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.  The final
release of 2.7 is currently scheduled for June 2010; the detailed
schedule is described in :pep:`373`.

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

.. _whatsnew27-python31:

Python 3.1 Features
=======================

Much as Python 2.6 incorporated features from Python 3.0,
version 2.7 incorporates some of the new features
in Python 3.1.  The 2.x series continues to provide tools
for migrating to the 3.x series.

A partial list of 3.1 features that were backported to 2.7:

* A version of the :mod:`io` library, rewritten in C for performance.
* The ordered-dictionary type described in :ref:`pep-0372`.
* The new format specifier described in :ref:`pep-0378`.
* The :class:`memoryview` object.
* A small subset of the :mod:`importlib` module `described below <#importlib-section>`__.
* Float-to-string and string-to-float conversions now round their
  results more correctly.  And :func:`repr` of a floating-point
  number *x* returns a result that's guaranteed to round back to the
  same number when converted back to a string.
* The :cfunc:`PyLong_AsLongAndOverflow` C API function.

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.

Other new Python3-mode warnings include:

* :func:`operator.isCallable` and :func:`operator.sequenceIncludes`,
  which are not supported in 3.x.

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

.. _pep-0372:

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

Regular Python dictionaries iterate over key/value pairs in arbitrary order.
Over the years, a number of authors have written alternative implementations
that remember the order that the keys were originally inserted.  Based on
the experiences from those implementations, a new
:class:`collections.OrderedDict` class has been introduced.

The :class:`OrderedDict` API is substantially the same as regular dictionaries
but will iterate over keys and values in a guaranteed order depending on
when a key was first inserted::

    >>> from collections import OrderedDict
    >>> d = OrderedDict([('first', 1), ('second', 2),
    ...                  ('third', 3)])
    >>> d.items()
    [('first', 1), ('second', 2), ('third', 3)]

If a new entry overwrites an existing entry, the original insertion
position is left unchanged::

    >>> d['second'] = 4
    >>> d.items()
    [('first', 1), ('second', 4), ('third', 3)]

Deleting an entry and reinserting it will move it to the end::

    >>> del d['second']
    >>> d['second'] = 5
    >>> d.items()
    [('first', 1), ('third', 3), ('second', 5)]

The :meth:`popitem` method has an optional *last* argument
that defaults to True.  If *last* is True, the most recently
added key is returned and removed; if it's False, the
oldest key is selected::

    >>> od = OrderedDict([(x,0) for x in range(20)])
    >>> od.popitem()
    (19, 0)
    >>> od.popitem()
    (18, 0)
    >>> od.popitem(False)
    (0, 0)
    >>> od.popitem(False)
    (1, 0)

Comparing two ordered dictionaries checks both the keys and values,
and requires that the insertion order was the same::

    >>> od1 = OrderedDict([('first', 1), ('second', 2),
    ...                    ('third', 3)])
    >>> od2 = OrderedDict([('third', 3), ('first', 1),
    ...                    ('second', 2)])
    >>> od1==od2
    False
    >>> # Move 'third' key to the end
    >>> del od2['third'] ; od2['third'] = 3
    >>> od1==od2
    True

Comparing an :class:`OrderedDict` with a regular dictionary
ignores the insertion order and just compares the keys and values.

How does the :class:`OrderedDict` work?  It maintains a doubly-linked
list of keys, appending new keys to the list as they're inserted.  A
secondary dictionary maps keys to their corresponding list node, so
deletion doesn't have to traverse the entire linked list and therefore
remains O(1).

.. XXX check O(1)-ness with Raymond

The standard library now supports use of ordered dictionaries in several
modules.  The :mod:`configparser` module uses them by default.  This lets
configuration files be read, modified, and then written back in their original
order.  The *_asdict()* method for :func:`collections.namedtuple` now
returns an ordered dictionary with the values appearing in the same order as
the underlying tuple indicies.  The :mod:`json` module is being built-out with
an *object_pairs_hook* to allow OrderedDicts to be built by the decoder.
Support was also added for third-party tools like `PyYAML <http://pyyaml.org/>`_.

