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

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

Release:
   2.7.3

Date:
   April 09, 2012

This article explains the new features in Python 2.7.  Python 2.7 was
released on July 3, 2010.

Numeric handling has been improved in many ways, for both floating-
point numbers and for the ``Decimal`` class. There are some useful
additions to the standard library, such as a greatly enhanced
``unittest`` module, the ``argparse`` module for parsing command-line
options, convenient ``OrderedDict`` and ``Counter`` classes in the
``collections`` module, and many other improvements.

Python 2.7 is planned to be the last of the 2.x releases, so we worked
on making it a good release for the long term.  To help with porting
to Python 3, several new features from the Python 3.x series have been
included in 2.7.

This article doesn't attempt to provide a complete specification of
the new features, but instead provides a convenient overview.  For
full details, you should refer to the documentation for Python 2.7 at
http://docs.python.org. If you want to understand the rationale for
the design and implementation, refer to the PEP for a particular new
feature or the issue on http://bugs.python.org in which a change was
discussed.  Whenever possible, "What's New in Python" links to the
bug/patch item for each change.


The Future for Python 2.x
=========================

Python 2.7 is intended to be the last major release in the 2.x series.
The Python maintainers are planning to focus their future efforts on
the Python 3.x series.

This means that 2.7 will remain in place for a long time, running
production systems that have not been ported to Python 3.x. Two
consequences of the long-term significance of 2.7 are:

* It's very likely the 2.7 release will have a longer period of
  maintenance compared to earlier 2.x versions.  Python 2.7 will
  continue to be maintained while the transition to 3.x continues, and
  the developers are planning to support Python 2.7 with bug-fix
  releases beyond the typical two years.

* A policy decision was made to silence warnings only of interest to
  developers.  ``DeprecationWarning`` and its descendants are now
  ignored unless otherwise requested, preventing users from seeing
  warnings triggered by an application.  This change was also made in
  the branch that will become Python 3.2. (Discussed on stdlib-sig and
  carried out in issue 7319.)

  In previous releases, ``DeprecationWarning`` messages were enabled
  by default, providing Python developers with a clear indication of
  where their code may break in a future major version of Python.

  However, there are increasingly many users of Python-based
  applications who are not directly involved in the development of
  those applications.  ``DeprecationWarning`` messages are irrelevant
  to such users, making them worry about an application that's
  actually working correctly and burdening application developers with
  responding to these concerns.

  You can re-enable display of ``DeprecationWarning`` messages by
  running Python with the *-Wdefault* (short form: *-Wd*) switch, or
  by setting the ``PYTHONWARNINGS`` environment variable to
  ``"default"`` (or ``"d"``) before running Python.  Python code can
  also re-enable them by calling ``warnings.simplefilter('default')``.


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:

* The syntax for set literals (``{1,2,3}`` is a mutable set).

* Dictionary and set comprehensions (``{i: i*2 for i in range(3)}``).

* Multiple context managers in a single ``with`` statement.

* A new version of the ``io`` library, rewritten in C for performance.

* The ordered-dictionary type described in *PEP 372: Adding an Ordered
  Dictionary to collections*.

* The new ``","`` format specifier described in *PEP 378: Format
  Specifier for Thousands Separator*.

* The ``memoryview`` object.

* A small subset of the ``importlib`` module, described below.

* The ``repr()`` of a float ``x`` is shorter in many cases: it's now
  based on the shortest decimal string that's guaranteed to round back
  to ``x``.  As in previous versions of Python, it's guaranteed that
  ``float(repr(x))`` recovers ``x``.

* Float-to-string and string-to-float conversions are correctly
  rounded. The ``round()`` function is also now correctly rounded.

* The ``PyCapsule`` type, used to provide a C API for extension
  modules.

* The ``PyLong_AsLongAndOverflow()`` C API function.

Other new Python3-mode warnings include:

* ``operator.isCallable()`` and ``operator.sequenceIncludes()``, which
  are not supported in 3.x, now trigger warnings.

* 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
====================================================

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, 2.7
introduces a new ``OrderedDict`` class in the ``collections`` module.

The ``OrderedDict`` API provides the same interface as regular
dictionaries but iterates 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 ``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(last=False)
   (0, 0)
   >>> od.popitem(last=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 ``OrderedDict`` with a regular dictionary ignores the
insertion order and just compares the keys and values.

How does the ``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).

The standard library now supports use of ordered dictionaries in
several modules.

* The ``ConfigParser`` module uses them by default, meaning that
  configuration files can now be read, modified, and then written back
  in their original order.

* The ``_asdict()`` method for ``collections.namedtuple()`` now
  returns an ordered dictionary with the values appearing in the same
  order as the underlying tuple indices.

* The ``json`` module's ``JSONDecoder`` class constructor was extended
  with an *object_pairs_hook* parameter to allow ``OrderedDict``
  instances to be built by the decoder. Support was also added for
  third-party tools like PyYAML.

See also:

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


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

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

The fully general solution for doing this is the ``locale`` module,
which can use different separators ("," in North America, "." in
Europe) and different grouping sizes, but ``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 ``str.format()`` method.  When formatting a
floating-point number, simply include a comma between the width and
the precision:

   >>> '{:20,.2f}'.format(18446744073709551616.0)
   '18,446,744,073,709,551,616.00'

When formatting an integer, include the comma after the width:

>>> '{:20,d}'.format(18446744073709551616)
'18,446,744,073,709,551,616'

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 ``locale`` module,
but it's easier to use.

See also:

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


PEP 389: The argparse Module for Parsing Command Lines
======================================================

The ``argparse`` module for parsing command-line arguments was added
as a more powerful replacement for the ``optparse`` module.

This means Python now supports three different modules for parsing
command-line arguments: ``getopt``, ``optparse``, and ``argparse``.
The ``getopt`` module closely resembles the C library's ``getopt()``
function, so it remains useful if you're writing a Python prototype
that will eventually be rewritten in C. ``optparse`` becomes
redundant, but there are no plans to remove it because there are many
scripts still using it, and there's no automated way to update these
scripts.  (Making the ``argparse`` API consistent with ``optparse``'s
interface was discussed but rejected as too messy and difficult.)

