cpython/Doc/whatsnew/3.3.rst

****************************
  What's New In Python 3.3
****************************

:Author: Raymond Hettinger
:Release: |release|
:Date: |today|

.. Rules for maintenance:

   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.

   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)

   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.

   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.

   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).

   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.

   * It's helpful to add the bug/patch number as a comment:

   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer in :issue:`12345`.)

   This saves the maintainer the effort of going through the Mercurial log
   when researching a change.

This article explains the new features in Python 3.3, compared to 3.2.


.. pep-3118-update:

PEP 3118: New memoryview implementation and buffer protocol documentation
=========================================================================

:issue:`10181` - memoryview bug fixes and features.
  Written by Stefan Krah.

The new memoryview implementation comprehensively fixes all ownership and
lifetime issues of dynamically allocated fields in the Py_buffer struct
that led to multiple crash reports. Additionally, several functions that
crashed or returned incorrect results for non-contiguous or multi-dimensional
input have been fixed.

The memoryview object now has a PEP-3118 compliant getbufferproc()
that checks the consumer's request type. Many new features have been
added, most of them work in full generality for non-contiguous arrays
and arrays with suboffsets.

The documentation has been updated, clearly spelling out responsibilities
for both exporters and consumers. Buffer request flags are grouped into
basic and compound flags. The memory layout of non-contiguous and
multi-dimensional NumPy-style arrays is explained.

Features
--------

* All native single character format specifiers in struct module syntax
  (optionally prefixed with '@') are now supported.

* With some restrictions, the cast() method allows changing of format and
  shape of C-contiguous arrays.

* Multi-dimensional list representations are supported for any array type.

* Multi-dimensional comparisons are supported for any array type.

* All array types are hashable if the exporting object is hashable
  and the view is read-only. (Contributed by Antoine Pitrou in
  :issue:`13411`)


* Arbitrary slicing of any 1-D arrays type is supported. For example, it
  is now possible to reverse a memoryview in O(1) by using a negative step.

API changes
-----------

* The maximum number of dimensions is officially limited to 64.

* The representation of empty shape, strides and suboffsets is now
  an empty tuple instead of None.

* Accessing a memoryview element with format 'B' (unsigned bytes)
  now returns an integer (in accordance with the struct module syntax).
  For returning a bytes object the view must be cast to 'c' first.

* For further changes see `Build and C API Changes`_ and `Porting C code`_ .

.. _pep-393:

PEP 393: Flexible String Representation
=======================================

The Unicode string type is changed to support multiple internal
representations, depending on the character with the largest Unicode ordinal
(1, 2, or 4 bytes) in the represented string.  This allows a space-efficient
representation in common cases, but gives access to full UCS-4 on all
systems.  For compatibility with existing APIs, several representations may
exist in parallel; over time, this compatibility should be phased out.

On the Python side, there should be no downside to this change.

On the C API side, PEP 393 is fully backward compatible.  The legacy API
should remain available at least five years.  Applications using the legacy
API will not fully benefit of the memory reduction, or - worse - may use
a bit more memory, because Python may have to maintain two versions of each
string (in the legacy format and in the new efficient storage).

Functionality
-------------

Changes introduced by :pep:`393` are the following:

* Python now always supports the full range of Unicode codepoints, including
  non-BMP ones (i.e. from ``U+0000`` to ``U+10FFFF``).  The distinction between
  narrow and wide builds no longer exists and Python now behaves like a wide
  build, even under Windows.

* With the death of narrow builds, the problems specific to narrow builds have
  also been fixed, for example:

  * :func:`len` now always returns 1 for non-BMP characters,
    so ``len('\U0010FFFF') == 1``;

  * surrogate pairs are not recombined in string literals,
    so ``'\uDBFF\uDFFF' != '\U0010FFFF'``;

  * indexing or slicing non-BMP characters returns the expected value,
    so ``'\U0010FFFF'[0]`` now returns ``'\U0010FFFF'`` and not ``'\uDBFF'``;

  * all other functions in the standard library now correctly handle
    non-BMP codepoints.

* The value of :data:`sys.maxunicode` is now always ``1114111`` (``0x10FFFF``
  in hexadecimal).  The :c:func:`PyUnicode_GetMax` function still returns
  either ``0xFFFF`` or ``0x10FFFF`` for backward compatibility, and it should
  not be used with the new Unicode API (see :issue:`13054`).

* The :file:`./configure` flag ``--with-wide-unicode`` has been removed.

Performance and resource usage
------------------------------

The storage of Unicode strings now depends on the highest codepoint in the string:

* pure ASCII and Latin1 strings (``U+0000-U+00FF``) use 1 byte per codepoint;

* BMP strings (``U+0000-U+FFFF``) use 2 bytes per codepoint;

* non-BMP strings (``U+10000-U+10FFFF``) use 4 bytes per codepoint.

The net effect is that for most applications, memory usage of string
storage should decrease significantly - especially compared to former
wide unicode builds - as, in many cases, strings will be pure ASCII
even in international contexts (because many strings store non-human
language data, such as XML fragments, HTTP headers, JSON-encoded data,
etc.).  We also hope that it will, for the same reasons, increase CPU
cache efficiency on non-trivial applications. The memory usage of
Python 3.3 is two to three times smaller than Python 3.2, and a little
bit better than Python 2.7, on a Django benchmark (see the PEP for
details).


PEP 3151: Reworking the OS and IO exception hierarchy
=====================================================

:pep:`3151` - Reworking the OS and IO exception hierarchy
 PEP written and implemented by Antoine Pitrou.

The hierarchy of exceptions raised by operating system errors is now both
simplified and finer-grained.

You don't have to worry anymore about choosing the appropriate exception
type between :exc:`OSError`, :exc:`IOError`, :exc:`EnvironmentError`,
:exc:`WindowsError`, :exc:`mmap.error`, :exc:`socket.error` or
:exc:`select.error`.  All these exception types are now only one:
:exc:`OSError`.  The other names are kept as aliases for compatibility
reasons.