.. seealso::

   :pep:`372` - Adding an ordered dictionary to collections
     PEP written by Armin Ronacher and Raymond Hettinger;
     implemented by Raymond Hettinger.

.. _pep-0378:

PEP 378: Format Specifier for Thousands Separator
====================================================

To make program output more readable, it can be useful to add
separators to large numbers and render them as
18,446,744,073,709,551,616 instead of 18446744073709551616.

The fully general solution for doing this is the :mod:`locale` module,
which can use different separators ("," in North America, "." in
Europe) and different grouping sizes, but :mod:`locale` is complicated
to use and unsuitable for multi-threaded applications where different
threads are producing output for different locales.

Therefore, a simple comma-grouping mechanism has been added to the
mini-language used by the string :meth:`format` method.  When
formatting a floating-point number, simply include a comma between the
width and the precision::

   >>> '{:20,.2}'.format(f)
   '18,446,744,073,709,551,616.00'

This mechanism is not adaptable at all; commas are always used as the
separator and the grouping is always into three-digit groups.  The
comma-formatting mechanism isn't as general as the :mod:`locale`
module, but it's easier to use.

.. XXX "Format String Syntax" in string.rst could use many more examples.

.. seealso::

   :pep:`378` - Format Specifier for Thousands Separator
     PEP written by Raymond Hettinger; implemented by Eric Smith.

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

Some smaller changes made to the core Python language are:

* The :keyword:`with` statement can now use multiple context managers
  in one statement.  Context managers are processed from left to right
  and each one is treated as beginning a new :keyword:`with` statement.
  This means that::

   with A() as a, B() as b:
       ... suite of statements ...

  is equivalent to::

   with A() as a:
       with B() as b:
           ... suite of statements ...

  The :func:`contextlib.nested` function provides a very similar
  function, so it's no longer necessary and has been deprecated.

  (Proposed in http://codereview.appspot.com/53094; implemented by
  Georg Brandl.)

* Conversions between floating-point numbers and strings are
  now correctly rounded on most platforms.  These conversions occur
  in many different places: :func:`str` on
  floats and complex numbers; the :class:`float` and :class:`complex`
  constructors;
  numeric formatting; serialization and
  deserialization of floats and complex numbers using the
  :mod:`marshal`, :mod:`pickle`
  and :mod:`json` modules;
  parsing of float and imaginary literals in Python code;
  and :class:`Decimal`-to-float conversion.

  Related to this, the :func:`repr` of a floating-point number *x*
  now returns a result based on the shortest decimal string that's
  guaranteed to round back to *x* under correct rounding (with
  round-half-to-even rounding mode).  Previously it gave a string
  based on rounding x to 17 decimal digits.

  The rounding library responsible for this improvement works on
  Windows, and on Unix platforms using the gcc, icc, or suncc
  compilers.  There may be a small number of platforms where correct
  operation of this code cannot be guaranteed, so the code is not
  used on such systems.

  Implemented by Mark Dickinson, using David Gay's :file:`dtoa.c` library;
  :issue:`7117`.

* The :meth:`str.format` method now supports automatic numbering of the replacement
  fields.  This makes using :meth:`str.format` more closely resemble using
  ``%s`` formatting::

    >>> '{}:{}:{}'.format(2009, 04, 'Sunday')
    '2009:4:Sunday'
    >>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
    '2009:4:Sunday'

  The auto-numbering takes the fields from left to right, so the first ``{...}``
  specifier will use the first argument to :meth:`str.format`, the next
  specifier will use the next argument, and so on.  You can't mix auto-numbering
  and explicit numbering -- either number all of your specifier fields or none
  of them -- but you can mix auto-numbering and named fields, as in the second
  example above.  (Contributed by Eric Smith; :issue:`5237`.)