In short, if you're writing a new script and don't need to worry about
compatibility with earlier versions of Python, use ``argparse``
instead of ``optparse``.

Here's an example:

   import argparse

   parser = argparse.ArgumentParser(description='Command-line example.')

   # Add optional switches
   parser.add_argument('-v', action='store_true', dest='is_verbose',
                       help='produce verbose output')
   parser.add_argument('-o', action='store', dest='output',
                       metavar='FILE',
                       help='direct output to FILE instead of stdout')
   parser.add_argument('-C', action='store', type=int, dest='context',
                       metavar='NUM', default=0,
                       help='display NUM lines of added context')

   # Allow any number of additional arguments.
   parser.add_argument(nargs='*', action='store', dest='inputs',
                       help='input filenames (default is stdin)')

   args = parser.parse_args()
   print args.__dict__

Unless you override it, *-h* and *--help* switches are automatically
added, and produce neatly formatted output:

   -> ./python.exe argparse-example.py --help
   usage: argparse-example.py [-h] [-v] [-o FILE] [-C NUM] [inputs [inputs ...]]

   Command-line example.

   positional arguments:
     inputs      input filenames (default is stdin)

   optional arguments:
     -h, --help  show this help message and exit
     -v          produce verbose output
     -o FILE     direct output to FILE instead of stdout
     -C NUM      display NUM lines of added context

As with ``optparse``, the command-line switches and arguments are
returned as an object with attributes named by the *dest* parameters:

   -> ./python.exe argparse-example.py -v
   {'output': None,
    'is_verbose': True,
    'context': 0,
    'inputs': []}

   -> ./python.exe argparse-example.py -v -o /tmp/output -C 4 file1 file2
   {'output': '/tmp/output',
    'is_verbose': True,
    'context': 4,
    'inputs': ['file1', 'file2']}

``argparse`` has much fancier validation than ``optparse``; you can
specify an exact number of arguments as an integer, 0 or more
arguments by passing ``'*'``, 1 or more by passing ``'+'``, or an
optional argument with ``'?'``.  A top-level parser can contain sub-
parsers to define subcommands that have different sets of switches, as
in ``svn commit``, ``svn checkout``, etc.  You can specify an
argument's type as ``FileType``, which will automatically open files
for you and understands that ``'-'`` means standard input or output.

See also:

   ``argparse`` documentation
      The documentation page of the argparse module.

   *Upgrading optparse code*
      Part of the Python documentation, describing how to convert code
      that uses ``optparse``.

   **PEP 389** - argparse - New Command Line Parsing Module
      PEP written and implemented by Steven Bethard.


PEP 391: Dictionary-Based Configuration For Logging
===================================================

The ``logging`` module is very flexible; applications can define a
tree of logging subsystems, and each logger in this tree can filter
out certain messages, format them differently, and direct messages to
a varying number of handlers.

All this flexibility can require a lot of configuration.  You can
write Python statements to create objects and set their properties,
but a complex set-up requires verbose but boring code. ``logging``
also supports a ``fileConfig()`` function that parses a file, but the
file format doesn't support configuring filters, and it's messier to
generate programmatically.

Python 2.7 adds a ``dictConfig()`` function that uses a dictionary to
configure logging.  There are many ways to produce a dictionary from
different sources: construct one with code; parse a file containing
JSON; or use a YAML parsing library if one is installed.  For more
information see *Configuration functions*.

The following example configures two loggers, the root logger and a
logger named "network".  Messages sent to the root logger will be sent
to the system log using the syslog protocol, and messages to the
"network" logger will be written to a ``network.log`` file that will
be rotated once the log reaches 1MB.

   import logging
   import logging.config

   configdict = {
    'version': 1,    # Configuration schema in use; must be 1 for now
    'formatters': {
        'standard': {
            'format': ('%(asctime)s %(name)-15s '
                       '%(levelname)-8s %(message)s')}},

    'handlers': {'netlog': {'backupCount': 10,
                        'class': 'logging.handlers.RotatingFileHandler',
                        'filename': '/logs/network.log',
                        'formatter': 'standard',
                        'level': 'INFO',
                        'maxBytes': 1000000},
                 'syslog': {'class': 'logging.handlers.SysLogHandler',
                            'formatter': 'standard',
                            'level': 'ERROR'}},

    # Specify all the subordinate loggers
    'loggers': {
                'network': {
                            'handlers': ['netlog']
                }
    },
    # Specify properties of the root logger
    'root': {
             'handlers': ['syslog']
    },
   }

   # Set up configuration
   logging.config.dictConfig(configdict)

   # As an example, log two error messages
   logger = logging.getLogger('/')
   logger.error('Database not found')

   netlogger = logging.getLogger('network')
   netlogger.error('Connection failed')

Three smaller enhancements to the ``logging`` module, all implemented
by Vinay Sajip, are:

* The ``SysLogHandler`` class now supports syslogging over TCP.  The
  constructor has a *socktype* parameter giving the type of socket to
  use, either ``socket.SOCK_DGRAM`` for UDP or ``socket.SOCK_STREAM``
  for TCP.  The default protocol remains UDP.

* ``Logger`` instances gained a ``getChild()`` method that retrieves a
  descendant logger using a relative path. For example, once you
  retrieve a logger by doing ``log = getLogger('app')``, calling
  ``log.getChild('network.listen')`` is equivalent to
  ``getLogger('app.network.listen')``.

* The ``LoggerAdapter`` class gained a ``isEnabledFor()`` method that
  takes a *level* and returns whether the underlying logger would
  process a message of that level of importance.

See also:

   **PEP 391** - Dictionary-Based Configuration For Logging
      PEP written and implemented by Vinay Sajip.


PEP 3106: Dictionary Views
==========================

The dictionary methods ``keys()``, ``values()``, and ``items()`` are
different in Python 3.x.  They return an object called a *view*
instead of a fully materialized list.