Also, it is now easier to catch a specific error condition.  Instead of
inspecting the ``errno`` attribute (or ``args[0]``) for a particular
constant from the :mod:`errno` module, you can catch the adequate
:exc:`OSError` subclass.  The available subclasses are the following:

* :exc:`BlockingIOError`
* :exc:`ChildProcessError`
* :exc:`ConnectionError`
* :exc:`FileExistsError`
* :exc:`FileNotFoundError`
* :exc:`InterruptedError`
* :exc:`IsADirectoryError`
* :exc:`NotADirectoryError`
* :exc:`PermissionError`
* :exc:`ProcessLookupError`
* :exc:`TimeoutError`

And the :exc:`ConnectionError` itself has finer-grained subclasses:

* :exc:`BrokenPipeError`
* :exc:`ConnectionAbortedError`
* :exc:`ConnectionRefusedError`
* :exc:`ConnectionResetError`

Thanks to the new exceptions, common usages of the :mod:`errno` can now be
avoided.  For example, the following code written for Python 3.2::

    from errno import ENOENT, EACCES, EPERM

    try:
        with open("document.txt") as f:
            content = f.read()
    except IOError as err:
        if err.errno == ENOENT:
            print("document.txt file is missing")
        elif err.errno in (EACCES, EPERM):
            print("You are not allowed to read document.txt")
        else:
            raise

can now be written without the :mod:`errno` import and without manual
inspection of exception attributes::

    try:
        with open("document.txt") as f:
            content = f.read()
    except FileNotFoundError:
        print("document.txt file is missing")
    except PermissionError:
        print("You are not allowed to read document.txt")


PEP 380: Syntax for Delegating to a Subgenerator
================================================

:pep:`380` - Syntax for Delegating to a Subgenerator
 PEP written by Greg Ewing.

PEP 380 adds the ``yield from`` expression, allowing a generator to delegate
part of its operations to another generator. This allows a section of code
containing 'yield' to be factored out and placed in another generator.
Additionally, the subgenerator is allowed to return with a value, and the
value is made available to the delegating generator.

While designed primarily for use in delegating to a subgenerator, the ``yield
from`` expression actually allows delegation to arbitrary subiterators.

For simple iterators, ``yield from iterable`` is essentially just a shortened
form of ``for item in iterable: yield item``::

    >>> def g(x):
    ...     yield from range(x, 0, -1)
    ...     yield from range(x)
    ...
    >>> list(g(5))
    [5, 4, 3, 2, 1, 0, 1, 2, 3, 4]

However, unlike an ordinary loop, ``yield from`` allows subgenerators to
receive sent and thrown values directly from the calling scope, and
return a final value to the outer generator::

    >>> def accumulate(start=0):
    ...     tally = start
    ...     while 1:
    ...         next = yield
    ...         if next is None:
    ...             return tally
    ...         tally += next
    ...
    >>> def gather_tallies(tallies, start=0):
    ...     while 1:
    ...         tally = yield from accumulate()
    ...         tallies.append(tally)
    ...
    >>> tallies = []
    >>> acc = gather_tallies(tallies)
    >>> next(acc) # Ensure the accumulator is ready to accept values
    >>> for i in range(10):
    ...     acc.send(i)
    ...
    >>> acc.send(None) # Finish the first tally
    >>> for i in range(5):
    ...     acc.send(i)
    ...
    >>> acc.send(None) # Finish the second tally
    >>> tallies
    [45, 10]

The main principle driving this change is to allow even generators that are
designed to be used with the ``send`` and ``throw`` methods to be split into
multiple subgenerators as easily as a single large function can be split into
multiple subfunctions.

(Implementation by Greg Ewing, integrated into 3.3 by Renaud Blanch, Ryan
Kelly and Nick Coghlan, documentation by Zbigniew Jędrzejewski-Szmek and
Nick Coghlan)


PEP 409: Suppressing exception context
======================================

:pep:`409` - Suppressing exception context
 PEP written by Ethan Furman, implemented by Ethan Furman and Nick Coghlan.

PEP 409 introduces new syntax that allows the display of the chained
exception context to be disabled. This allows cleaner error messages in
applications that convert between exception types::

    >>> class D:
    ...     def __init__(self, extra):
    ...         self._extra_attributes = extra
    ...     def __getattr__(self, attr):
    ...         try:
    ...             return self._extra_attributes[attr]
    ...         except KeyError:
    ...             raise AttributeError(attr) from None
    ...
    >>> D({}).x
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 8, in __getattr__
    AttributeError: x

Without the ``from None`` suffix to suppress the cause, the original
exception would be displayed by default::

    >>> class C:
    ...     def __init__(self, extra):
    ...         self._extra_attributes = extra
    ...     def __getattr__(self, attr):
    ...         try:
    ...             return self._extra_attributes[attr]
    ...         except KeyError:
    ...             raise AttributeError(attr)
    ...
    >>> C({}).x
    Traceback (most recent call last):
      File "<stdin>", line 6, in __getattr__
    KeyError: 'x'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 8, in __getattr__
    AttributeError: x

No debugging capability is lost, as the original exception context remains
available if needed (for example, if an intervening library has incorrectly
suppressed valuable underlying details)::

    >>> try:
    ...     D({}).x
    ... except AttributeError as exc:
    ...     print(repr(exc.__context__))
    ...
    KeyError('x',)


PEP 414: Explicit Unicode literals
======================================

:pep:`414` - Explicit Unicode literals
 PEP written by Armin Ronacher.

To ease the transition from Python 2 for Unicode aware Python applications
that make heavy use of Unicode literals, Python 3.3 once again supports the
"``u``" prefix for string literals. This prefix has no semantic significance
in Python 3, it is provided solely to reduce the number of purely mechanical
changes in migrating to Python 3, making it easier for developers to focus on
the more significant semantic changes (such as the stricter default
separation of binary and text data).


PEP 3155: Qualified name for classes and functions
==================================================

:pep:`3155` - Qualified name for classes and functions
 PEP written and implemented by Antoine Pitrou.

Functions and class objects have a new ``__qualname__`` attribute representing
the "path" from the module top-level to their definition.  For global functions
and classes, this is the same as ``__name__``.  For other functions and classes,
it provides better information about where they were actually defined, and
how they might be accessible from the global scope.

Example with (non-bound) methods::

   >>> class C:
   ...     def meth(self):
   ...         pass
   >>> C.meth.__name__
   'meth'
   >>> C.meth.__qualname__
   'C.meth'

Example with nested classes::

   >>> class C:
   ...     class D:
   ...         def meth(self):
   ...             pass
   ...
   >>> C.D.__name__
   'D'
   >>> C.D.__qualname__
   'C.D'
   >>> C.D.meth.__name__
   'meth'
   >>> C.D.meth.__qualname__
   'C.D.meth'

Example with nested functions::

   >>> def outer():
   ...     def inner():
   ...         pass
   ...     return inner
   ...
   >>> outer().__name__
   'inner'
   >>> outer().__qualname__
   'outer.<locals>.inner'

The string representation of those objects is also changed to include the
new, more precise information::

   >>> str(C.D)
   "<class '__main__.C.D'>"
   >>> str(C.D.meth)
   '<function C.D.meth at 0x7f46b9fe31e0>'


Using importlib as the Implementation of Import
===============================================
:issue:`2377` - Replace __import__ w/ importlib.__import__
:issue:`13959` - Re-implement parts of :mod:`imp` in pure Python
:issue:`14605` - Make import machinery explicit
:issue:`14646` - Require loaders set __loader__ and __package__

(Written by Brett Cannon)

The :func:`__import__` function is now powered by :func:`importlib.__import__`.
This work leads to the completion of "phase 2" of :pep:`302`. There are
multiple benefits to this change. First, it has allowed for more of the
machinery powering import to be exposed instead of being implicit and hidden
within the C code. It also provides a single implementation for all Python VMs
supporting Python 3.3 to use, helping to end any VM-specific deviations in
import semantics. And finally it eases the maintenance of import, allowing for
future growth to occur.

