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

[Abstract copied from the PEP: 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).  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.]

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 little bit more
memory, because Python may have to maintain two versions of each string (in
the legacy format and in the new efficient storage).

XXX Add list of changes introduced by :pep:`393` here:

* 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.

* 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 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
   <http://mail.python.org/pipermail/python-dev/2011-September/113714.html>`_.
   XXX The result should be moved in the PEP and a small summary about
   performances and a link to the PEP should be added here.

* Some of the problems visible on narrow builds have 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 a non-BMP characters doesn't return surrogates anymore,
    so ``'\U0010FFFF'[0]`` now returns ``'\U0010FFFF'`` and not ``'\uDBFF'``;

  * several other functions in the stdlib now handle correctly 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.

XXX mention new and deprecated functions and macros


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.

New subclasses of :exc:`OSError` exceptions:

 * :exc:`BlockingIOError`
 * :exc:`ChildProcessError`
 * :exc:`ConnectionError`

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

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

The following exceptions have been merged into :exc:`OSError`:

 * :exc:`EnvironmentError`
 * :exc:`IOError`
 * :exc:`WindowsError`
 * :exc:`VMSError`
 * :exc:`socket.error`
 * :exc:`select.error`
 * :exc:`mmap.error`

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: ::

    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")


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

Some smaller changes made to the core Python language are:

* Stub


New, Improved, and Deprecated Modules
=====================================

* Stub

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`)


codecs
------

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`)

Don't reset incremental encoders of CJK codecs at each call to their encode()
method anymore. 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`)

crypt
-----

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

(:issue:`10924`)

curses
------

 * 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`)

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
plaintex.  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`)


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`.)


packaging
---------

:mod:`distutils` has undergone additions and refactoring under a new name,
:mod:`packaging`, to allow developers to break 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 2.4+ and 3.1+ will be made available on PyPI under the
name :mod:`distutils2`.

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


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.


sys
---

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

  (:issue:`11223`)


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.

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`)


ssl
---

The :mod:`ssl` module has new functions:

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


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`)


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

Major performance enhancements have been added:

* Stub


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

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

* Stub


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.


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

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

* 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 #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: -Q command-line flags are related artifacts have been
   removed.  Code checking sys.flags.division_warning will need updating.
   Contributed by Éric Araujo.

* :pep:`393`: The :c:type:`Py_UNICODE` type and all functions using this type
  are deprecated. To fully benefit of the memory footprint reduction provided
  by the PEP 393, you have to convert your code to the new Unicode API. Read
  the porting guide: XXX.