It's not possible to change the return values of ``keys()``,
``values()``, and ``items()`` in Python 2.7 because too much code
would break.  Instead the 3.x versions were added under the new names
``viewkeys()``, ``viewvalues()``, and ``viewitems()``.

   >>> d = dict((i*10, chr(65+i)) for i in range(26))
   >>> d
   {0: 'A', 130: 'N', 10: 'B', 140: 'O', 20: ..., 250: 'Z'}
   >>> d.viewkeys()
   dict_keys([0, 130, 10, 140, 20, 150, 30, ..., 250])

Views can be iterated over, but the key and item views also behave
like sets.  The ``&`` operator performs intersection, and ``|``
performs a union:

   >>> d1 = dict((i*10, chr(65+i)) for i in range(26))
   >>> d2 = dict((i**.5, i) for i in range(1000))
   >>> d1.viewkeys() & d2.viewkeys()
   set([0.0, 10.0, 20.0, 30.0])
   >>> d1.viewkeys() | range(0, 30)
   set([0, 1, 130, 3, 4, 5, 6, ..., 120, 250])

The view keeps track of the dictionary and its contents change as the
dictionary is modified:

   >>> vk = d.viewkeys()
   >>> vk
   dict_keys([0, 130, 10, ..., 250])
   >>> d[260] = '&'
   >>> vk
   dict_keys([0, 130, 260, 10, ..., 250])

However, note that you can't add or remove keys while you're iterating
over the view:

   >>> for k in vk:
   ...     d[k*2] = k
   ...
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   RuntimeError: dictionary changed size during iteration

You can use the view methods in Python 2.x code, and the 2to3
converter will change them to the standard ``keys()``, ``values()``,
and ``items()`` methods.

See also:

   **PEP 3106** - Revamping dict.keys(), .values() and .items()
      PEP written by Guido van Rossum. Backported to 2.7 by Alexandre
      Vassalotti; issue 1967.


PEP 3137: The memoryview Object
===============================

The ``memoryview`` object provides a view of another object's memory
content that matches the ``bytes`` type's interface.

>>> import string
>>> m = memoryview(string.letters)
>>> m
<memory at 0x37f850>
>>> len(m)           # Returns length of underlying object
52
>>> m[0], m[25], m[26]   # Indexing returns one byte
('a', 'z', 'A')
>>> m2 = m[0:26]         # Slicing returns another memoryview
>>> m2
<memory at 0x37f080>

The content of the view can be converted to a string of bytes or a
list of integers:

>>> m2.tobytes()
'abcdefghijklmnopqrstuvwxyz'
>>> m2.tolist()
[97, 98, 99, 100, 101, 102, 103, ... 121, 122]
>>>

``memoryview`` objects allow modifying the underlying object if it's a
mutable object.

>>> m2[0] = 75
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot modify read-only memory
>>> b = bytearray(string.letters)  # Creating a mutable object
>>> b
bytearray(b'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
>>> mb = memoryview(b)
>>> mb[0] = '*'         # Assign to view, changing the bytearray.
>>> b[0:5]              # The bytearray has been changed.
bytearray(b'*bcde')
>>>

See also:

   **PEP 3137** - Immutable Bytes and Mutable Buffer
      PEP written by Guido van Rossum. Implemented by Travis Oliphant,
      Antoine Pitrou and others. Backported to 2.7 by Antoine Pitrou;
      issue 2396.


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

Some smaller changes made to the core Python language are:

* The syntax for set literals has been backported from Python 3.x.
  Curly brackets are used to surround the contents of the resulting
  mutable set; set literals are distinguished from dictionaries by not
  containing colons and values. ``{}`` continues to represent an empty
  dictionary; use ``set()`` for an empty set.

  >>> {1, 2, 3, 4, 5}
  set([1, 2, 3, 4, 5])
  >>> set() # empty set
  set([])
  >>> {}    # empty dict
  {}

  Backported by Alexandre Vassalotti; issue 2335.

* Dictionary and set comprehensions are another feature backported
  from 3.x, generalizing list/generator comprehensions to use the
  literal syntax for sets and dictionaries.

  >>> {x: x*x for x in range(6)}
  {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  >>> {('a'*x) for x in range(6)}
  set(['', 'a', 'aa', 'aaa', 'aaaa', 'aaaaa'])

  Backported by Alexandre Vassalotti; issue 2333.

* The ``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 ``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 ``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: ``str()`` on floats and complex numbers; the
  ``float`` and ``complex`` constructors; numeric formatting;
  serializing and deserializing floats and complex numbers using the
  ``marshal``, ``pickle`` and ``json`` modules; parsing of float and
  imaginary literals in Python code; and ``Decimal``-to-float
  conversion.

  Related to this, the ``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.  You can find out which code is being used by
  checking ``sys.float_repr_style``,  which will be ``short`` if the
  new code is in use and ``legacy`` if it isn't.

  Implemented by Eric Smith and Mark Dickinson, using David Gay's
  ``dtoa.c`` library; issue 7117.

* 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.)

* Implicit coercion for complex numbers has been removed; the
  interpreter will no longer ever attempt to call a ``__coerce__()``
  method on complex objects.  (Removed by Meador Inge and Mark
  Dickinson; issue 5211.)

* The ``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.)

  Complex numbers now correctly support usage with ``format()``, and
  default to being right-aligned. 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 and issue 7988.)

  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.)

  A low-level change: the ``object.__format__()`` method now triggers
  a ``PendingDeprecationWarning`` if it's passed a format string,
  because the ``__format__()`` method for ``object`` converts the
  object to a string representation and formats that.  Previously the
  method silently applied the format string to the string
  representation, but that could hide mistakes in Python code.  If
  you're supplying formatting information such as an alignment or
  precision, presumably you're expecting the formatting to be applied
  in some object-specific way.  (Fixed by Eric Smith; issue 7994.)