  Complex numbers now correctly support usage with :func:`format`.
  Specifying a precision or comma-separation applies to both the real
  and imaginary parts of the number, but a specified field width and
  alignment is applied to the whole of the resulting ``1.5+3j``
  output.  (Contributed by Eric Smith; :issue:`1588`.)

  The 'F' format code now always formats its output using uppercase characters,
  so it will now produce 'INF' and 'NAN'.
  (Contributed by Eric Smith; :issue:`3382`.)

* 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`.)

* Conversions from long integers and regular integers to floating
  point now round differently, returning the floating-point number
  closest to the number.  This doesn't matter for small integers that
  can be converted exactly, but for large numbers that will
  unavoidably lose precision, Python 2.7 now approximates more
  closely.  For example, Python 2.6 computed the following::

    >>> n = 295147905179352891391
    >>> float(n)
    2.9514790517935283e+20
    >>> n - long(float(n))
    65535L

  Python 2.7's floating-point result is larger, but much closer to the
  true value::

    >>> n = 295147905179352891391
    >>> float(n)
    2.9514790517935289e+20
    >>> n-long(float(n)
    ... )
    -1L

  (Implemented by Mark Dickinson; :issue:`3166`.)

  Integer division is also more accurate in its rounding behaviours.  (Also
  implemented by Mark Dickinson; :issue:`1811`.)

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

* When using ``@classmethod`` and ``@staticmethod`` to wrap
  methods as class or static methods, the wrapper object now
  exposes the wrapped function as their :attr:`__func__` attribute.
  (Contributed by Amaury Forgeot d'Arc, after a suggestion by
  George Sakkis; :issue:`5982`.)

* A new encoding named "cp720", used primarily for Arabic text, is now
  supported.  (Contributed by Alexander Belchenko and Amaury Forgeot
  d'Arc; :issue:`1616979`.)

* The :class:`file` object will now set the :attr:`filename` attribute
  on the :exc:`IOError` exception when trying to open a directory
  on POSIX platforms.  (Noted by Jan Kaliszewski; :issue:`4764`.)

* Extra parentheses in function definitions are illegal in Python 3.x,
  meaning that you get a syntax error from ``def f((x)): pass``.  In
  Python3-warning mode, Python 2.7 will now warn about this odd usage.
  (Noted by James Lingard; :issue:`7362`.)

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


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.

* A new opcode was added to perform the initial setup for
  :keyword:`with` statements, looking up the :meth:`__enter__` and
  :meth:`__exit__` methods.  (Contributed by Benjamin Peterson.)

* The garbage collector now performs better for one common usage
  pattern: when many objects are being allocated without deallocating
  any of them.  This would previously take quadratic
  time for garbage collection, but now the number of full garbage collections
  is reduced as the number of objects on the heap grows.
  The new logic is to only perform a full garbage collection pass 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.  (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`.)

* Long 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`.)

* The :mod:`pickle` and :mod:`cPickle` modules now automatically
  intern the strings used for attribute names, reducing memory usage
  of the objects resulting from unpickling.  (Contributed by Jake
  McGuire; :issue:`5084`.)

* The :mod:`cPickle` module now special-cases dictionaries,
  nearly halving the time required to pickle them.
  (Contributed by Collin Winter; :issue:`5670`.)

* Converting an integer or long integer to a decimal string was made
  faster by special-casing base 10 instead of using a generalized
  conversion function that supports arbitrary bases.
  (Patch by Gawain Bolton; :issue:`6713`.)

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

New and Improved 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:`bdb` module's base debugging class :class:`Bdb`
  gained a feature for skipping modules.  The constructor
  now takes an iterable containing glob-style patterns such as
  ``django.*``; the debugger will not step into stack frames
  from a module that matches one of these patterns.
  (Contributed by Maru Newby after a suggestion by
  Senthil Kumaran; :issue:`5142`.)

* 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`.)

* New class: the :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`:

  .. doctest::
     :options: +NORMALIZE_WHITESPACE

     >>> 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 new `OrderedDict` class is described in the earlier section
  :ref:`pep-0372`.

