cpython/Doc/whatsnew/2.7.rst

****************************
  What's New in Python 2.7
****************************

:Author: A.M. Kuchling (amk at amk.ca)
:Release: |release|
:Date: |today|

.. Fix accents on Kristjan Valur Jonsson, Fuerstenau.

.. $Id$
   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 in a parenthetical comment.

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

   This saves the maintainer some effort going through the SVN logs
   when researching a change.

This article explains the new features in Python 2.7.
No release schedule has been decided yet for 2.7.

.. Compare with previous release in 2 - 3 sentences here.
   add hyperlink when the documentation becomes available online.

.. ========================================================================
.. Large, PEP-level features and changes should be described here.
.. Should there be a new section here for 3k migration?
.. Or perhaps a more general section describing module changes/deprecation?
.. ========================================================================



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

Some smaller changes made to the core Python language are:

* The :func:`int` and :func:`long` types gained a ``bit_length``
  method that returns the number of bits necessary to represent
  its argument in binary::

      >>> n = 37
      >>> bin(37)
      '0b100101'
      >>> n.bit_length()
      6
      >>> n = 2**123-1
      >>> n.bit_length()
      123
      >>> (n+1).bit_length()
      124

  (Contributed by Fredrik Johansson and Victor Stinner; :issue:`3439`.)


* Integers are now stored internally either in base 2**15 or in base
  2**30, the base being determined at build time.  Previously, they
  were always stored in base 2**15.  Using base 2**30 gives
  significant performance improvements on 64-bit machines, but
  benchmark results on 32-bit machines have been mixed.  Therefore,
  the default is to use base 2**30 on 64-bit machines and base 2**15
  on 32-bit machines; on Unix, there's a new configure option
  --enable-big-digits that can be used to override this default.

  Apart from the performance improvements this change should be
  invisible to end users, with one exception: for testing and
  debugging purposes there's a new structseq ``sys.long_info`` that
  provides information about the internal format, giving the number of
  bits per digit and the size in bytes of the C type used to store
  each digit::

     >>> import sys
     >>> sys.long_info
     sys.long_info(bits_per_digit=30, sizeof_digit=4)


  (Contributed by Mark Dickinson; :issue:`4258`.)


.. ======================================================================


Optimizations
-------------

A few performance enhancements have been added:

* The garbage collector now performs better when many objects are
  being allocated without deallocating any.  A full garbage collection
  pass is only performed when the middle generation has been collected
  10 times and when the number of survivor objects from the middle
  generation exceeds 10% of the number of objects in the oldest
  generation.  The second condition was added to reduce the number
  of full garbage collections as the number of objects on the heap grows,
  avoiding quadratic performance when allocating very many objects.
  (Suggested by Martin von Loewis and implemented by Antoine Pitrou;
  :issue:`4074`.)

* The garbage collector tries to avoid tracking simple containers which
  can't be part of a cycle. As of now, this is true for tuples and dicts
  containing atomic types (such as ints, strings, etc.). Transitively, a dict
  containing tuples of atomic types won't be tracked either. This helps bring
  down the individual cost of each garbage collection, since it decreases the
  number of objects to be considered and traversed by the collector.

  To help diagnosing this optimization, a new function in the :mod:`gc`
  module, :func:`is_tracked`, returns True if a given instance is tracked
  by the garbage collector, False otherwise.
  (Contributed by Antoine Pitrou; :issue:`4688`.)


.. ======================================================================

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

As in every release, Python's standard library received a number of
enhancements and bug fixes.  Here's a partial list of the most notable
changes, sorted alphabetically by module name. Consult the
:file:`Misc/NEWS` file in the source tree for a more complete list of
changes, or look through the Subversion logs for all the details.

* In Distutils, distutils.sdist.add_defaults now uses package_dir and data_files
  to feed MANIFEST.

* It is not mandatory anymore to store clear text passwords in the
  :file:`.pypirc` file when registering and uploading packages to PyPI. As long
  as the username is present in that file, the :mod:`distutils` package will
  prompt for the password if not present.  (Added by tarek, with the initial
  contribution of Nathan Van Gheem; :issue:`4394`.)

* The :mod:`bz2` module's :class:`BZ2File` now supports the context
  management protocol, so you can write ``with bz2.BZ2File(...) as f: ...``.
  (Contributed by Hagen Fuerstenau; :issue:`3860`.)