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

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

* The ``import`` statement will no longer try an absolute import if a
  relative import (e.g. ``from .os import sep``) fails.  This fixes a
  bug, but could possibly break certain ``import`` statements that
  were only working by accident.  (Fixed by Meador Inge; issue 7902.)

* It's now possible for a subclass of the built-in ``unicode`` type to
  override the ``__unicode__()`` method.  (Implemented by Victor
  Stinner; issue 1583863.)

* The ``bytearray`` type's ``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 ``__func__`` attribute. (Contributed by Amaury
  Forgeot d'Arc, after a suggestion by George Sakkis; issue 5982.)

* When a restricted set of attributes were set using ``__slots__``,
  deleting an unset attribute would not raise ``AttributeError`` as
  you would expect.  Fixed by Benjamin Peterson; issue 7604.)

* Two new encodings are now supported: "cp720", used primarily for
  Arabic text; and "cp858", a variant of CP 850 that adds the euro
  symbol.  (CP720 contributed by Alexander Belchenko and Amaury
  Forgeot d'Arc in issue 1616979; CP858 contributed by Tim Hatch in
  issue 8016.)

* The ``file`` object will now set the ``filename`` attribute on the
  ``IOError`` exception when trying to open a directory on POSIX
  platforms (noted by Jan Kaliszewski; issue 4764), and now explicitly
  checks for and forbids writing to read-only file objects instead of
  trusting the C library to catch and report the error (fixed by
  Stefan Krah; issue 5677).

* The Python tokenizer now translates line endings itself, so the
  ``compile()`` built-in function now accepts code using any line-
  ending convention.  Additionally, it no longer requires that the
  code end in a newline.

* 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.)

* It's now possible to create weak references to old-style class
  objects.  New-style classes were always weak-referenceable.  (Fixed
  by Antoine Pitrou; issue 8268.)

* When a module object is garbage-collected, the module's dictionary
  is now only cleared if no one else is holding a reference to the
  dictionary (issue 7140).


Interpreter Changes
-------------------

A new environment variable, ``PYTHONWARNINGS``, allows controlling
warnings.  It should be set to a string containing warning settings,
equivalent to those used with the *-W* switch, separated by commas.
(Contributed by Brian Curtin; issue 7301.)

For example, the following setting will print warnings every time they
occur, but turn warnings from the ``Cookie`` module into an error.
(The exact syntax for setting an environment variable varies across
operating systems and shells.)

   export PYTHONWARNINGS=all,error:::Cookie:0


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

Several performance enhancements have been added:

* A new opcode was added to perform the initial setup for ``with``
  statements, looking up the ``__enter__()`` and ``__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
  only performs 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 Löwis 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
  *--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.)
  Bitwise operations are also significantly faster (initial patch by
  Gregory Smith; issue 1087418).

* 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.)

* 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.)

* The ``split()``, ``replace()``, ``rindex()``, ``rpartition()``, and
  ``rsplit()`` methods of string-like types (strings, Unicode strings,
  and ``bytearray`` objects) now use a fast reverse-search algorithm
  instead of a character-by-character scan.  This is sometimes faster
  by a factor of 10.  (Added by Florent Xicluna; issue 7462 and issue
  7622.)

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


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
``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 ``bdb`` module's base debugging 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 ``binascii`` module now supports the buffer API, so it can be
  used with ``memoryview`` instances and other similar buffer objects.
  (Backported from 3.x by Florent Xicluna; issue 7703.)

* Updated module: the ``bsddb`` module has been updated from
  4.7.2devel9 to version 4.8.4 of the pybsddb package. The new version
  features better Python 3.x compatibility, various bug fixes, and
  adds several new BerkeleyDB flags and methods. (Updated by Jesús Cea
  Avión; issue 8156.  The pybsddb changelog can be read at
  http://hg.jcea.es/pybsddb/file/tip/ChangeLog.)

* The ``bz2`` module's ``BZ2File`` now supports the context management
  protocol, so you can write ``with bz2.BZ2File(...) as f:``.
  (Contributed by Hagen Fürstenau; 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 three additional ``Counter`` methods. ``most_common()``
  returns the N most common elements and their counts.  ``elements()``
  returns an iterator over the contained elements, repeating each
  element as many times as its count. ``subtract()`` takes an iterable
  and subtracts one for each element instead of adding; if the
  argument is a dictionary or another ``Counter``, the counts are
  subtracted.

     >>> 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'
     >>> c['e']
     5
     >>> c.subtract('very heavy on the letter e')
     >>> c['e']    # Count is now lower
     -1

  Contributed by Raymond Hettinger; issue 1696199.

  New class: ``OrderedDict`` is described in the earlier section *PEP
  372: Adding an Ordered Dictionary to collections*.

  New method: The ``deque`` data type now has a ``count()`` method
  that returns the number of contained elements equal to the supplied
  argument *x*, and a ``reverse()`` method that reverses the elements
  of the deque in-place.  ``deque`` also exposes its maximum length as
  the read-only ``maxlen`` attribute. (Both features added by Raymond
  Hettinger.)

  The ``namedtuple`` class now has an optional *rename* parameter. If
  *rename* is true, field names that are invalid because they've been
  repeated or 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.)

  Finally, the ``Mapping`` abstract base class now returns
  ``NotImplemented`` if a mapping is compared to another type that
  isn't a ``Mapping``. (Fixed by Daniel Stutzbach; issue 8729.)

* Constructors for the parsing classes in the ``ConfigParser`` module
  now take a *allow_no_value* parameter, defaulting to false; if true,
  options without values will be allowed.  For example:

     >>> import ConfigParser, StringIO
     >>> sample_config = """
     ... [mysqld]
     ... user = mysql
     ... pid-file = /var/run/mysqld/mysqld.pid
     ... skip-bdb
     ... """
     >>> config = ConfigParser.RawConfigParser(allow_no_value=True)
     >>> config.readfp(StringIO.StringIO(sample_config))
     >>> config.get('mysqld', 'user')
     'mysql'
     >>> print config.get('mysqld', 'skip-bdb')
     None
     >>> print config.get('mysqld', 'unknown')
     Traceback (most recent call last):
       ...
     NoOptionError: No option 'unknown' in section: 'mysqld'

  (Contributed by Mats Kindahl; issue 7005.)