  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:

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

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

  The :class:`deque` data type now exposes its maximum length as the
  read-only :attr:`maxlen` attribute, and has a
  :meth:`reverse` method that reverses the elements of the deque in-place.
  (Added by Raymond Hettinger.)

* The :mod:`copy` module's :func:`deepcopy` function will now
  correctly copy bound instance methods.  (Implemented by
  Robert Collins; :issue:`1515`.)

* The :mod:`ctypes` module now always converts ``None`` to a C NULL
  pointer for arguments declared as pointers.  (Changed by Thomas
  Heller; :issue:`4606`.)

* New method: the :mod:`datetime` module's :class:`timedelta` class
  gained a :meth:`total_seconds` method that returns the number of seconds
  in the duration.  (Contributed by Brian Quinlan; :issue:`5788`.)

* 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`.)

  The constructor for :class:`Decimal` now accepts non-European
  Unicode characters, such as Arabic-Indic digits.  (Contributed by
  Mark Dickinson; :issue:`6595`.)

  When using :class:`Decimal` instances with a string's
  :meth:`format` method, the default alignment was previously
  left-alignment.  This has been changed to right-alignment, which seems
  more sensible for numeric types.  (Changed by Mark Dickinson; :issue:`6857`.)

* Distutils is being more actively developed, thanks to Tarek Ziade
  who has taken over maintenance of the package.  A new
  :file:`setup.py` subcommand, ``check``, will
  check that the arguments being passed to the :func:`setup` function
  are complete and correct (:issue:`5732`).

  :func:`distutils.sdist.add_defaults` now uses
  *package_dir* and *data_files* to create the MANIFEST file.
  :mod:`distutils.sysconfig` now reads the :envvar:`AR` and
  :envvar:`ARFLAGS` environment variables.

  .. ARFLAGS done in #5941

  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`.)

  A Distutils setup can now specify that a C extension is optional by
  setting the *optional* option setting to true.  If this optional is
  supplied, failure to build the extension will not abort the build
  process, but instead simply not install the failing extension.
  (Contributed by Georg Brandl; :issue:`5583`.)

  The :class:`distutils.dist.DistributionMetadata` class'
  :meth:`read_pkg_file` method will read the contents of a package's
  :file:`PKG-INFO` metadata file.  For an example of its use,
  XXX link to  file:///MacDev/svn.python.org/python-trunk/Doc/build/html/distutils/examples.html#reading-the-metadata
  (Contributed by Tarek Ziade; :issue:`7457`.)

* The :class:`Fraction` class now accepts two rational numbers
  as arguments to its constructor.
  (Implemented by Mark Dickinson; :issue:`5812`.)

* The :mod:`ftplib` module gained the ability to establish secure FTP
  connections using TLS encapsulation of authentication as well as
  subsequent control and data transfers.  This is provided by the new
  :class:`ftplib.FTP_TLS` class.
  (Contributed by Giampaolo Rodola', :issue:`2054`.)  The :meth:`storbinary`
  method for binary uploads can now restart uploads thanks to an added
  *rest* parameter (patch by Pablo Mouzo; :issue:`6845`.)

* New function: the :mod:`gc` module's :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 :mod:`hashlib` module was inconsistent about accepting
  input as a Unicode object or an object that doesn't support
  the buffer protocol.  The behavior was different depending on
  whether :mod:`hashlib` was using an external OpenSSL library
  or its built-in implementations.  Python 2.7 makes the
  behavior consistent, always rejecting such objects by raising a
  :exc:`TypeError`.  (Fixed by Gregory P. Smith; :issue:`3745`.)

* The default :class:`HTTPResponse` class used by the :mod:`httplib` module now
  supports buffering, resulting in much faster reading of HTTP responses.
  (Contributed by Kristjan Valur Jonsson; :issue:`4879`.)

* The :mod:`imaplib` module now supports IPv6 addresses.
  (Contributed by Derek Morr; :issue:`1655`.)

