were logged.  (Contributed by Antoine Pitrou in
:issue:`18937`.)

Test discovery now works with namespace packages (Contributed by Claudiu Popa
in :issue:`17457`.)


venv
----

:mod:`venv` now includes activation scripts for the ``csh`` and ``fish``
shells (Contributed by Andrew Svetlov in :issue:`15417`.)


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`)

The callback, if any, associated with a :class:`~weakref.ref` is now
exposed via the :attr:`~weakref.ref.__callback__` attribute.  (Contributed
by Mark Dickinson in :issue:`17643`.)


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

A new parser, :class:`~xml.etree.ElementTree.XMLPullParser`, allows a
non-blocking applications to parse XML documents.  An example can be
seen at :ref:`elementtree-pull-parsing`.  (Contributed by Antoine
Pitrou in :issue:`17741`.)

The :mod:`xml.etree.ElementTree` :func:`~xml.etree.ElementTree.tostring` and
:func:`~xml.etree.ElementTree.tostringlist` functions, and the
:class:`~xml.etree.ElementTree.ElementTree`
:meth:`~xml.etree.ElementTree.ElementTree.write` method, now have a
*short_empty_elements* :ref:`keyword-only parameter <keyword-only_parameter>`
providing control over whether elements with no content are written in
abbreviated (``<tag />``) or expanded (``<tag></tag>``) form.  (Contributed by
Ariel Poliak and Serhiy Storchaka in :issue:`14377`.)


zipfile
-------

The :meth:`~zipfile.PyZipFile.writepy` method of the
:class:`~zipfile.PyZipFile` class has a new *filterfunc* option that can be
used to control which directories and files are added to the archive.  For
example, this could be used to exclude test files from the archive.
(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.

Some standard library extension modules have been converted to use Argument
Clinic in Python 3.4, and :mod:`inspect` has been updated accordingly.

.. 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
-----------------------------

* The new :c:func:`PyType_GetSlot` function has been added to the stable ABI,
  allowing retrieval of function pointers from named type slots when using
  the limited API. (Contributed by Martin von Löwis in :issue:`17162`)

* 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).

* :c:func:`PyUnicode_FromFormat` now supports width and precision
  specifications for ``%s``, ``%A``, ``%U``, ``%V``, ``%S``, and ``%R``.
  (Contributed by Ysj Ray and Victor Stinner in :issue:`7330`.)

* New function :c:func:`PyStructSequence_InitType2` supplements the
  existing :c:func:`PyStructSequence_InitType` function.  The difference
  is that it returns ``0`` on success and ``-1`` on failure.

* The CPython source can now be compiled using the address sanity checking
  features of recent versions of GCC and clang:  the false alarms in the small
  object allocator have been silenced.  (Contributed by Dhiru Kholia in
  :issue:`18596`.)

* New function :c:func:`Py_SetStandardStreamEncoding` allows an application
  that is embedding Python to do the equivalent of setting
  :envvar:`PYTHONIOENCODING`.  Its arguments override the equivalent
  values from :envvar:`PYTHONIOENCODING` if it exists.  (Contributed
  by Bastien Montagne and Nick Coghlan in :issue:`16129`.)

* The Windows build now uses `Address Space Layout Randomization
  <http://en.wikipedia.org/wiki/ASLR>`_ and `Data Execution Prevention
  <http://en.wikipedia.org/wiki/Data_Execution_Prevention>`_.  (Contributed by
  Christian Heimes in :issue:`16632`.)


.. _other-improvements-3.4:

Other Improvements
------------------

.. _whatsnew-isolated-mode:

* The :ref:`python <using-on-cmdline>` command has a new :ref:`option
  <using-on-misc-options>`, ``-I``, which causes it to run in "isolated mode",
  which means that :data:`sys.path` contains neither the script's directory nor
  the user's ``site-packages`` directory, and all :envvar:`PYTHON*` environment
  variables are ignored (it implies both ``-s`` and ``-E``).  Other
  restrictions may also be applied in the future, with the goal being to
  isolate the execution of a script from the user's environment.  This is
  appropriate, for example, when Python is used to run a system script.  On
  most POSIX systems it can and should be used in the ``#!`` line of system
  scripts.  (Contributed by Christian Heimes in :issue:`16499`.)