* Deprecated function: ``contextlib.nested()``, which allows handling
  more than one context manager with a single ``with`` statement, has
  been deprecated, because the ``with`` statement now supports
  multiple context managers.

* The ``cookielib`` module now ignores cookies that have an invalid
  version field, one that doesn't contain an integer value.  (Fixed by
  John J. Lee; issue 3924.)

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

* The ``ctypes`` module now always converts ``None`` to a C NULL
  pointer for arguments declared as pointers.  (Changed by Thomas
  Heller; issue 4606.)  The underlying libffi library has been updated
  to version 3.0.9, containing various fixes for different platforms.
  (Updated by Matthias Klose; issue 8142.)

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

* New method: the ``Decimal`` class gained a ``from_float()`` class
  method that performs an exact conversion of a floating-point number
  to a ``Decimal``. This exact conversion 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.)

  Comparing instances of ``Decimal`` with floating-point numbers now
  produces sensible results based on the numeric values of the
  operands.  Previously such comparisons would fall back to Python's
  default rules for comparing objects, which produced arbitrary
  results based on their type.  Note that you still cannot combine
  ``Decimal`` and floating-point in other operations such as addition,
  since you should be explicitly choosing how to convert between float
  and ``Decimal``.  (Fixed by Mark Dickinson; issue 2531.)

  The constructor for ``Decimal`` now accepts floating-point numbers
  (added by Raymond Hettinger; issue 8257) and non-European Unicode
  characters such as Arabic-Indic digits (contributed by Mark
  Dickinson; issue 6595).

  Most of the methods of the ``Context`` class now accept integers as
  well as ``Decimal`` instances; the only exceptions are the
  ``canonical()`` and ``is_canonical()`` methods.  (Patch by Juan José
  Conti; issue 7633.)

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

  Comparisons involving a signaling NaN value (or ``sNAN``) now signal
  ``InvalidOperation`` instead of silently returning a true or false
  value depending on the comparison operator.  Quiet NaN values (or
  ``NaN``) are now hashable.  (Fixed by Mark Dickinson; issue 7279.)

* The ``difflib`` module now produces output that is more compatible
  with modern **diff**/**patch** tools through one small change, using
  a tab character instead of spaces as a separator in the header
  giving the filename.  (Fixed by Anatoly Techtonik; issue 7585.)

* The Distutils ``sdist`` command now always regenerates the
  ``MANIFEST`` file, since even if the ``MANIFEST.in`` or ``setup.py``
  files haven't been modified, the user might have created some new
  files that should be included. (Fixed by Tarek Ziadé; issue 8688.)

* The ``doctest`` module's ``IGNORE_EXCEPTION_DETAIL`` flag will now
  ignore the name of the module containing the exception being tested.
  (Patch by Lennart Regebro; issue 7490.)

* The ``email`` module's ``Message`` class will now accept a Unicode-
  valued payload, automatically converting the payload to the encoding
  specified by ``output_charset``. (Added by R. David Murray; issue
  1368247.)

* The ``Fraction`` class now accepts a single float or ``Decimal``
  instance, or two rational numbers, as arguments to its constructor.
  (Implemented by Mark Dickinson; rationals added in issue 5812, and
  float/decimal in issue 8294.)

  Ordering comparisons (``<``, ``<=``, ``>``, ``>=``) between
  fractions and complex numbers now raise a ``TypeError``. This fixes
  an oversight, making the ``Fraction`` match the other numeric types.

* New class: ``FTP_TLS`` in the ``ftplib`` module provides secure FTP
  connections using TLS encapsulation of authentication as well as
  subsequent control and data transfers. (Contributed by Giampaolo
  Rodola; issue 2054.)

  The ``storbinary()`` method for binary uploads can now restart
  uploads thanks to an added *rest* parameter (patch by Pablo Mouzo;
  issue 6845.)

* New class decorator: ``total_ordering()`` in the ``functools``
  module takes a class that defines an ``__eq__()`` method and one of
  ``__lt__()``, ``__le__()``, ``__gt__()``, or ``__ge__()``, and
  generates the missing comparison methods.  Since the ``__cmp__()``
  method is being deprecated in Python 3.x, this decorator makes it
  easier to define ordered classes. (Added by Raymond Hettinger; issue
  5479.)

  New function: ``cmp_to_key()`` will take an old-style comparison
  function that expects two arguments and return a new callable that
  can be used as the *key* parameter to functions such as
  ``sorted()``, ``min()`` and ``max()``, etc.  The primary intended
  use is to help with making code compatible with Python 3.x. (Added
  by Raymond Hettinger.)

* 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 Fürstenau; issue 3860), and it now
  implements the ``io.BufferedIOBase`` ABC, so you can wrap it with
  ``io.BufferedReader`` for faster processing (contributed by Nir
  Aides; issue 7471). It's also 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.)

  Files in gzip format can be padded with trailing zero bytes; the
  ``gzip`` module will now consume these trailing bytes.  (Fixed by
  Tadek Pietraszek and Brian Curtin; issue 2846.)

* New attribute: the ``hashlib`` module now has an ``algorithms``
  attribute containing a tuple naming the supported algorithms. In
  Python 2.7, ``hashlib.algorithms`` contains ``('md5', 'sha1',
  'sha224', 'sha256', 'sha384', 'sha512')``. (Contributed by Carl
  Chenet; issue 7418.)

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

  The ``HTTPConnection`` and ``HTTPSConnection`` classes now support a
  *source_address* parameter, a ``(host, port)`` 2-tuple giving the
  source address that will be used for the connection. (Contributed by
  Eldon Ziegler; issue 3972.)

* The ``ihooks`` module now supports relative imports.  Note that
  ``ihooks`` is an older module for customizing imports, superseded by
  the ``imputil`` module added in Python 2.0. (Relative import support
  added by Neil Schemenauer.)