* The :mod:`io` library has been upgraded to the version shipped with
  Python 3.1.  For 3.1, the I/O library was entirely rewritten in C
  and is 2 to 20 times faster depending on the task at hand.  The
  original Python version was renamed to the :mod:`_pyio` module.

  One minor resulting change: the :class:`io.TextIOBase` class now
  has an :attr:`errors` attribute giving the error setting
  used for encoding and decoding errors (one of ``'strict'``, ``'replace'``,
  ``'ignore'``).

  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`.)

* New functions: the :mod:`math` module gained
  :func:`erf` and :func:`erfc` for the error function and the complementary error function,
  :func:`expm1` which computes ``e**x - 1`` with more precision than
  using :func:`exp` and subtracting 1,
  :func:`gamma` for the Gamma function, and
  :func:`lgamma` for the natural log of the Gamma function.
  (Contributed by Mark Dickinson and nirinA raseliarison; :issue:`3366`.)

* The :mod:`multiprocessing` module's :class:`Manager*` classes
  can now be passed a callable that will be called whenever
  a subprocess is started, along with a set of arguments that will be
  passed to the callable.
  (Contributed by lekma; :issue:`5585`.)

* The :mod:`nntplib` module now supports IPv6 addresses.
  (Contributed by Derek Morr; :issue:`1664`.)

* New functions: the :mod:`os` module wraps the following POSIX system
  calls: :func:`getresgid` and :func:`getresuid`, which return the
  real, effective, and saved GIDs and UIDs;
  :func:`setresgid` and :func:`setresuid`, which set
  real, effective, and saved GIDs and UIDs to new values;
  :func:`initgroups`.  (GID/UID functions
  contributed by Travis H.; :issue:`6508`.  Support for initgroups added
  by Jean-Paul Calderone; :issue:`7333`.)

* 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`.)

* The :mod:`re` module's :func:`split`, :func:`sub`, and :func:`subn`
  now accept an optional *flags* argument, for consistency with the
  other functions in the module.  (Added by Gregory P. Smith.)

* The :mod:`shutil` module's :func:`copyfile` and :func:`copytree`
  functions now raises a :exc:`SpecialFileError` exception when
  asked to copy a named pipe.  Previously the code would treat
  named pipes like a regular file by opening them for reading, and
  this would block indefinitely.  (Fixed by Antoine Pitrou; :issue:`3002`.)

* New functions: in the :mod:`site` module, three new functions
  return various site- and user-specific paths.
  :func:`getsitepackages` returns a list containing all
  global site-packages directories, and
  :func:`getusersitepackages` returns the path of the user's
  site-packages directory.
  :func:`getuserbase` returns the value of the :envvar:`USER_BASE`
  environment variable, giving the path to a directory that can be used
  to store data.
  (Contributed by Tarek Ziade; :issue:`6693`.)

* The :mod:`SocketServer` module's :class:`TCPServer` class now
  has a :attr:`disable_nagle_algorithm` class attribute.
  The default value is False; if overridden to be True,
  new request connections will have the TCP_NODELAY option set to
  prevent buffering many small sends into a single TCP packet.
  (Contributed by Kristjan Valur Jonsson; :issue:`6192`.)

* The :mod:`struct` module will no longer silently ignore overflow
  errors when a value is too large for a particular integer format
  code (one of ``bBhHiIlLqQ``); it now always raises a
  :exc:`struct.error` exception.  (Changed by Mark Dickinson;
  :issue:`1523`.)

* New function: the :mod:`subprocess` module's
  :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.)

* New function: :func:`is_declared_global` in the :mod:`symtable` module
  returns true for variables that are explicitly declared to be global,
  false for ones that are implicitly global.
  (Contributed by Jeremy Hylton.)

* 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:`tarfile` module now supports filtering the :class:`TarInfo`
  objects being added to a tar file.  When you call :meth:`TarFile.add`,
  instance, you may supply an optional *filter* argument
  that's a callable.  The *filter* callable will be passed the
  :class:`TarInfo` for every file being added, and can modify and return it.
  If the callable returns ``None``, the file will be excluded from the
  resulting archive.  This is more powerful than the existing
  *exclude* argument, which has therefore been deprecated.
  (Added by Lars Gustaebel; :issue:`6856`.)