* 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`.)

* A new ``make`` target `coverage-report
  <http://docs.python.org/devguide/coverage.html#measuring-coverage-of-c-code-with-gcov-and-lcov>`_
  will build python, run the test suite, and generate an HTML coverage report
  for the C codebase using ``gcov`` and `lcov
  <http://ltp.sourceforge.net/coverage/lcov.php>`_.

* The ``-R`` option to the :ref:`python regression test suite <regrtest>` now
  also checks for memory allocation leaks, using
  :func:`sys.getallocatedblocks()`.  (Contributed by Antoine Pitrou in
  :issue:`13390`).

* ``python -m`` now works with namespace packages.

* The :mod:`stat` module is now implemented in C, which means it gets the
  values for its constants from the C header files, instead of having the
  values hard-coded in the python module as was previously the case.

* Loading multiple python modules from a single OS module (``.so``, ``.dll``)
  now works correctly (previously it silently returned the first python
  module in the file).  (Contributed by Václav Šmilauer in :issue:`16421`.)

* A new opcode, :opcode:`LOAD_CLASSDEREF`, has been added to fix a bug in the
  loading of free variables in class bodies that could be triggered by certain
  uses of :ref:`__prepare__ <prepare>`.  (Contributed by Benjamin Peterson in
  :issue:`17853`.)

* A number of MemoryError-related crashes were identified and fixed by Victor
  Stinner using his :pep:`445`-based ``pyfailmalloc`` tool (:issue:`18408`,
  :issue:`18520`).

* The :ref:`pyvenv <scripts-pyvenv>` command now accepts a ``--copies`` option
  to use copies rather than symlinks even on systems where symlinks are the
  default.  (Contributed by Vinay Sajip in :issue:`18807`.)

* The encoding name is now optional in the value set for the
  :envvar:`PYTHONIOENCODING` environment variable.  This makes it possible to
  set just the error handler, without changing the default encoding.
  (Contributed by Serhiy Storchaka in :issue:`18818`.)

* The :mod:`bz2`, :mod:`lzma`, and :mod:`gzip` module ``open`` functions now
  support ``x`` (exclusive creation) mode.  (Contributed by Tim Heaney and
  Vajrasky Kok in :issue:`19201`, :issue:`19222`, and :issue:`19223`.)


Significant Optimizations
-------------------------

* The UTF-32 decoder is now 3x to 4x faster.  (Contributed by Serhiy Storchaka
  in :issue:`14625`.)

* 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`)

* :class:`bz2.BZ2File` is now as fast or faster than the Python2 version for
  most cases.  :class:`lzma.LZMAFile` has also been optimized.  (Contributed by
  Serhiy Storchaka and Nadeem Vawda in :issue:`16034`.)

* :func:`random.getrandbits` is 20%-40% faster for small integers (the most
  common use case).  (Contributed by Serhiy Storchaka in :issue:`16674`).

* By taking advantage of the new storage format for strings, pickling of
  strings is now significantly faster.  (Contributed by Victor Stinner and
  Antoine Pitrou in :issue:`15596`.)

* A performance issue in :meth:`io.FileIO.readall` has been solved.  This
  particularly affects Windows, and significantly speeds up the case of piping
  significant amounts of data through :mod:`subprocess`.  (Contributed
  by Richard Oudkerk in :issue:`15758`.)

* :func:`html.escape` is now 10x faster.  (Contributed by Matt Bryant in
  :issue:`18020`.)

* On Windows, the native ``VirtualAlloc`` is now used instead of the CRT
  ``malloc`` in ``obmalloc``.  Artificial benchmarks show about a 3% memory
  savings.

* :func:`os.urandom` now uses a lazily-opened persistent file descriptor
  so as to avoid using many file descriptors when run in parallel from
  multiple threads. (Contributed by Antoine Pitrou in :issue:`18756`.)


Deprecated
==========

This section covers various APIs and other features that have been deprecated
in Python 3.4, and will be removed in Python 3.5 or later.  In most (but not
all) cases, using the deprecated APIs will produce a :exc:`DeprecationWarning`
when the interpreter is run with deprecation warnings enabled (for example, by
using ``-Wd``).


