#6398 typo: versio. -> version.

[python.git] / 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:

* :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`.)

* 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 will now approximate 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`.)

* 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`.)

* 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 :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.  (Added by Raymond Hettinger.)

* In Distutils, :func:`distutils.sdist.add_defaults` now uses
  *package_dir* and *data_files* to create the MANIFEST file.
  :mod:`distutils.sysconfig` will now read the :envvar:`AR`
  environment variable.

  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`.)

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

* 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 :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:`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:`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.)

* 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:`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 XXX; :issue:`1674032`.)

* The :mod:`unittest` module was enhanced in several ways.
  The progress messages will now show '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`.)

  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*.

  * 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.

* 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
------------------------------

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 user 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.

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


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`.)

* 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`.)

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

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 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 now 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 to shorten 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:

* 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`.)

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


.. _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.