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, Tarek Ziade.

.. $Id$
   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.

Python 3.1
================

Much as Python 2.6 incorporated features from Python 3.0,
version 2.7 is influenced by features from 3.1.

XXX mention importlib; anything else?

One porting change: the :option:`-3` switch now automatically
enables the :option:`-Qwarn` switch that causes warnings
about using classic division with integers and long integers.

.. ========================================================================
.. Large, PEP-level features and changes should be described here.
.. ========================================================================

PEP 372: Adding an ordered dictionary to collections
====================================================

XXX write this

Several modules will now use :class:`OrderedDict` by default.  The
:mod:`ConfigParser` module uses :class:`OrderedDict` for the list
of sections and the options within a section.
The :meth:`namedtuple._asdict` method returns an :class:`OrderedDict`
as well.


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`.)

* The :class:`bytearray` type's :meth:`translate` method will
  now accept None as its first argument.  (Fixed by Georg Brandl;
  :issue:`4759`.)

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


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

Several performance enhancements have been added:

.. * A new :program:`configure` option, :option:`--with-computed-gotos`,
   compiles the main bytecode interpreter loop using a new dispatch
   mechanism that gives speedups of up to 20%, depending on the system
   and benchmark.  The new mechanism is only supported on certain
   compilers, such as gcc, SunPro, and icc.

* 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. In Python 2.7, this is now 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 reduce the cost of each
  garbage collection by decreasing the number of objects to be
  considered and traversed by the collector.
  (Contributed by Antoine Pitrou; :issue:`4688`.)

* 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
  :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`.)

  Another set of changes made long objects a few bytes smaller: 2 bytes
  smaller on 32-bit systems and 6 bytes on 64-bit.
  (Contributed by Mark Dickinson; :issue:`5260`.)

* The division algorithm for long integers has been made faster
  by tightening the inner loop, doing shifts instead of multiplications,
  and fixing an unnecessary extra iteration.
  Various benchmarks show speedups of between 50% and 150% for long
  integer divisions and modulo operations.
  (Contributed by Mark Dickinson; :issue:`5512`.)

* The implementation of ``%`` checks for the left-side operand being
  a Python string and special-cases it; this results in a 1-3%
  performance increase for applications that frequently use ``%``
  with strings, such as templating libraries.
  (Implemented by Collin Winter; :issue:`5176`.)

* List comprehensions with an ``if`` condition are compiled into
  faster bytecode.  (Patch by Antoine Pitrou, back-ported to 2.7
  by Jeffrey Yasskin; :issue:`4715`.)

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

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.

* 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 :class:`namedtuple` class now has an optional *rename* parameter.
  If *rename* is True, field names that are invalid because they've
  been repeated or that aren't legal Python identifiers will be
  renamed to legal names that are derived from the field's
  position within the list of fields:

     >>> T=namedtuple('T', ['field1', '$illegal', 'for', 'field2'], rename=True)
     >>> T._fields
     ('field1', '_1', '_2', 'field2')

  (Added by Raymond Hettinger; :issue:`1818`.)

* In Distutils, :func:`distutils.sdist.add_defaults` now uses
  *package_dir* and *data_files* to create the MANIFEST file.

  It is no longer mandatory 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 Ziade,
  based on an initial contribution by Nathan Van Gheem; :issue:`4394`.)

* New method: the :class:`Decimal` class gained a
  :meth:`from_float` class method that performs an exact conversion
  of a floating-point number to a :class:`Decimal`.
  Note that this is an **exact** conversion that strives for the
  closest decimal approximation to the floating-point representation's value;
  the resulting decimal value will therefore still include the inaccuracy,
  if any.
  For example, ``Decimal.from_float(0.1)`` returns
  ``Decimal('0.1000000000000000055511151231257827021181583404541015625')``.
  (Implemented by Raymond Hettinger; :issue:`4796`.)

* 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`.)

* 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`.)
  It's now possible to override the modification time
  recorded in a gzipped file by providing an optional timestamp to
  the constructor.  (Contributed by Jacques Frechet; :issue:`4272`.)