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.

* As mentioned in :ref:`whatsnew-pep-451`, a number of :mod:`importilb`
  methods and functions are deprecated: :meth:`importlib.find_loader` is
  replaced by :func:`importlib.util.find_spec`;
  :meth:`importlib.machinery.PathFinder.find_module` is replaced by
  :meth:`importlib.machinery.PathFinder.find_spec`;
  :meth:`importlib.abc.MetaPathFinder.find_module` is replaced by
  :meth:`importlib.abc.MetaPathFinder.find_spec`;
  :meth:`importlib.abc.PathEntryFinder.find_loader` and
  :meth:`~importlib.abc.PathEntryFinder.find_module` are replaced by
  :meth:`importlib.abc.PathEntryFinder.find_spec`; all of the ``xxxLoader`` ABC
  ``load_module`` methods (:meth:`importlib.abc.Loader.load_module`,
  :meth:`importlib.abc.InspectLoader.load_module`,
  :meth:`importlib.abc.FileLoader.load_module`,
  :meth:`importlib.abc.SourceLoader.load_module`) should no longer be
  implemented, instead loaders should implement an
  ``exec_module`` method
  (:meth:`importlib.abc.Loader.exec_module`,
  :meth:`importlib.abc.InspectLoader.exec_module`
  :meth:`importlib.abc.SourceLoader.exec_module`) and let the import system
  take care of the rest; and
  :meth:`importlib.abc.Loader.module_repr`,
  :meth:`importlib.util.module_for_loader`, :meth:`importlib.util.set_loader`,
  and :meth:`importlib.util.set_package` are no longer needed because their
  functions are now handled automatically by the import system.

* 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.

* The internal ``Netrc`` class in the :mod:`ftplib` module has been documented
  as deprecated in its docstring for quite some time.  It now emits a
  :exc:`DeprecationWarning` and will be removed completely in Python 3.5.

* The undocumented *endtime* argument to :meth:`subprocess.Popen.wait` should
  not have been exposed and is hopefully not in use; it is deprecated and
  will mostly likely be removed in Python 3.5.

* The *strict* argument of :class:`~html.parser.HTMLParser` is deprecated.


Deprecated Functions and Types in the C API
-------------------------------------------

XXX: None so far


Deprecated Features
-------------------

* Running :ref:`idle` with the ``-n`` flag (no subprocess) is deprecated.
  However, the feature will not be removed until :issue:`18823` is resolved.

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



Removed
=======


Operating Systems No Longer Supported
-------------------------------------

Support for the following operating systems has been removed from the source
and build tools:

* OS/2 (:issue:`16135`).
* Windows 2000 (changeset e52df05b496a).
* Windows systems where ``COMSPEC`` points to ``command.com`` (:issue:`14470`).
* VMS (:issue:`16136`).


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

The following obsolete and previously deprecated APIs and features have been
removed:

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

* The ``SO`` makefile macro is removed (it was replaced by the
  ``SHLIB_SUFFIX`` and ``EXT_SUFFIX`` macros) (:issue:`16754`).

* The ``PyThreadState.tick_counter`` field has been removed; its value has
  been meaningless since Python 3.2, when the "new GIL" was introduced
  (:issue:`19199`).

* ``PyLoader`` and ``PyPycLoader`` have been removed from :mod:`importlib`.
  (Contributed by Taras Lyapun in :issue:`15641`.)

* The *strict* argument to :class:`~http.client.HTTPConnection` and
  :class:`~http.client.HTTPSConnection` has been removed.  HTTP 0.9-style
  "Simple Responses" are no longer supported.

* The deprecated :mod:`urllib.request.Request` getter and setter methods
  ``add_data``, ``has_data``, ``get_data``, ``get_type``, ``get_host``,
  ``get_selector``, ``set_proxy``, ``get_origin_req_host``, and
  ``is_unverifiable`` have been removed (use direct attribute access instead).

* Support for loading the deprecated ``TYPE_INT64`` has been removed from
  :mod:`marshal`.  (Contributed by Dan Riti in :issue:`15480`.)