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

* New function: the ``inspect`` module's ``getcallargs()`` takes a
  callable and its positional and keyword arguments, and figures out
  which of the callable's parameters will receive each argument,
  returning a dictionary mapping argument names to their values.  For
  example:

     >>> from inspect import getcallargs
     >>> def f(a, b=1, *pos, **named):
     ...     pass
     >>> getcallargs(f, 1, 2, 3)
     {'a': 1, 'b': 2, 'pos': (3,), 'named': {}}
     >>> getcallargs(f, a=2, x=4)
     {'a': 2, 'b': 1, 'pos': (), 'named': {'x': 4}}
     >>> getcallargs(f)
     Traceback (most recent call last):
     ...
     TypeError: f() takes at least 1 argument (0 given)

  Contributed by George Sakkis; issue 3135.

* Updated module: The ``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
  being performed.  The original Python version was renamed to the
  ``_pyio`` module.

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

  The ``io.FileIO`` class now raises an ``OSError`` when passed an
  invalid file descriptor.  (Implemented by Benjamin Peterson; issue
  4991.)  The ``truncate()`` method now preserves the file position;
  previously it would change the file position to the end of the new
  file.  (Fixed by Pascal Chambon; issue 6939.)

* 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()`` previously
  raised ``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.)

* Updated module: 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 ``collections.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 ``mailbox`` module's ``Maildir`` class now records the timestamp
  on the directories it reads, and only re-reads them if the
  modification time has subsequently changed.  This improves
  performance by avoiding unneeded directory scans.  (Fixed by A.M.
  Kuchling and Antoine Pitrou; issue 1607951, issue 6896.)

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

* 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 ``Pool`` class, which controls a pool of worker processes, now
  has an optional *maxtasksperchild* parameter.  Worker processes will
  perform the specified number of tasks and then exit, causing the
  ``Pool`` to start a new worker.  This is useful if tasks may leak
  memory or other resources, or if some tasks will cause the worker to
  become very large. (Contributed by Charles Cazabon; issue 6963.)

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

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

  The ``os.fork()`` function now re-initializes the import lock in the
  child process; this fixes problems on Solaris when ``fork()`` is
  called from a thread.  (Fixed by Zsolt Cserna; issue 7242.)

* In the ``os.path`` module, the ``normpath()`` and ``abspath()``
  functions now preserve Unicode; if their input path is a Unicode
  string, the return value is also a Unicode string. (``normpath()``
  fixed by Matt Giuca in issue 5827; ``abspath()`` fixed by Ezio
  Melotti in issue 3426.)

* 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: ``run_path()`` in the ``runpy`` module will execute
  the code at a provided *path* argument.  *path* can be the path of a
  Python source file (``example.py``), a compiled bytecode file
  (``example.pyc``), a directory (``./package/``), or a zip archive
  (``example.zip``).  If a directory or zip path is provided, it will
  be added to the front of ``sys.path`` and the module ``__main__``
  will be imported.  It's expected that the directory or zip contains
  a ``__main__.py``; if it doesn't, some other ``__main__.py`` might
  be imported from a location later in ``sys.path``.  This makes more
  of the machinery of ``runpy`` available to scripts that want to
  mimic the way Python's command line processes an explicit path name.
  (Added by Nick Coghlan; issue 6816.)

* New function: in the ``shutil`` module, ``make_archive()`` takes a
  filename, archive type (zip or tar-format), and a directory path,
  and creates an archive containing the directory's contents. (Added
  by Tarek Ziadé.)

  ``shutil``'s ``copyfile()`` and ``copytree()`` functions now raise a
  ``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.)

* The ``signal`` module no longer re-installs the signal handler
  unless this is truly necessary, which fixes a bug that could make it
  impossible to catch the EINTR signal robustly.  (Fixed by Charles-
  Francois Natali; issue 8354.)

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

  The ``site`` module now reports exceptions occurring when the
  ``sitecustomize`` module is imported, and will no longer catch and
  swallow the ``KeyboardInterrupt`` exception.  (Fixed by Victor
  Stinner; issue 3137.)

* The ``create_connection()`` function gained a *source_address*
  parameter, a ``(host, port)`` 2-tuple giving the source address that
  will be used for the connection. (Contributed by Eldon Ziegler;
  issue 3972.)

  The ``recv_into()`` and ``recvfrom_into()`` methods will now write
  into objects that support the buffer API, most usefully the
  ``bytearray`` and ``memoryview`` objects.  (Implemented by Antoine
  Pitrou; issue 8104.)

* The ``SocketServer`` module's ``TCPServer`` class now supports
  socket timeouts and disabling the Nagle algorithm. The
  ``disable_nagle_algorithm`` class attribute defaults to 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. The ``timeout`` class attribute can hold a
  timeout in seconds that will be applied to the request socket; if no
  request is received within that time, ``handle_timeout()`` will be
  called and ``handle_request()`` will return. (Contributed by
  Kristján Valur Jónsson; issue 6192 and issue 6267.)

* Updated module: the ``sqlite3`` module has been updated to version
  2.6.0 of the pysqlite package. Version 2.6.0 includes a number of
  bugfixes, and adds the ability to load SQLite extensions from shared
  libraries. Call the ``enable_load_extension(True)`` method to enable
  extensions, and then call ``load_extension()`` to load a particular
  shared library. (Updated by Gerhard Häring.)

* The ``ssl`` module's ``SSLSocket`` objects now support the buffer
  API, which fixed a test suite failure (fix by Antoine Pitrou; issue
  7133) and automatically set OpenSSL's ``SSL_MODE_AUTO_RETRY``, which
  will prevent an error code being returned from ``recv()`` operations
  that trigger an SSL renegotiation (fix by Antoine Pitrou; issue
  8222).

  The ``ssl.wrap_socket()`` constructor function now takes a *ciphers*
  argument that's a string listing the encryption algorithms to be
  allowed; the format of the string is described in the OpenSSL
  documentation. (Added by Antoine Pitrou; issue 8322.)