* The :mod:`threading` module's :meth:`Event.wait` method now returns
  the internal flag on exit.  This means the method will usually
  return true because :meth:`wait` is supposed to block until the
  internal flag becomes true.  The return value will only be false if
  a timeout was provided and the operation timed out.
  (Contributed by Tim Lesher; :issue:`1674032`.)

* The :func:`is_zipfile` function in the :mod:`zipfile` module now
  accepts 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

Unit Testing Enhancements
---------------------------------

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

.. XXX describe test discovery (Contributed by Michael Foord; :issue:`6001`.)

The error messages for :meth:`assertEqual`,
:meth:`assertTrue`, and :meth:`assertFalse`
failures now provide more information.  If you set the
:attr:`longMessage` attribute of your :class:`TestCase` classes to
true, both the standard error message and any additional message you
provide will be printed for failures.  (Added by Michael Foord; :issue:`5663`.)

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 methods :meth:`addCleanup` and :meth:`doCleanups` were added.
:meth:`addCleanup` allows you to add cleanup functions that
will be called unconditionally (after :meth:`setUp` if
:meth:`setUp` fails, otherwise after :meth:`tearDown`). This allows
for much simpler resource allocation and deallocation during tests.
:issue:`5679`

A number of new methods were added that provide more specialized
tests.  Many of these methods were written by Google engineers
for use in their test suites; Gregory P. Smith, Michael Foord, and
GvR worked on merging them into Python's version of :mod:`unittest`.

* :meth:`assertIsNone` and :meth:`assertIsNotNone` take one
  expression and verify that the result is or is not ``None``.

* :meth:`assertIs` and :meth:`assertIsNot` take two values and check
  whether the two values evaluate to the same object or not.
  (Added by Michael Foord; :issue:`2578`.)

* :meth:`assertGreater`, :meth:`assertGreaterEqual`,
  :meth:`assertLess`, and :meth:`assertLessEqual` compare
  two quantities.

* :meth:`assertMultiLineEqual` compares two strings, and if they're
  not equal, displays a helpful comparison that highlights the
  differences in the two strings.

* :meth:`assertRegexpMatches` checks whether its first argument is a
  string matching a regular expression provided as its second argument.

* :meth:`assertRaisesRegexp` checks whether a particular exception
  is raised, and then also checks that the string representation of
  the exception matches the provided regular expression.

* :meth:`assertIn` and :meth:`assertNotIn` tests whether
  *first* is or is not in  *second*.

* :meth:`assertSameElements` tests whether two provided sequences
  contain the same elements.

* :meth:`assertSetEqual` compares whether two sets are equal, and
  only reports the differences between the sets in case of error.

* Similarly, :meth:`assertListEqual` and :meth:`assertTupleEqual`
  compare the specified types and explain the differences.
  More generally, :meth:`assertSequenceEqual` compares two sequences
  and can optionally check whether both sequences are of a
  particular type.

* :meth:`assertDictEqual` compares two dictionaries and reports the
  differences.  :meth:`assertDictContainsSubset` checks whether
  all of the key/value pairs in *first* are found in *second*.

* :meth:`assertAlmostEqual` and :meth:`assertNotAlmostEqual` short-circuit
  (automatically pass or fail without checking decimal places) if the objects
  are equal.

* :meth:`loadTestsFromName` properly honors the ``suiteClass`` attribute of
  the :class:`TestLoader`. (Fixed by Mark Roddy; :issue:`6866`.)

* A new hook, :meth:`addTypeEqualityFunc` takes a type object and a
  function.  The :meth:`assertEqual` method will use the function
  when both of the objects being compared are of the specified type.
  This function should compare the two objects and raise an
  exception if they don't match; it's a good idea for the function
  to provide additional information about why the two objects are
  matching, much as the new sequence comparison methods do.