* :class:`inspect.Signature`: positional-only parameters are now required
  to have a valid name.

* :meth:`object.__format__` no longer accepts non-empty format strings, it now
  raises a :exc:`TypeError` instead.  Using a non-empty string has been
  deprecated since Python 3.2.  This change has been made to prevent a
  situation where previously working (but incorrect) code would start failing
  if an object gained a __format__ method, which means that your code may now
  raise a :exc:`TypeError` if you are using an ``'s'`` format code with objects
  that do not have a __format__ method that handles it.  See :issue:`7994` for
  background.


Code Cleanups
-------------

* The unused and undocumented internal ``Scanner`` class has been removed from
  the :mod:`pydoc` module.

* The private and effectively unused ``_gestalt`` module has been removed,
  along with the private :mod:`platform` functions ``_mac_ver_lookup``,
  ``_mac_ver_gstalt``, and ``_bcd2str``, which would only have ever been called
  on badly broken OSX systems (see :issue:`18393`).

* The hardcoded copies of certain :mod:`stat` constants that were included in
  the :mod:`tarfile` module namespace have been removed.



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

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


Changes in 'python' command behavior
------------------------------------

* In a posix shell, setting the :envvar:`PATH` environment variable to
  an empty value is equivalent to not setting it at all.  However, setting
  :envvar:`PYTHONPATH` to an empty value was *not* equivalent to not setting it
  at all: setting :envvar:`PYTHONPATH` to an empty value was equivalent to
  setting it to ``.``, which leads to confusion when reasoning by analogy to
  how :envvar:`PATH` works.  The behavior now conforms to the posix convention
  for :envvar:`PATH`.

* The [X refs, Y blocks] output of a debug (``--with-pydebug``) build of the
  CPython interpreter is now off by default.  It can be re-enabled using the
  ``-X showrefcount`` option. (Contributed by Ezio Melotti in :issue:`17323`.)

* The python command and most stdlib scripts (as well as :mod:`argparse`) now
  output ``--version`` information to ``stdout`` instead of ``stderr`` (for
  issue list see :ref:`other-improvements-3.4` above).


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

* 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``.  (:issue:`17115`.)

* :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.
  Note that this restores a pre-3.3 behavior in that it means a module is
  re-found when re-loaded (:issue:`19413` and XXX).

* Frozen packages no longer set ``__path__`` to a list containing the package
  name, they now set it to an empty list.  The previous behavior could cause
  the import system to do the wrong thing on submodule imports if there was
  also a directory with the same name as the frozen package.  The correct way
  to determine if a module is a package or not is to use``hasattr(module,
  '__path__')`` (:issue:`18065`).

* :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 wrapped, 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) (:issue:`18416`).

* The removal of the *strict* argument to :class:`~http.client.HTTPConnection`
  and :class:`~http.client.HTTPSConnection` changes the meaning of the
  remaining arguments if you are specifying them positionally rather than by
  keyword.  If you've been paying attention to deprecation warnings your code
  should already be specifying any additional arguments via keywords.

* Strings between ``from __future__ import ...`` statements now *always* raise
  a :exc:`SyntaxError`.  Previously if there was no leading docstring, an
  interstitial string would sometimes be ignored.  This brings CPython into
  compliance with the language spec; Jython and PyPy already were.
  (:issue:`17434`).

* :meth:`ssl.SSLSocket.getpeercert` and :meth:`ssl.SSLSocket.do_handshake`
  now raise an :exc:`OSError` with ``ENOTCONN`` when the ``SSLSocket`` is not
  connected, instead of the previous behavior of raising an
  :exc:`AttributError`.  In addition, :meth:`~ssl.SSLSocket.getpeercert`
  will raise a :exc:`ValueError` if the handshake has not yet been done.

* :func:`base64.b32decode` now raises a :exc:`binascii.Error` when the
  input string contains non-b32-alphabet characters, instead of a
  :exc:`TypeError`.  This particular :exc:`TypeError` was missed when the other
  :exc:`TypeError`\ s were converted.  (Contributed by Serhiy Storchaka in
  :issue:`18011`.)  Note: this change was also inadvertently applied in Python
  3.3.3.