* A new :class:`Counter` class in the :mod:`collections` module is
  useful for tallying data.  :class:`Counter` instances behave mostly
  like dictionaries but return zero for missing keys instead of
  raising a :exc:`KeyError`::

    >>> from collections import Counter
    >>> c=Counter()
    >>> for letter in 'here is a sample of english text':
    ...   c[letter] += 1
    ...
    >>> c
    Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2,
    'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1,
    'p': 1, 'r': 1, 'x': 1})
    >>> c['e']
    5
    >>> c['z']
    0

  There are two additional :class:`Counter` methods: :meth:`most_common`
  returns the N most common elements and their counts, and :meth:`elements`
  returns an iterator over the contained element, repeating each element
  as many times as its count::

    >>> c.most_common(5)
    [(' ', 6), ('e', 5), ('s', 3), ('a', 2), ('i', 2)]
    >>> c.elements() ->
       'a', 'a', ' ', ' ', ' ', ' ', ' ', ' ',
       'e', 'e', 'e', 'e', 'e', 'g', 'f', 'i', 'i',
       'h', 'h', 'm', 'l', 'l', 'o', 'n', 'p', 's',
       's', 's', 'r', 't', 't', 'x']

  Contributed by Raymond Hettinger; :issue:`1696199`.

* The :mod:`gzip` module's :class:`GzipFile` now supports the context
  management protocol, so you can write ``with gzip.GzipFile(...) as f: ...``.
  (Contributed by Hagen Fuerstenau; :issue:`3860`.)

* The :class:`io.FileIO` class now raises an :exc:`OSError` when passed
  an invalid file descriptor.  (Implemented by Benjamin Peterson;
  :issue:`4991`.)

* The :mod:`pydoc` module now has help for the various symbols that Python
  uses.  You can now do ``help('<<')`` or ``help('@')``, for example.
  (Contributed by David Laban; :issue:`4739`.)

* A new function in the :mod:`subprocess` module,
  :func:`check_output`, runs a command with a specified set of arguments
  and returns the command's output as a string if the command runs without
  error, or raises a :exc:`CalledProcessError` exception otherwise.

  ::

    >>> subprocess.check_output(['df', '-h', '.'])
    'Filesystem     Size   Used  Avail Capacity  Mounted on\n
    /dev/disk0s2    52G    49G   3.0G    94%    /\n'

    >>> subprocess.check_output(['df', '-h', '/bogus'])
      ...
    subprocess.CalledProcessError: Command '['df', '-h', '/bogus']' returned non-zero exit status 1

  (Contributed by Gregory P. Smith.)

* The :func:`is_zipfile` function in the :mod:`zipfile` module will now
  accept a file object, in addition to the path names accepted in earlier
  versions.  (Contributed by Gabriel Genellina; :issue:`4756`.)

.. ======================================================================
.. whole new modules get described in subsections here

ttk: Themed Widgets for Tk
--------------------------

Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk
widgets but have a more customizable appearance and can therefore more
closely resemble the native platform's widgets.  This widget
set was originally called Tile, but was renamed to Ttk (for "themed Tk")
on being added to Tcl/Tck release 8.5.

XXX write a brief discussion and an example here.

The :mod:`ttk` module was written by Guilherme Polo and added in
:issue:`2983`.  An alternate version called ``Tile.py``, written by
Martin Franklin and maintained by Kevin Walzer, was proposed for
inclusion in :issue:`2618`, but the authors argued that Guilherme
Polo's work was more comprehensive.

.. ======================================================================


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

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

* If you use the :file:`.gdbinit` file provided with Python,
  the "pyo" macro in the 2.7 version will now work when the thread being
  debugged doesn't hold the GIL; the macro will now acquire it before printing.
  (Contributed by Victor Stinner; :issue:`3632`.)

* :cfunc:`Py_AddPendingCall` is now thread safe, letting any
  worker thread submit notifications to the main Python thread.  This
  is particularly useful for asynchronous IO operations.
  (Contributed by Kristjan Valur Jonsson; :issue:`4293`.)


.. ======================================================================

Port-Specific Changes: Windows
-----------------------------------

* The :mod:`msvcrt` module now contains some constants from
  the :file:`crtassem.h` header file:
  :data:`CRT_ASSEMBLY_VERSION`,
  :data:`VC_ASSEMBLY_PUBLICKEYTOKEN`,
  and :data:`LIBRARIES_ASSEMBLY_NAME_PREFIX`.
  (Contributed by David Cournapeau; :issue:`4365`.)

* The new :cfunc:`_beginthreadex` API is used to start threads, and
  the native thread-local storage functions are now used.
  (Contributed by Kristjan Valur Jonsson; :issue:`3582`.)

.. ======================================================================

Port-Specific Changes: Mac OS X
-----------------------------------


.. ======================================================================

Porting to Python 2.7
=====================

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

To be written.

.. ======================================================================


.. _acks27:

Acknowledgements
================

The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: no one yet.