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

* New function: ``itertools.compress(*data*, *selectors*)`` takes two
  iterators.  Elements of *data* are returned if the corresponding
  value in *selectors* is True::

    itertools.compress('ABCDEF', [1,0,1,0,1,1]) =>
      A, C, E, F

  New function: ``itertools.combinations_with_replacement(*iter*, *r*)``
  returns all the possible *r*-length combinations of elements from the
  iterable *iter*.  Unlike :func:`combinations`, individual elements
  can be repeated in the generated combinations::

    itertools.combinations_with_replacement('abc', 2) =>
      ('a', 'a'), ('a', 'b'), ('a', 'c'),
      ('b', 'b'), ('b', 'c'), ('c', 'c')

  Note that elements are treated as unique depending on their position
  in the input, not their actual values.

  The :class:`itertools.count` function now has a *step* argument that
  allows incrementing by values other than 1.  :func:`count` also
  now allows keyword arguments, and using non-integer values such as
  floats or :class:`Decimal` instances.  (Implemented by Raymond
  Hettinger; :issue:`5032`.)

  :func:`itertools.combinations` and :func:`itertools.product` were
  previously raising :exc:`ValueError` for values of *r* larger than
  the input iterable.  This was deemed a specification error, so they
  now return an empty iterator.  (Fixed by Raymond Hettinger; :issue:`4816`.)

* The :mod:`json` module was upgraded to version 2.0.9 of the
  simplejson package, which includes a C extension that makes
  encoding and decoding faster.
  (Contributed by Bob Ippolito; :issue:`4136`.)

  To support the new :class:`OrderedDict` type, :func:`json.load`
  now has an optional *object_pairs_hook* parameter that will be called
  with any object literal that decodes to a list of pairs.
  (Contributed by Raymond Hettinger; :issue:`5381`.)

* 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 when 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 ``sys.version_info`` value is now a named tuple, with attributes
  named ``major``, ``minor``, ``micro``, ``releaselevel``, and ``serial``.
  (Contributed by Ross Light; :issue:`4285`.)

* The :mod:`unittest` module was enhanced in several ways.
  Test cases can raise the :exc:`SkipTest` exception to skip a test.
  (:issue:`1034053`.)
  It will now use 'x' for expected failures
  and 'u' for unexpected successes when run in its verbose mode.
  (Contributed by Benjamin Peterson.)

  The :meth:`assertRaises` and :meth:`failUnlessRaises` methods now
  return a context handler when called without providing a callable
  object to run.  For example, you can write this::

      with self.assertRaises(KeyError):
          raise ValueError

  (Implemented by Antoine Pitrou; :issue:`4444`.)

* 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`.)

  :mod:`zipfile` now supports archiving empty directories and
  extracts them correctly.  (Fixed by Kuba Wieczorek; :issue:`4710`.)

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

importlib: Importing Modules
------------------------------

XXX write this

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`.)

* The :program:`configure` script now checks for floating-point rounding bugs
  on certain 32-bit Intel chips and defines a :cmacro:`X87_DOUBLE_ROUNDING`
  preprocessor definition.  No code currently uses this definition,
  but it's available if anyone wishes to use it.
  (Added by Mark Dickinson; :issue:`2937`.)

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

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
-----------------------------------

* The ``/Library/Python/2.7/site-packages`` is now appended to
  ``sys.path``, in order to share added packages between the system
  installation and a user-installed copy of the same version.
  (Changed by Ronald Oussoren; :issue:`4865`.)


Other Changes and Fixes
=======================

* When importing a module from a :file:`.pyc` or :file:`.pyo` file
  with an existing :file:`.py` counterpart, the :attr:`co_filename`
  attributes of all code objects if the original filename is obsolete,
  which can happen if the file has been renamed, moved, or is accessed
  through different paths.  (Patch by Ziga Seilnacht and Jean-Paul
  Calderone; :issue:`1180193`.)

* The :file:`regrtest.py` script now takes a :option:`--randseed=`
  switch that takes an integer that will be used as the random seed
  for the :option:`-r` option that executes tests in random order.
  The :option:`-r` option also now reports the seed that was used
  (Added by Collin Winter.)


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

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.