:func:`unittest.main` now takes an optional ``exit`` argument.
If False ``main`` doesn't call :func:`sys.exit` allowing it to
be used from the interactive interpreter. :issue:`3379`.

:class:`TestResult` has new :meth:`startTestRun` and
:meth:`stopTestRun` methods; called immediately before
and after a test run. :issue:`5728` by Robert Collins.

With all these changes, the :file:`unittest.py` was becoming awkwardly
large, so the module was turned into a package and the code split into
several files (by Benjamin Peterson).  This doesn't affect how the
module is imported.


.. _importlib-section:

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

Python 3.1 includes the :mod:`importlib` package, a re-implementation
of the logic underlying Python's :keyword:`import` statement.
:mod:`importlib` is useful for implementors of Python interpreters and
to users who wish to write new importers that can participate in the
import process.  Python 2.7 doesn't contain the complete
:mod:`importlib` package, but instead has a tiny subset that contains
a single function, :func:`import_module`.

``import_module(name, package=None)`` imports a module.  *name* is
a string containing the module or package's name.  It's possible to do
relative imports by providing a string that begins with a ``.``
character, such as ``..utils.errors``.  For relative imports, the
*package* argument must be provided and is the name of the package that
will be used as the anchor for
the relative import.  :func:`import_module` both inserts the imported
module into ``sys.modules`` and returns the module object.

Here are some examples::

    >>> from importlib import import_module
    >>> anydbm = import_module('anydbm')  # Standard absolute import
    >>> anydbm
    <module 'anydbm' from '/p/python/Lib/anydbm.py'>
    >>> # Relative import
    >>> sysconfig = import_module('..sysconfig', 'distutils.command')
    >>> sysconfig
    <module 'distutils.sysconfig' from '/p/python/Lib/distutils/sysconfig.pyc'>

:mod:`importlib` was implemented by Brett Cannon and introduced in
Python 3.1.


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.


Deprecations and Removals
=========================

* :func:`contextlib.nested`, which allows handling more than one context manager
  with one :keyword:`with` statement, has been deprecated; :keyword:`with`
  supports multiple context managers syntactically now.

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


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 now works correctly when the thread being
  debugged doesn't hold the GIL; the macro now acquires 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`.)

* New function: :cfunc:`PyCode_NewEmpty` creates an empty code object;
  only the filename, function name, and first line number are required.
  This is useful to extension modules that are attempting to
  construct a more useful traceback stack.  Previously such
  extensions needed to call :cfunc:`PyCode_New`, which had many
  more arguments.  (Added by Jeffrey Yasskin.)

* New function: :cfunc:`PyErr_NewExceptionWithDoc` creates a new
  exception class, just as the existing :cfunc:`PyErr_NewException` does,
  but takes an extra ``char *`` argument containing the docstring for the
  new exception class.  (Added by the 'lekma' user on the Python bug tracker;
  :issue:`7033`.)

* New function: :cfunc:`PyFrame_GetLineNumber` takes a frame object
  and returns the line number that the frame is currently executing.
  Previously code would need to get the index of the bytecode
  instruction currently executing, and then look up the line number
  corresponding to that address.  (Added by Jeffrey Yasskin.)

* New function: :cfunc:`PyLong_AsLongAndOverflow` approximates a Python long
  integer as a C :ctype:`long`.  If the number is too large to fit into
  a :ctype:`long`, an *overflow* flag is set and returned to the caller.
  (Contributed by Case Van Horsen; :issue:`7528`.)

* New macros: the Python header files now define the following macros:
  :cmacro:`Py_ISALNUM`,
  :cmacro:`Py_ISALPHA`,
  :cmacro:`Py_ISDIGIT`,
  :cmacro:`Py_ISLOWER`,
  :cmacro:`Py_ISSPACE`,
  :cmacro:`Py_ISUPPER`,
  :cmacro:`Py_ISXDIGIT`,
  and :cmacro:`Py_TOLOWER`, :cmacro:`Py_TOUPPER`.
  All of these functions are analogous to the C
  standard macros for classifying characters, but ignore the current
  locale setting, because in
  several places Python needs to analyze characters in a
  locale-independent way.  (Added by Eric Smith;
  :issue:`5793`.)