* The :attr:`~cgi.FieldStorage.file` attribute is now automatically closed when
  the creating :class:`cgi.FieldStorage` instance is garbage collected. If you
  were pulling the file object out separately from the :class:`cgi.FieldStorage`
  instance and not keeping the instance alive, then you should either store the
  entire :class:`cgi.FieldStorage` instance or read the contents of the file
  before the :class:`cgi.FieldStorage` instance is garbage collected.

* Calling ``read`` or ``write`` on a closed SSL socket now raises an
  informative :exc:`ValueError` rather than the previous more mysterious
  :exc:`AttributeError` (:issue:`9177`).

* :meth:`slice.indices` no longer produces an :exc:`OverflowError` for huge
  values.  As a consequence of this fix, :meth:`slice.indices` now raises a
  :exc:`ValueError` if given a negative length; previously it returned nonsense
  values (:issue:`14794`).

* The :class:`complex` constructor, unlike the :mod:`cmath` functions, was
  incorrectly accepting :class:`float` values if an object's ``__complex__``
  special method returned one.  This now raises a :exc:`TypeError`.
  (:issue:`16290`.)

* The :class:`int` constructor in 3.2 and 3.3 erroneously accepts :class:`float`
  values for the *base* parameter.  It is unlikely anyone was doing this, but
  if so, it will now raise a :exc:`TypeError` (:issue:`16772`).

* Defaults for keyword-only arguments are now evaluated *after* defaults for
  regular keyword arguments, instead of before.  Hopefully no one wrote any
  code that depends on the previous buggy behavior (:issue:`16967`).

* Stale thread states are now cleared after :func:`~os.fork`.  This may cause
  some system resources to be released that previously were incorrectly kept
  perpetually alive (for example, database connections kept in thread-local
  storage).  (:issue:`17094`.)

* Parameter names in ``__annotations__`` dict are now mangled properly,
  similarly to ``__kwdefaults__``. (Contributed by Yury Selivanov in
  :issue:`20625`).

* :attr:`hashlib.hash.name` now always returns the identifier in lower case.
  Previously some builtin hashes had uppercase names, but now that it is a
  formal public interface the naming has been made consistent (:issue:`18532`).

* Because :mod:`unittest.TestSuite` now drops references to tests after they
  are run, test harnesses that re-use a :class:`~unittest.TestSuite` to re-run
  a set of tests may fail.   Test suites should not be re-used in this fashion
  since it means state is retained between test runs, breaking the test
  isolation that :mod:`unittest` is designed to provide.  However, if the lack
  of isolation is considered acceptable, the old behavior can be restored by
  creating a :mod:`~unittest.TestSuite` subclass that defines a
  ``_removeTestAtIndex`` method that does nothing (see
  :meth:`.TestSuite.__iter__`) (:issue:`11798`).

* :mod:`unittest` now uses :mod:`argparse` for command line parsing.  There are
  certain invalid command forms that used to work that are no longer allowed;
  in theory this should not cause backward compatibility issues since the
  disallowed command forms didn't make any sense and are unlikely to be in use.

* The :func:`re.split`, :func:`re.findall`, and :func:`re.sub` functions, and
  the :meth:`~re.match.group` and :meth:`~re.match.groups` methods of
  :class:``match`` objects now always return a *bytes* object when the string
  to be matched is a :term:`bytes-like object`.  Previously the return type
  matched the input type, so if your code was depending on the return value
  being, say, a ``bytearray``, you will need to change your code.


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

* :c:func:`PyEval_EvalFrameEx`, :c:func:`PyObject_Repr`, and
  :c:func:`PyObject_Str`, along with some other internal C APIs, now include
  a debugging assertion that ensures they are not used in situations where
  they may silently discard a currently active exception. In cases where
  discarding the active exception is expected and desired (for example,
  because it has already been saved locally with :c:func:`PyErr_Fetch` or
  is being deliberately replaced with a different exception), an explicit
  :c:func:`PyErr_Clear` call will be needed to avoid triggering the
  assertion when invoking these operations (directly or indirectly) and
  running against a version of Python that is compiled with assertions
  enabled.

* :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`
  (:issue:`16742`)

* :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.