For the common user, this change should result in no visible change in
semantics. Any possible changes required in one's code to handle this change
should read the `Porting Python code`_ section of this document to see what
needs to be changed, but it will only affect those that currently manipulate
import or try calling it programmatically.

New APIs
--------
One of the large benefits of this work is the exposure of what goes into
making the import statement work. That means the various importers that were
once implicit are now fully exposed as part of the :mod:`importlib` package.

In terms of finders, * :class:`importlib.machinery.FileFinder` exposes the
mechanism used to search for source and bytecode files of a module. Previously
this class was an implicit member of :attr:`sys.path_hooks`.

For loaders, the new abstract base class :class:`importlib.abc.FileLoader` helps
write a loader that uses the file system as the storage mechanism for a module's
code. The loader for source files
(:class:`importlib.machinery.SourceFileLoader`), sourceless bytecode files
(:class:`importlib.machinery.SourcelessFileLoader`), and extension modules
(:class:`importlib.machinery.ExtensionFileLoader`) are now available for
direct use.

:exc:`ImportError` now has ``name`` and ``path`` attributes which are set when
there is relevant data to provide. The message for failed imports will also
provide the full name of the module now instead of just the tail end of the
module's name.

The :func:`importlib.invalidate_caches` function will now call the method with
the same name on all finders cached in :attr:`sys.path_importer_cache` to help
clean up any stored state as necessary.

Visible Changes
---------------
[For potential required changes to code, see the `Porting Python code`_
section]

Beyond the expanse of what :mod:`importlib` now exposes, there are other
visible changes to import. The biggest is that :attr:`sys.meta_path` and
:attr:`sys.path_hooks` now store all of the finders used by import explicitly.
Previously the finders were implicit and hidden within the C code of import
instead of being directly exposed. This means that one can now easily remove or
change the order of the various finders to fit one's needs.

Another change is that all modules have a ``__loader__`` attribute, storing the
loader used to create the module. :pep:`302` has been updated to make this
attribute mandatory for loaders to implement, so in the future once 3rd-party
loaders have been updated people will be able to rely on the existence of the
attribute. Until such time, though, import is setting the module post-load.

Loaders are also now expected to set the ``__package__`` attribute from
:pep:`366`. Once again, import itself is already setting this on all loaders
from :mod:`importlib` and import itself is setting the attribute post-load.

``None`` is now inserted into :attr:`sys.path_importer_cache` when no finder
can be found on :attr:`sys.path_hooks`. Since :class:`imp.NullImporter` is not
directly exposed on :attr:`sys.path_hooks` it could no longer be relied upon to
always be available to use as a value representing no finder found.

All other changes relate to semantic changes which should be taken into
consideration when updating code for Python 3.3, and thus should be read about
in the `Porting Python code`_ section of this document.


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

Some smaller changes made to the core Python language are:

* Added support for Unicode name aliases and named sequences.
  Both :func:`unicodedata.lookup()` and ``'\N{...}'`` now resolve name aliases,
  and :func:`unicodedata.lookup()` resolves named sequences too.

  (Contributed by Ezio Melotti in :issue:`12753`)

* Equality comparisons on :func:`range` objects now return a result reflecting
  the equality of the underlying sequences generated by those range objects.

  (:issue:`13201`)

* The ``count()``, ``find()``, ``rfind()``, ``index()`` and ``rindex()``
  methods of :class:`bytes` and :class:`bytearray` objects now accept an
  integer between 0 and 255 as their first argument.

  (:issue:`12170`)

* A dict lookup now raises a :exc:`RuntimeError` if the dict is modified during
  the lookup. If you implement your own comparison function for objects used
  as dict keys and the dict is shared by multiple threads, access to the dict
  should be protected by a lock.

  (:issue:`14205`)

* New methods have been added to :class:`list` and :class:`bytearray`:
  ``copy()`` and ``clear()``.

  (:issue:`10516`)

.. XXX mention new error messages for passing wrong number of arguments to functions

New and Improved Modules
========================

abc
---

Improved support for abstract base classes containing descriptors composed with
abstract methods. The recommended approach to declaring abstract descriptors is
now to provide :attr:`__isabstractmethod__` as a dynamically updated
property. The built-in descriptors have been updated accordingly.

  * :class:`abc.abstractproperty` has been deprecated, use :class:`property`
    with :func:`abc.abstractmethod` instead.
  * :class:`abc.abstractclassmethod` has been deprecated, use
    :class:`classmethod` with :func:`abc.abstractmethod` instead.
  * :class:`abc.abstractstaticmethod` has been deprecated, use
    :class:`staticmethod` with :func:`abc.abstractmethod` instead.

(Contributed by Darren Dale in :issue:`11610`)

array
-----

The :mod:`array` module supports the :c:type:`long long` type using ``q`` and
``Q`` type codes.

(Contributed by Oren Tirosh and Hirokazu Yamamoto in :issue:`1172711`)


bz2
---

The :mod:`bz2` module has been rewritten from scratch. In the process, several
new features have been added:

* :class:`bz2.BZ2File` can now read from and write to arbitrary file-like
  objects, by means of its constructor's *fileobj* argument.

  (Contributed by Nadeem Vawda in :issue:`5863`)

* :class:`bz2.BZ2File` and :func:`bz2.decompress` can now decompress
  multi-stream inputs (such as those produced by the :program:`pbzip2` tool).
  :class:`bz2.BZ2File` can now also be used to create this type of file, using
  the ``'a'`` (append) mode.

  (Contributed by Nir Aides in :issue:`1625`)

* :class:`bz2.BZ2File` now implements all of the :class:`io.BufferedIOBase` API,
  except for the :meth:`detach` and :meth:`truncate` methods.


codecs
------

The :mod:`~encodings.mbcs` codec has been rewritten to handle correctly
``replace`` and ``ignore`` error handlers on all Windows versions.  The
:mod:`~encodings.mbcs` codec now supports all error handlers, instead of only
``replace`` to encode and ``ignore`` to decode.

