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

Author:
   A.M. Kuchling (amk at amk.ca)

Release:
   3.1

Date:
   June 26, 2009

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


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 *-3* switch now automatically enables the
*-Qwarn* switch that causes warnings about using classic division with
integers and long integers.


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

XXX write this

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


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

Some smaller changes made to the core Python language are:

* ``str.format()`` method now supports automatic numbering of the
  replacement fields.  This makes using ``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 ``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 ``int()`` and ``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 ``bytearray`` type's ``translate()`` method will now accept
  ``None`` as its first argument.  (Fixed by Georg Brandl; issue
  4759.)


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

Several performance enhancements have been added:

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

* The garbage collector tries to avoid tracking simple containers
  which can't be part of a cycle. 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
  *--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
``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 ``bz2`` module's ``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 ``Counter`` class in the ``collections`` module is
  useful for tallying data.  ``Counter`` instances behave mostly like
  dictionaries but return zero for missing keys instead of raising a
  ``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 ``Counter`` methods: ``most_common()``
  returns the N most common elements and their counts, and
  ``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 ``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 ``deque`` data type now exposes its maximum length as the read-
  only ``maxlen`` attribute.  (Added by Raymond Hettinger.)

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

  It is no longer mandatory to store clear-text passwords in the
  ``.pypirc`` file when registering and uploading packages to PyPI. As
  long as the username is present in that file, the ``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 ``Decimal`` class gained a ``from_float()`` class
  method that performs an exact conversion of a floating-point number
  to a ``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('0.1000000000000000055
  511151231257827021181583404541015625')``. (Implemented by Raymond
  Hettinger; issue 4796.)

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

* New function: the ``gc`` module's ``is_tracked()`` returns true if a
  given instance is tracked by the garbage collector, false otherwise.
  (Contributed by Antoine Pitrou; issue 4688.)

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

  ``itertools.combinations()`` and ``itertools.product()`` were
  previously raising ``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 ``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 ``OrderedDict`` type, ``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 ``multiprocessing`` module's ``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 ``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 ``re`` module's ``split()``, ``sub()``, and ``subn()`` now
  accept an optional *flags* argument, for consistency with the other
  functions in the module.  (Added by Gregory P. Smith.)

* New function: the ``subprocess`` module's ``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
  ``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: ``is_declared_global()`` in the ``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 ``threading`` module's ``Event.wait()`` method now returns the
  internal flag on exit.  This means the method will usually return
  true because ``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 ``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 ``SkipTest`` exception
  to skip a test. (issue 1034053.)

  The error messages for ``assertEqual()``, ``assertTrue()``, and
  ``assertFalse()`` failures now provide more information.  If you set
  the ``longMessage`` attribute of your ``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 ``assertRaises()`` and ``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 ``addCleanup()`` and ``doCleanups()`` were added.
  ``addCleanup()`` allows you to add cleanup functions that will be
  called unconditionally (after ``setUp()`` if ``setUp()`` fails,
  otherwise after ``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 ``unittest``.

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

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

  * ``assertGreater()``, ``assertGreaterEqual()``, ``assertLess()``,
    and ``assertLessEqual()`` compare two quantities.

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

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

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

  * ``assertIn()`` and ``assertNotIn()`` tests whether *first* is or
    is not in  *second*.

  * ``assertSameElements()`` tests whether two provided sequences
    contain the same elements.

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

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

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

  * A new hook, ``addTypeEqualityFunc()`` takes a type object and a
    function.  The ``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.

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

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

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

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


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

Python 3.1 includes the ``importlib`` package, a re-implementation of
the logic underlying Python's ``import`` statement. ``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 ``importlib`` package, but
instead has a tiny subset that contains a single function,
``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.
``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'>

``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 ``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 ``.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.)

* ``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 ``ctypes`` module are now prefixed
  with ``Py`, or with ``_ctypes``.  (Implemented by Thomas Heller;
  issue 3102.)

* The **configure** script now checks for floating-point rounding bugs
  on certain 32-bit Intel chips and defines a ``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 ``msvcrt`` module now contains some constants from the
  ``crtassem.h`` header file: ``CRT_ASSEMBLY_VERSION``,
  ``VC_ASSEMBLY_PUBLICKEYTOKEN``, and
  ``LIBRARIES_ASSEMBLY_NAME_PREFIX``. (Contributed by David
  Cournapeau; issue 4365.)

* The new ``_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 ``.pyc`` or ``.pyo`` file with an
  existing ``.py`` counterpart, the ``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 ``regrtest.py`` script now takes a *--randseed=* switch that
  takes an integer that will be used as the random seed for the *-r*
  option that executes tests in random order. The *-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:

* Because of an optimization for the ``with`` statement, the special
  methods ``__enter__()`` and ``__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 ``object``)
  and C extension types.  (issue 6101.)


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.