  Another change makes the extension load all of OpenSSL's ciphers and
  digest algorithms so that they're all available.  Some SSL
  certificates couldn't be verified, reporting an "unknown algorithm"
  error.  (Reported by Beda Kosata, and fixed by Antoine Pitrou; issue
  8484.)

  The version of OpenSSL being used is now available as the module
  attributes ``ssl.OPENSSL_VERSION`` (a string),
  ``ssl.OPENSSL_VERSION_INFO`` (a 5-tuple), and
  ``ssl.OPENSSL_VERSION_NUMBER`` (an integer).  (Added by Antoine
  Pitrou; issue 8321.)

* The ``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 ``struct.error``
  exception.  (Changed by Mark Dickinson; issue 1523.)  The ``pack()``
  function will also attempt to use ``__index__()`` to convert and
  pack non-integers before trying the ``__int__()`` method or
  reporting an error. (Changed by Mark Dickinson; issue 8300.)

* 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.)

  The ``subprocess`` module will now retry its internal system calls
  on receiving an ``EINTR`` signal.  (Reported by several people;
  final patch by Gregory P. Smith in issue 1068268.)

* 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 ``syslog`` module will now use the value of ``sys.argv[0]`` as
  the identifier instead of the previous default value of
  ``'python'``. (Changed by Sean Reifschneider; issue 8451.)

* 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.)

  ``sys.getwindowsversion()`` also returns a named tuple, with
  attributes named ``major``, ``minor``, ``build``, ``platform``,
  ``service_pack``, ``service_pack_major``, ``service_pack_minor``,
  ``suite_mask``, and ``product_type``.  (Contributed by Brian Curtin;
  issue 7766.)

* The ``tarfile`` module's default error handling has changed, to no
  longer suppress fatal errors.  The default error level was
  previously 0, which meant that errors would only result in a message
  being written to the debug log, but because the debug log is not
  activated by default, these errors go unnoticed.  The default error
  level is now 1, which raises an exception if there's an error.
  (Changed by Lars Gustäbel; issue 7357.)

  ``tarfile`` now supports filtering the ``TarInfo`` objects being
  added to a tar file.  When you call ``add()``, you may supply an
  optional *filter* argument that's a callable.  The *filter* callable
  will be passed the ``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 Gustäbel; issue 6856.) The ``TarFile``
  class also now supports the context manager protocol. (Added by Lars
  Gustäbel; issue 7232.)

* The ``wait()`` method of the ``threading.Event`` class 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
  Tim Lesher; issue 1674032.)

* The Unicode database provided by the ``unicodedata`` module is now
  used internally to determine which characters are numeric,
  whitespace, or represent line breaks.  The database also includes
  information from the ``Unihan.txt`` data file (patch by Anders
  Chrigström and Amaury Forgeot d'Arc; issue 1571184) and has been
  updated to version 5.2.0 (updated by Florent Xicluna; issue 8024).

* The ``urlparse`` module's ``urlsplit()`` now handles unknown URL
  schemes in a fashion compliant with **RFC 3986**: if the URL is of
  the form ``"<something>://..."``, the text before the ``://`` is
  treated as the scheme, even if it's a made-up scheme that the module
  doesn't know about.  This change may break code that worked around
  the old behaviour.  For example, Python 2.6.4 or 2.5 will return the
  following:

  >>> import urlparse
  >>> urlparse.urlsplit('invented://host/filename?query')
  ('invented', '', '//host/filename?query', '', '')

  Python 2.7 (and Python 2.6.5) will return:

  >>> import urlparse
  >>> urlparse.urlsplit('invented://host/filename?query')
  ('invented', 'host', '/filename?query', '', '')

  (Python 2.7 actually produces slightly different output, since it
  returns a named tuple instead of a standard tuple.)

  The ``urlparse`` module also supports IPv6 literal addresses as
  defined by **RFC 2732** (contributed by Senthil Kumaran; issue
  2987).

     >>> urlparse.urlparse('http://[1080::8:800:200C:417A]/foo')
     ParseResult(scheme='http', netloc='[1080::8:800:200C:417A]',
                 path='/foo', params='', query='', fragment='')

* New class: the ``WeakSet`` class in the ``weakref`` module is a set
  that only holds weak references to its elements; elements will be
  removed once there are no references pointing to them. (Originally
  implemented in Python 3.x by Raymond Hettinger, and backported to
  2.7 by Michael Foord.)

* The ElementTree library, ``xml.etree``, no longer escapes ampersands
  and angle brackets when outputting an XML processing instruction
  (which looks like ``<?xml-stylesheet href="#style1"?>``) or comment
  (which looks like ``<!-- comment -->``). (Patch by Neil Muller;
  issue 2746.)

* The XML-RPC client and server, provided by the ``xmlrpclib`` and
  ``SimpleXMLRPCServer`` modules, have improved performance by
  supporting HTTP/1.1 keep-alive and by optionally using gzip encoding
  to compress the XML being exchanged.  The gzip compression is
  controlled by the ``encode_threshold`` attribute of
  ``SimpleXMLRPCRequestHandler``, which contains a size in bytes;
  responses larger than this will be compressed. (Contributed by
  Kristján Valur Jónsson; issue 6267.)

* The ``zipfile`` module's ``ZipFile`` now supports the context
  management protocol, so you can write ``with zipfile.ZipFile(...) as
  f:``. (Contributed by Brian Curtin; issue 5511.)

  ``zipfile`` now also supports archiving empty directories and
  extracts them correctly.  (Fixed by Kuba Wieczorek; issue 4710.)
  Reading files out of an archive is faster, and interleaving
  ``read()`` and ``readline()`` now works correctly. (Contributed by
  Nir Aides; issue 7610.)

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

  The ``writestr()`` method now has an optional *compress_type*
  parameter that lets you override the default compression method
  specified in the ``ZipFile`` constructor.  (Contributed by Ronald
  Oussoren; issue 6003.)