A new Windows-only codec has been added: ``cp65001`` (:issue:`13216`). It is the
Windows code page 65001 (Windows UTF-8, ``CP_UTF8``).  For example, it is used
by ``sys.stdout`` if the console output code page is set to cp65001 (e.g., using
``chcp 65001`` command).

Multibyte CJK decoders now resynchronize faster.  They only ignore the first
byte of an invalid byte sequence. For example, ``b'\xff\n'.decode('gb2312',
'replace')`` now returns a ``\n`` after the replacement character.

(:issue:`12016`)

Incremental CJK codec encoders are no longer reset at each call to their
encode() methods. For example::

    $ ./python -q
    >>> import codecs
    >>> encoder = codecs.getincrementalencoder('hz')('strict')
    >>> b''.join(encoder.encode(x) for x in '\u52ff\u65bd\u65bc\u4eba\u3002 Bye.')
    b'~{NpJ)l6HK!#~} Bye.'

This example gives ``b'~{Np~}~{J)~}~{l6~}~{HK~}~{!#~} Bye.'`` with older Python
versions.

(:issue:`12100`)

The ``unicode_internal`` codec has been deprecated.


collections
-----------

Addition of a new :class:`~collections.ChainMap` class to allow treating a
number of mappings as a single unit.

(Written by Raymond Hettinger for :issue:`11089`, made public in
:issue:`11297`)

The abstract base classes have been moved in a new :mod:`collections.abc`
module, to better differentiate between the abstract and the concrete
collections classes.  Aliases for ABCs are still present in the
:mod:`collections` module to preserve existing imports.

(:issue:`11085`)

.. XXX addition of __slots__ to ABCs not recorded here: internal detail


crypt
-----

Addition of salt and modular crypt format and the :func:`~crypt.mksalt`
function to the :mod:`crypt` module.

(:issue:`10924`)

curses
------

 * If the :mod:`curses` module is linked to the ncursesw library, use Unicode
   functions when Unicode strings or characters are passed (e.g.
   :c:func:`waddwstr`), and bytes functions otherwise (e.g. :c:func:`waddstr`).
 * Use the locale encoding instead of ``utf-8`` to encode Unicode strings.
 * :class:`curses.window` has a new :attr:`curses.window.encoding` attribute.
 * The :class:`curses.window` class has a new :meth:`~curses.window.get_wch`
   method to get a wide character
 * The :mod:`curses` module has a new :meth:`~curses.unget_wch` function to
   push a wide character so the next :meth:`~curses.window.get_wch` will return
   it

(Contributed by Iñigo Serna in :issue:`6755`)

decimal
-------

:issue:`7652` - integrate fast native decimal arithmetic.
   C-module and libmpdec written by Stefan Krah.

The new C version of the decimal module integrates the high speed libmpdec
library for arbitrary precision correctly-rounded decimal floating point
arithmetic. libmpdec conforms to IBM's General Decimal Arithmetic Specification.

Performance gains range from 10x for database applications to 100x for
numerically intensive applications. These numbers are expected gains
for standard precisions used in decimal floating point arithmetic. Since
the precision is user configurable, the exact figures may vary. For example,
in integer bignum arithmetic the differences can be significantly higher.

The following table is meant as an illustration. Benchmarks are available
at http://www.bytereef.org/mpdecimal/quickstart.html.

   +---------+-------------+--------------+-------------+
   |         |  decimal.py |   _decimal   |   speedup   |
   +=========+=============+==============+=============+
   |   pi    |    38.89s   |    0.38s     |    100x     |
   +---------+-------------+--------------+-------------+
   | telco   |   172.19s   |    5.68s     |     30x     |
   +---------+-------------+--------------+-------------+
   | psycopg |     3.57s   |    0.29s     |     12x     |
   +---------+-------------+--------------+-------------+

Features
~~~~~~~~

* The :exc:`~decimal.FloatOperation` signal optionally enables stricter
  semantics for mixing floats and Decimals.

* If Python is compiled without threads, the C version automatically
  disables the expensive thread local context machinery. In this case,
  the variable :data:`~decimal.HAVE_THREADS` is set to False.

API changes
~~~~~~~~~~~

* The C module has the following context limits, depending on the machine
  architecture:

   +-------------------+---------------------+------------------------------+
   |                   |       32-bit        |            64-bit            |
   +===================+=====================+==============================+
   | :const:`MAX_PREC` | :const:`425000000`  | :const:`999999999999999999`  |
   +-------------------+---------------------+------------------------------+
   | :const:`MAX_EMAX` | :const:`425000000`  | :const:`999999999999999999`  |
   +-------------------+---------------------+------------------------------+
   | :const:`MIN_EMIN` | :const:`-425000000` | :const:`-999999999999999999` |
   +-------------------+---------------------+------------------------------+

* In the context templates (:class:`~decimal.DefaultContext`,
  :class:`~decimal.BasicContext` and :class:`~decimal.ExtendedContext`)
  the magnitude of :attr:`~decimal.Context.Emax` and
  :attr:`~decimal.Context.Emin` has changed to :const:`999999`.

* The :class:`~decimal.Decimal` constructor in decimal.py does not observe
  the context limits and converts values with arbitrary exponents or precision
  exactly. Since the C version has internal limits, the following scheme is
  used: If possible, values are converted exactly, otherwise
  :exc:`~decimal.InvalidOperation` is raised and the result is NaN. In the
  latter case it is always possible to use :meth:`~decimal.Context.create_decimal`
  in order to obtain a rounded or inexact value.


* The power function in decimal.py is always correctly-rounded. In the
  C version, it is defined in terms of the correctly-rounded
  :meth:`~decimal.Decimal.exp` and :meth:`~decimal.Decimal.ln` functions,
  but the final result is only "almost always correctly rounded".


* In the C version, the context dictionary containing the signals is a
  :class:`~collections.abc.MutableMapping`.  For speed reasons,
  :attr:`~decimal.Context.flags` and :attr:`~decimal.Context.traps` always
  refer to the same :class:`~collections.abc.MutableMapping` that the context
  was initialized with. If a new signal dictionary is assigned,
  :attr:`~decimal.Context.flags` and :attr:`~decimal.Context.traps`
  are updated with the new values, but they do not reference the RHS
  dictionary.


* Pickling a :class:`~decimal.Context` produces a different output in order
  to have a common interchange format for the Python and C versions.


* The order of arguments in the :class:`~decimal.Context` constructor has been
  changed to match the order displayed by :func:`repr`.