  .. XXX these macros don't seem to be described in the c-api docs.

* New format codes: the :cfunc:`PyFormat_FromString`,
  :cfunc:`PyFormat_FromStringV`, and :cfunc:`PyErr_Format` now
  accepts ``%lld`` and ``%llu`` format codes for displaying values of
  C's :ctype:`long long` types.
  (Contributed by Mark Dickinson; :issue:`7228`.)

* The complicated interaction between threads and process forking has
  been changed.  Previously, the child process created by
  :func:`os.fork` might fail because the child is created with only a
  single thread running, the thread performing the :func:`os.fork`.
  If other threads were holding a lock, such as Python's import lock,
  when the fork was performed, the lock would still be marked as
  "held" in the new process.  But in the child process nothing would
  ever release the lock, since the other threads weren't replicated,
  and the child process would no longer be able to perform imports.

  Python 2.7 now acquires the import lock before performing an
  :func:`os.fork`, and will also clean up any locks created using the
  :mod:`threading` module.  C extension modules that have internal
  locks, or that call :cfunc:`fork()` themselves, will not benefit
  from this clean-up.

  (Fixed by Thomas Wouters; :issue:`1590864`.)

* Global symbols defined by the :mod:`ctypes` module are now prefixed
  with ``Py``, or with ``_ctypes``.  (Implemented by Thomas
  Heller; :issue:`3102`.)

* 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`.)

* The build process now creates the necessary files for pkg-config
  support.  (Contributed by Clinton Roy; :issue:`3585`.)

* The build process now supports Subversion 1.7.  (Contributed by
  Arfrever Frehtes Taifersar Arahesis; :issue:`6094`.)

* Compiling Python with the :option:`--with-valgrind` option will now
  disable the pymalloc allocator, which is difficult for the Valgrind to
  analyze correctly.  Valgrind will therefore be better at detecting
  memory leaks and overruns. (Contributed by James Henstridge; :issue:`2422`.)


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

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`.)

* The :func:`os.listdir` function now correctly fails
  for an empty path.  (Fixed by Hirokazu Yamamoto; :issue:`5913`.)

* The :mod:`mimelib` module will now read the MIME database from
  the Windows registry when initializing.
  (Patch by Gabriel Genellina; :issue:`4969`.)

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

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

* The path ``/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 the resulting code objects are overwritten when the
  original filename is obsolete.  This 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 reports the seed that was used
  (Added by Collin Winter.)

* The :file:`regrtest.py` script now takes a :option:`-j` switch
  that takes an integer specifying how many tests run in parallel. This
  allows reducing the total runtime on multi-core machines.
  This option is compatible with several other options, including the
  :option:`-R` switch which is known to produce long runtimes.
  (Added by Antoine Pitrou, :issue:`6152`.)

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

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

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

* When using :class:`Decimal` instances with a string's
  :meth:`format` method, the default alignment was previously
  left-alignment.  This has been changed to right-alignment, which might
  change the output of your programs.
  (Changed by Mark Dickinson; :issue:`6857`.)

  Another :meth:`format`-related change: the default precision used
  for floating-point and complex numbers was changed from 6 decimal
  places to 12, which matches the precision used by :func:`str`.
  (Changed by Eric Smith; :issue:`5920`.)

* Because of an optimization for the :keyword:`with` statement, the special
  methods :meth:`__enter__` and :meth:`__exit__` must belong to the object's
  type, and cannot be directly attached to the object's instance.  This
  affects new-style classes (derived from :class:`object`) and C extension
  types.  (:issue:`6101`.)

* The :meth:`readline` method of :class:`StringIO` objects now does
  nothing when a negative length is requested, as other file-like
  objects do.  (:issue:`7348`).

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


.. _acks27:

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

The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: Ryan Lovett, Hugh Secker-Walker.