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

New module: :mod:`faulthandler`.

 * :envvar:`PYTHONFAULTHANDLER`
 * :option:`-X` ``faulthandler``

ftplib
------

The :class:`~ftplib.FTP_TLS` class now provides a new
:func:`~ftplib.FTP_TLS.ccc` function to revert control channel back to
plaintext.  This can be useful to take advantage of firewalls that know how to
handle NAT with non-secure FTP without opening fixed ports.

(Contributed by Giampaolo Rodolà in :issue:`12139`)


imaplib
-------

The :class:`~imaplib.IMAP4_SSL` constructor now accepts an SSLContext
parameter to control parameters of the secure channel.

(Contributed by Sijin Joseph in :issue:`8808`)


io
--

The :func:`~io.open` function has a new ``'x'`` mode that can be used to
exclusively create a new file, and raise a :exc:`FileExistsError` if the file
already exists. It is based on the C11 'x' mode to fopen().

(Contributed by David Townshend in :issue:`12760`)


lzma
----

The newly-added :mod:`lzma` module provides data compression and decompression
using the LZMA algorithm, including support for the ``.xz`` and ``.lzma``
file formats.

(Contributed by Nadeem Vawda and Per Øyvind Karlsen in :issue:`6715`)


math
----

The :mod:`math` module has a new function:

  * :func:`~math.log2`: return the base-2 logarithm of *x*
    (Written by Mark Dickinson in :issue:`11888`).


nntplib
-------

The :class:`nntplib.NNTP` class now supports the context manager protocol to
unconditionally consume :exc:`socket.error` exceptions and to close the NNTP
connection when done::

  >>> from nntplib import NNTP
  >>> with NNTP('news.gmane.org') as n:
  ...     n.group('gmane.comp.python.committers')
  ...
  ('211 1755 1 1755 gmane.comp.python.committers', 1755, 1, 1755, 'gmane.comp.python.committers')
  >>>

(Contributed by Giampaolo Rodolà in :issue:`9795`)


os
--

* The :mod:`os` module has a new :func:`~os.pipe2` function that makes it
  possible to create a pipe with :data:`~os.O_CLOEXEC` or
  :data:`~os.O_NONBLOCK` flags set atomically. This is especially useful to
  avoid race conditions in multi-threaded programs.

* The :mod:`os` module has a new :func:`~os.sendfile` function which provides
  an efficent "zero-copy" way for copying data from one file (or socket)
  descriptor to another. The phrase "zero-copy" refers to the fact that all of
  the copying of data between the two descriptors is done entirely by the
  kernel, with no copying of data into userspace buffers. :func:`~os.sendfile`
  can be used to efficiently copy data from a file on disk to a network socket,
  e.g. for downloading a file.

  (Patch submitted by Ross Lagerwall and Giampaolo Rodolà in :issue:`10882`.)

* The :mod:`os` module has two new functions: :func:`~os.getpriority` and
  :func:`~os.setpriority`. They can be used to get or set process
  niceness/priority in a fashion similar to :func:`os.nice` but extended to all
  processes instead of just the current one.

  (Patch submitted by Giampaolo Rodolà in :issue:`10784`.)

* The :mod:`os` module has a new :func:`~os.fwalk` function similar to
  :func:`~os.walk` except that it also yields file descriptors referring to the
  directories visited. This is especially useful to avoid symlink races.

* "at" functions (:issue:`4761`):

  * :func:`~os.faccessat`
  * :func:`~os.fchmodat`
  * :func:`~os.fchownat`
  * :func:`~os.fstatat`
  * :func:`~os.futimesat`
  * :func:`~os.linkat`
  * :func:`~os.mkdirat`
  * :func:`~os.mkfifoat`
  * :func:`~os.mknodat`
  * :func:`~os.openat`
  * :func:`~os.readlinkat`
  * :func:`~os.renameat`
  * :func:`~os.symlinkat`
  * :func:`~os.unlinkat`
  * :func:`~os.utimensat`

* extended attributes (:issue:`12720`):

  * :func:`~os.fgetxattr`
  * :func:`~os.flistxattr`
  * :func:`~os.fremovexattr`
  * :func:`~os.fsetxattr`
  * :func:`~os.getxattr`
  * :func:`~os.lgetxattr`
  * :func:`~os.listxattr`
  * :func:`~os.llistxattr`
  * :func:`~os.lremovexattr`
  * :func:`~os.lsetxattr`
  * :func:`~os.removexattr`
  * :func:`~os.setxattr`

* Scheduler functions (:issue:`12655`):

  * :func:`~os.sched_get_priority_max`
  * :func:`~os.sched_get_priority_min`
  * :func:`~os.sched_getaffinity`
  * :func:`~os.sched_getparam`
  * :func:`~os.sched_getscheduler`
  * :func:`~os.sched_rr_get_interval`
  * :func:`~os.sched_setaffinity`
  * :func:`~os.sched_setparam`
  * :func:`~os.sched_setscheduler`
  * :func:`~os.sched_yield`

* Add some extra posix functions to the os module (:issue:`10812`):

  * :func:`~os.fexecve`
  * :func:`~os.futimens`
  * :func:`~os.futimes`
  * :func:`~os.lockf`
  * :func:`~os.lutimes`
  * :func:`~os.posix_fadvise`
  * :func:`~os.posix_fallocate`
  * :func:`~os.pread`
  * :func:`~os.pwrite`
  * :func:`~os.readv`
  * :func:`~os.sync`
  * :func:`~os.truncate`
  * :func:`~os.waitid`
  * :func:`~os.writev`

* Other new functions:

  * :func:`~os.flistdir` (:issue:`10755`)
  * :func:`~os.getgrouplist` (:issue:`9344`)


packaging
---------

:mod:`distutils` has undergone additions and refactoring under a new name,
:mod:`packaging`, to allow developers to make far-reaching changes without
being constrained by backward compatibility.
:mod:`distutils` is still provided in the standard library, but users are
encouraged to transition to :mod:`packaging`.  For older versions of Python, a
backport compatible with Python 2.5 and newer and 3.2 is available on PyPI
under the name `distutils2 <http://pypi.python.org/pypi/Distutils2>`_.

.. TODO add examples and howto to the packaging docs and link to them


pdb
---

* Tab-completion is now available not only for command names, but also their
  arguments.  For example, for the ``break`` command, function and file names
  are completed.  (Contributed by Georg Brandl in :issue:`14210`)


pydoc
-----

The Tk GUI and the :func:`~pydoc.serve` function have been removed from the
:mod:`pydoc` module: ``pydoc -g`` and :func:`~pydoc.serve` have been deprecated
in Python 3.2.


sched
-----

* :meth:`~sched.scheduler.run` now accepts a *blocking* parameter which when
  set to False makes the method execute the scheduled events due to expire
  soonest (if any) and then return immediately.
  This is useful in case you want to use the :class:`~sched.scheduler` in
  non-blocking applications.  (Contributed by Giampaolo Rodolà in :issue:`13449`)

* :class:`~sched.scheduler` class can now be safely used in multi-threaded
  environments.  (Contributed by Josiah Carlson and Giampaolo Rodolà in
  :issue:`8684`)

* *timefunc* and *delayfunct* parameters of :class:`~sched.scheduler` class
  constructor are now optional and defaults to :func:`time.time` and
  :func:`time.sleep` respectively.  (Contributed by Chris Clark in
  :issue:`13245`)

* :meth:`~sched.scheduler.enter` and :meth:`~sched.scheduler.enterabs`
  *argument* parameter is now optional.  (Contributed by Chris Clark in
  :issue:`13245`)

* :meth:`~sched.scheduler.enter` and :meth:`~sched.scheduler.enterabs`
  now accept a *kwargs* parameter.  (Contributed by Chris Clark in
  :issue:`13245`)


shutil
------

* The :mod:`shutil` module has these new fuctions:

  * :func:`~shutil.disk_usage`: provides total, used and free disk space
    statistics. (Contributed by Giampaolo Rodolà in :issue:`12442`)
  * :func:`~shutil.chown`: allows one to change user and/or group of the given
    path also specifying the user/group names and not only their numeric
    ids. (Contributed by Sandro Tosi in :issue:`12191`)


signal
------

* The :mod:`signal` module has new functions:

  * :func:`~signal.pthread_sigmask`: fetch and/or change the signal mask of the
    calling thread (Contributed by Jean-Paul Calderone in :issue:`8407`) ;
  * :func:`~signal.pthread_kill`: send a signal to a thread ;
  * :func:`~signal.sigpending`: examine pending functions ;
  * :func:`~signal.sigwait`: wait a signal.
  * :func:`~signal.sigwaitinfo`: wait for a signal, returning detailed
    information about it.
  * :func:`~signal.sigtimedwait`: like :func:`~signal.sigwaitinfo` but with a
    timeout.

* The signal handler writes the signal number as a single byte instead of
  a nul byte into the wakeup file descriptor. So it is possible to wait more
  than one signal and know which signals were raised.

* :func:`signal.signal` and :func:`signal.siginterrupt` raise an OSError,
  instead of a RuntimeError: OSError has an errno attribute.

smtplib
-------

The :class:`~smtplib.SMTP_SSL` constructor and the :meth:`~smtplib.SMTP.starttls`
method now accept an SSLContext parameter to control parameters of the secure
channel.

(Contributed by Kasun Herath in :issue:`8809`)


socket
------

* The :class:`~socket.socket` class now exposes additional methods to process
  ancillary data when supported by the underlying platform:

  * :func:`~socket.socket.sendmsg`
  * :func:`~socket.socket.recvmsg`
  * :func:`~socket.socket.recvmsg_into`

  (Contributed by David Watson in :issue:`6560`, based on an earlier patch by
  Heiko Wundram)

* The :class:`~socket.socket` class now supports the PF_CAN protocol family
  (http://en.wikipedia.org/wiki/Socketcan), on Linux
  (http://lwn.net/Articles/253425).

  (Contributed by Matthias Fuchs, updated by Tiago Gonçalves in :issue:`10141`)

* The :class:`~socket.socket` class now supports the PF_RDS protocol family
  (http://en.wikipedia.org/wiki/Reliable_Datagram_Sockets and
  http://oss.oracle.com/projects/rds/).


ssl
---

* The :mod:`ssl` module has two new random generation functions:

  * :func:`~ssl.RAND_bytes`: generate cryptographically strong
    pseudo-random bytes.
  * :func:`~ssl.RAND_pseudo_bytes`: generate pseudo-random bytes.

  (Contributed by Victor Stinner in :issue:`12049`)

* The :mod:`ssl` module now exposes a finer-grained exception hierarchy
  in order to make it easier to inspect the various kinds of errors.

  (Contributed by Antoine Pitrou in :issue:`11183`)

* :meth:`~ssl.SSLContext.load_cert_chain` now accepts a *password* argument
  to be used if the private key is encrypted.

  (Contributed by Adam Simpkins in :issue:`12803`)

* Diffie-Hellman key exchange, both regular and Elliptic Curve-based, is
  now supported through the :meth:`~ssl.SSLContext.load_dh_params` and
  :meth:`~ssl.SSLContext.set_ecdh_curve` methods.

  (Contributed by Antoine Pitrou in :issue:`13626` and :issue:`13627`)

* SSL sockets have a new :meth:`~ssl.SSLSocket.get_channel_binding` method
  allowing the implementation of certain authentication mechanisms such as
  SCRAM-SHA-1-PLUS.

  (Contributed by Jacek Konieczny in :issue:`12551`)

* You can query the SSL compression algorithm used by an SSL socket, thanks
  to its new :meth:`~ssl.SSLSocket.compression` method.

  (Contributed by Antoine Pitrou in :issue:`13634`)


sys
---

* The :mod:`sys` module has a new :data:`~sys.thread_info` :term:`struct
  sequence` holding informations about the thread implementation.

  (:issue:`11223`)


time
----

The :pep:`418` added new functions to the :mod:`time` module:

* :func:`~time.get_clock_info`: Get information on a clock.
* :func:`~time.monotonic`: Monotonic clock (cannot go backward), not affected
  by system clock updates.
* :func:`~time.perf_counter`: Performance counter with the highest available
  resolution to measure a short duration.
* :func:`~time.process_time`: Sum of the system and user CPU time of the
  current process.

Other new functions:

* :func:`~time.clock_getres`, :func:`~time.clock_gettime` and
  :func:`~time.clock_settime` functions with ``CLOCK_xxx`` constants.
  (Contributed by Victor Stinner in :issue:`10278`)


types
-----

Add a new :class:`types.MappingProxyType` class: Read-only proxy of a mapping.
(:issue:`14386`)


urllib
------

The :class:`~urllib.request.Request` class, now accepts a *method* argument
used by :meth:`~urllib.request.Request.get_method` to determine what HTTP method
should be used.  For example, this will send a ``'HEAD'`` request::

   >>> urlopen(Request('http://www.python.org', method='HEAD'))

(:issue:`1673007`)


webbrowser
----------

The :mod:`webbrowser` module supports more browsers: Google Chrome (named
:program:`chrome`, :program:`chromium`, :program:`chrome-browser` or
:program:`chromium-browser` depending on the version and operating system) as
well as the the generic launchers :program:`xdg-open` from the FreeDesktop.org
project and :program:`gvfs-open` which is the default URI handler for GNOME 3.

(:issue:`13620` and :issue:`14493`)


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

Major performance enhancements have been added:

* Thanks to :pep:`393`, some operations on Unicode strings have been optimized:

  * the memory footprint is divided by 2 to 4 depending on the text
  * encode an ASCII string to UTF-8 doesn't need to encode characters anymore,
    the UTF-8 representation is shared with the ASCII representation
  * the UTF-8 encoder has been optimized
  * repeating a single ASCII letter and getting a substring of a ASCII strings
    is 4 times faster


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

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

* New :pep:`3118` related function:

  * :c:func:`PyMemoryView_FromMemory`

* :pep:`393` added new Unicode types, macros and functions:

  * High-level API:

    * :c:func:`PyUnicode_CopyCharacters`
    * :c:func:`PyUnicode_FindChar`
    * :c:func:`PyUnicode_GetLength`, :c:macro:`PyUnicode_GET_LENGTH`
    * :c:func:`PyUnicode_New`
    * :c:func:`PyUnicode_Substring`
    * :c:func:`PyUnicode_ReadChar`, :c:func:`PyUnicode_WriteChar`

  * Low-level API:

    * :c:type:`Py_UCS1`, :c:type:`Py_UCS2`, :c:type:`Py_UCS4` types
    * :c:type:`PyASCIIObject` and :c:type:`PyCompactUnicodeObject` structures
    * :c:macro:`PyUnicode_READY`
    * :c:func:`PyUnicode_FromKindAndData`
    * :c:func:`PyUnicode_AsUCS4`, :c:func:`PyUnicode_AsUCS4Copy`
    * :c:macro:`PyUnicode_DATA`, :c:macro:`PyUnicode_1BYTE_DATA`,
      :c:macro:`PyUnicode_2BYTE_DATA`, :c:macro:`PyUnicode_4BYTE_DATA`
    * :c:macro:`PyUnicode_KIND` with :c:type:`PyUnicode_Kind` enum:
      :c:data:`PyUnicode_WCHAR_KIND`, :c:data:`PyUnicode_1BYTE_KIND`,
      :c:data:`PyUnicode_2BYTE_KIND`, :c:data:`PyUnicode_4BYTE_KIND`
    * :c:macro:`PyUnicode_READ`, :c:macro:`PyUnicode_READ_CHAR`, :c:macro:`PyUnicode_WRITE`
    * :c:macro:`PyUnicode_MAX_CHAR_VALUE`



Deprecated
==========

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

OS/2 and VMS are no longer supported due to the lack of a maintainer.

Windows 2000 and Windows platforms which set ``COMSPEC`` to ``command.com``
are no longer supported due to maintenance burden.


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

* The :mod:`distutils` module has been deprecated.  Use the new
  :mod:`packaging` module instead.
* The ``unicode_internal`` codec has been deprecated because of the
  :pep:`393`, use UTF-8, UTF-16 (``utf-16-le`` or ``utf-16-be``), or UTF-32
  (``utf-32-le`` or ``utf-32-be``)
* :meth:`ftplib.FTP.nlst` and :meth:`ftplib.FTP.dir`: use
  :meth:`ftplib.FTP.mlsd`
* :func:`platform.popen`: use the :mod:`subprocess` module. Check especially
  the :ref:`subprocess-replacements` section.
* :issue:`13374`: The Windows bytes API has been deprecated in the :mod:`os`
  module. Use Unicode filenames, instead of bytes filenames, to not depend on
  the ANSI code page anymore and to support any filename.
* :issue:`13988`: The :mod:`xml.etree.cElementTree` module is deprecated.  The
  accelerator is used automatically whenever available.
* The behaviour of :func:`time.clock` depends on the platform: use the new
  :func:`time.perf_counter` or :func:`time.process_time` function instead,
  depending on your requirements, to have a well defined behaviour.


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

The :c:type:`Py_UNICODE` has been deprecated by :pep:`393` and will be
removed in Python 4. All functions using this type are deprecated:

Unicode functions and methods using :c:type:`Py_UNICODE` and
:c:type:`Py_UNICODE*` types:

 * :c:macro:`PyUnicode_FromUnicode`: use :c:func:`PyUnicode_FromWideChar` or
   :c:func:`PyUnicode_FromKindAndData`
 * :c:macro:`PyUnicode_AS_UNICODE`, :c:func:`PyUnicode_AsUnicode`,
   :c:func:`PyUnicode_AsUnicodeAndSize`: use :c:func:`PyUnicode_AsWideCharString`
 * :c:macro:`PyUnicode_AS_DATA`: use :c:macro:`PyUnicode_DATA` with
   :c:macro:`PyUnicode_READ` and :c:macro:`PyUnicode_WRITE`
 * :c:macro:`PyUnicode_GET_SIZE`, :c:func:`PyUnicode_GetSize`: use
   :c:macro:`PyUnicode_GET_LENGTH` or :c:func:`PyUnicode_GetLength`
 * :c:macro:`PyUnicode_GET_DATA_SIZE`: use
   ``PyUnicode_GET_LENGTH(str) * PyUnicode_KIND(str)`` (only work on ready
   strings)
 * :c:func:`PyUnicode_AsUnicodeCopy`: use :c:func:`PyUnicode_AsUCS4Copy` or
   :c:func:`PyUnicode_AsWideCharString`
 * :c:func:`PyUnicode_GetMax`


Functions and macros manipulating Py_UNICODE* strings:

 * :c:macro:`Py_UNICODE_strlen`: use :c:func:`PyUnicode_GetLength` or
   :c:macro:`PyUnicode_GET_LENGTH`
 * :c:macro:`Py_UNICODE_strcat`: use :c:func:`PyUnicode_CopyCharacters` or
   :c:func:`PyUnicode_FromFormat`
 * :c:macro:`Py_UNICODE_strcpy`, :c:macro:`Py_UNICODE_strncpy`,
   :c:macro:`Py_UNICODE_COPY`: use :c:func:`PyUnicode_CopyCharacters` or
   :c:func:`PyUnicode_Substring`
 * :c:macro:`Py_UNICODE_strcmp`: use :c:func:`PyUnicode_Compare`
 * :c:macro:`Py_UNICODE_strncmp`: use :c:func:`PyUnicode_Tailmatch`
 * :c:macro:`Py_UNICODE_strchr`, :c:macro:`Py_UNICODE_strrchr`: use
   :c:func:`PyUnicode_FindChar`
 * :c:macro:`Py_UNICODE_FILL`: use :c:func:`PyUnicode_Fill`
 * :c:macro:`Py_UNICODE_MATCH`

Encoders:

 * :c:func:`PyUnicode_Encode`: use :c:func:`PyUnicode_AsEncodedObject`
 * :c:func:`PyUnicode_EncodeUTF7`
 * :c:func:`PyUnicode_EncodeUTF8`: use :c:func:`PyUnicode_AsUTF8` or
   :c:func:`PyUnicode_AsUTF8String`
 * :c:func:`PyUnicode_EncodeUTF32`
 * :c:func:`PyUnicode_EncodeUTF16`
 * :c:func:`PyUnicode_EncodeUnicodeEscape:` use
   :c:func:`PyUnicode_AsUnicodeEscapeString`
 * :c:func:`PyUnicode_EncodeRawUnicodeEscape:` use
   :c:func:`PyUnicode_AsRawUnicodeEscapeString`
 * :c:func:`PyUnicode_EncodeLatin1`: use :c:func:`PyUnicode_AsLatin1String`
 * :c:func:`PyUnicode_EncodeASCII`: use :c:func:`PyUnicode_AsASCIIString`
 * :c:func:`PyUnicode_EncodeCharmap`
 * :c:func:`PyUnicode_TranslateCharmap`
 * :c:func:`PyUnicode_EncodeMBCS`: use :c:func:`PyUnicode_AsMBCSString` or
   :c:func:`PyUnicode_EncodeCodePage` (with ``CP_ACP`` code_page)
 * :c:func:`PyUnicode_EncodeDecimal`,
   :c:func:`PyUnicode_TransformDecimalToASCII`


Porting to Python 3.3
=====================

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

Porting Python code
-------------------

.. XXX add a point about hash randomization and that it's always on in 3.3

* :issue:`14205`: A dict lookup now raises a :exc:`RuntimeError` if the dict is
  modified during the lookup. If you implement your own comparison function for
  objects used as dict keys and the dict is shared by multiple threads, access
  to the dict should be protected by a lock.

* :issue:`12326`: On Linux, sys.platform doesn't contain the major version
  anymore. It is now always 'linux', instead of 'linux2' or 'linux3' depending
  on the Linux version used to build Python. Replace sys.platform == 'linux2'
  with sys.platform.startswith('linux'), or directly sys.platform == 'linux' if
  you don't need to support older Python versions.

* :issue:`13847`, :issue:`14180`: :mod:`time` and :mod:`datetime`:
  :exc:`OverflowError` is now raised instead of :exc:`ValueError` if a
  timestamp is out of range. :exc:`OSError` is now raised if C functions
  :c:func:`gmtime` or :c:func:`localtime` failed.

* The default finders used by import now utilize a cache of what is contained
  within a specific directory. If you create a Python source file or sourceless
  bytecode file, make sure to call :func:`importlib.invalidate_caches` to clear
  out the cache for the finders to notice the new file.

* :exc:`ImportError` now uses the full name of the module that was attemped to
  be imported. Doctests that check ImportErrors' message will need to be
  updated to use the full name of the module instead of just the tail of the
  name.

* The **index** argument to :func:`__import__` now defaults to 0 instead of -1
  and no longer support negative values. It was an oversight when :pep:`328` was
  implemented that the default value remained -1. If you need to continue to
  perform a relative import followed by an absolute import, then perform the
  relative import using an index of 1, followed by another import using an
  index of 0. It is preferred, though, that you use
  :func:`importlib.import_module` rather than call :func:`__import__` directly.

* :func:`__import__` no longer allows one to use an index value other than 0
  for top-level modules. E.g. ``__import__('sys', level=1)`` is now an error.

* Because :attr:`sys.meta_path` and :attr:`sys.path_hooks` now have finders on
  them by default, you will most likely want to use :meth:`list.insert` instead
  of :meth:`list.append` to add to those lists.

* Because ``None`` is now inserted into :attr:`sys.path_importer_cache`, if you
  are clearing out entries in the dictionary of paths that do not have a
  finder, you will need to remove keys paired with values of ``None`` **and**
  :class:`imp.NullImporter` to be backwards-compatible. This will need to extra
  overhead on older versions of Python that re-insert ``None`` into
  :attr:`sys.path_importer_cache` where it repesents the use of implicit
  finders, but semantically it should not change anything.

* :meth:`importlib.abc.SourceLoader.path_mtime` is now deprecated in favour of
  :meth:`importlib.abc.SourceLoader.path_stats` as bytecode files now store
  both the modification time and size of the source file the bytecode file was
  compiled from.


Porting C code
--------------

* In the course of changes to the buffer API the undocumented
  :c:member:`~Py_buffer.smalltable` member of the
  :c:type:`Py_buffer` structure has been removed and the
  layout of the :c:type:`PyMemoryViewObject` has changed.

  All extensions relying on the relevant parts in ``memoryobject.h``
  or ``object.h`` must be rebuilt.

* Due to :ref:`PEP 393 <pep-393>`, the :c:type:`Py_UNICODE` type and all
  functions using this type are deprecated (but will stay available for
  at least five years).  If you were using low-level Unicode APIs to
  construct and access unicode objects and you want to benefit of the
  memory footprint reduction provided by PEP 393, you have to convert
  your code to the new :doc:`Unicode API <../c-api/unicode>`.

  However, if you only have been using high-level functions such as
  :c:func:`PyUnicode_Concat()`, :c:func:`PyUnicode_Join` or
  :c:func:`PyUnicode_FromFormat()`, your code will automatically take
  advantage of the new unicode representations.

Building C extensions
---------------------

* The range of possible file names for C extensions has been narrowed.
  Very rarely used spellings have been suppressed: under POSIX, files
  named ``xxxmodule.so``, ``xxxmodule.abi3.so`` and
  ``xxxmodule.cpython-*.so`` are no longer recognized as implementing
  the ``xxx`` module.  If you had been generating such files, you have
  to switch to the other spellings (i.e., remove the ``module`` string
  from the file names).

  (implemented in :issue:`14040`.)


Other issues
------------

.. Issue #11591: When :program:`python` was started with :option:`-S`,
   ``import site`` will not add site-specific paths to the module search
   paths.  In previous versions, it did.  See changeset for doc changes in
   various files.  Contributed by Carl Meyer with editions by Éric Araujo.

.. Issue #10998: the -Q command-line flag and related artifacts have been
   removed.  Code checking sys.flags.division_warning will need updating.
   Contributed by Éric Araujo.