New module: importlib
---------------------

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 users 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
   >>> file_util = import_module('..file_util', 'distutils.command')
   >>> file_util
   <module 'distutils.file_util' from '/python/Lib/distutils/file_util.pyc'>

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


New module: sysconfig
---------------------

The ``sysconfig`` module has been pulled out of the Distutils package,
becoming a new top-level module in its own right. ``sysconfig``
provides functions for getting information about Python's build
process: compiler switches, installation paths, the platform name, and
whether Python is running from its source directory.

Some of the functions in the module are:

* ``get_config_var()`` returns variables from Python's Makefile and
  the ``pyconfig.h`` file.

* ``get_config_vars()`` returns a dictionary containing all of the
  configuration variables.

* ``get_path()`` returns the configured path for a particular type of
  module: the standard library, site-specific modules, platform-
  specific modules, etc.

* ``is_python_build()`` returns true if you're running a binary from a
  Python source tree, and false otherwise.

Consult the ``sysconfig`` documentation for more details and for a
complete list of functions.

The Distutils package and ``sysconfig`` are now maintained by Tarek
Ziadé, who has also started a Distutils2 package (source repository at
http://hg.python.org/distutils2/) for developing a next-generation
version of Distutils.


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.

To learn more, read the ``ttk`` module documentation.  You may also
wish to read the Tcl/Tk manual page describing the Ttk theme engine,
available at http://www.tcl.tk/man/tcl8.5/TkCmd/ttk_intro.htm. Some
screenshots of the Python/Ttk code in use are at
http://code.google.com/p/python-ttk/wiki/Screenshots.

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.


Updated module: unittest
------------------------

The ``unittest`` module was greatly enhanced; many new features were
added.  Most of these features were implemented by Michael Foord,
unless otherwise noted.  The enhanced version of the module is
downloadable separately for use with Python versions 2.4 to 2.6,
packaged as the ``unittest2`` package, from
http://pypi.python.org/pypi/unittest2.

When used from the command line, the module can automatically discover
tests.  It's not as fancy as py.test or nose, but provides a simple
way to run tests kept within a set of package directories.  For
example, the following command will search the ``test/`` subdirectory
for any importable test files named ``test*.py``:

   python -m unittest discover -s test

Consult the ``unittest`` module documentation for more details.
(Developed in issue 6001.)

The ``main()`` function supports some other new options:

* *-b* or *--buffer* will buffer the standard output and standard
  error streams during each test.  If the test passes, any resulting
  output will be discarded; on failure, the buffered output will be
  displayed.

* *-c* or *--catch* will cause the control-C interrupt to be handled
  more gracefully.  Instead of interrupting the test process
  immediately, the currently running test will be completed and then
  the partial results up to the interruption will be reported. If
  you're impatient, a second press of control-C will cause an
  immediate interruption.

  This control-C handler tries to avoid causing problems when the code
  being tested or the tests being run have defined a signal handler of
  their own, by noticing that a signal handler was already set and
  calling it.  If this doesn't work for you, there's a
  ``removeHandler()`` decorator that can be used to mark tests that
  should have the control-C handling disabled.

* *-f* or *--failfast* makes test execution stop immediately when a
  test fails instead of continuing to execute further tests.
  (Suggested by Cliff Dyer and implemented by Michael Foord; issue
  8074.)

The progress messages 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()`` method now returns a context handler when
called without providing a callable object to run.  For example, you
can write this:

   with self.assertRaises(KeyError):
       {}['foo']

(Implemented by Antoine Pitrou; issue 4444.)

Module- and class-level setup and teardown fixtures are now supported.
Modules can contain ``setUpModule()`` and ``tearDownModule()``
functions.  Classes can have ``setUpClass()`` and ``tearDownClass()``
methods that must be defined as class methods (using ``@classmethod``
or equivalent).  These functions and methods are invoked when the test
runner switches to a test case in a different module or class.

The methods ``addCleanup()`` and ``doCleanups()`` were added.
``addCleanup()`` lets you 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.)

* ``assertIsInstance()`` and ``assertNotIsInstance()`` check whether
  the resulting object is an instance of a particular class, or of one
  of a tuple of classes.  (Added by Georg Brandl; issue 7031.)

* ``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.  This comparison is now used by default when
  Unicode strings are compared with ``assertEqual()``.

* ``assertRegexpMatches()`` and ``assertNotRegexpMatches()`` checks
  whether the first argument is a string matching or not matching the
  regular expression provided as the second argument (issue 8038).

* ``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*.

* ``assertItemsEqual()`` 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 any differences without necessarily
  printing their full values; these methods are now used by default
  when comparing lists and tuples using ``assertEqual()``. 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; it's now used by default when you compare two
  dictionaries using ``assertEqual()``.
  ``assertDictContainsSubset()`` checks whether all of the key/value
  pairs in *first* are found in *second*.

* ``assertAlmostEqual()`` and ``assertNotAlmostEqual()`` test whether
  *first* and *second* are approximately equal.  This method can
  either round their difference to an optionally-specified number of
  *places* (the default is 7) and compare it to zero, or require the
  difference to be smaller than a supplied *delta* value.

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

* A new hook lets you extend the ``assertEqual()`` method to handle
  new data types.  The ``addTypeEqualityFunc()`` method takes a type
  object and a function. The function will be used 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 aren't 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 ``main()`` to
be used from the interactive interpreter. (Contributed by J. Pablo
Fernández; issue 3379.)

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

With all these changes, the ``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 or used.

See also:

   http://www.voidspace.org.uk/python/articles/unittest2.shtml
      Describes the new features, how to use them, and the rationale
      for various design decisions.  (By Michael Foord.)


Updated module: ElementTree 1.3
-------------------------------

The version of the ElementTree library included with Python was
updated to version 1.3.  Some of the new features are:

* The various parsing functions now take a *parser* keyword argument
  giving an ``XMLParser`` instance that will be used.  This makes it
  possible to override the file's internal encoding:

     p = ET.XMLParser(encoding='utf-8')
     t = ET.XML("""<root/>""", parser=p)

  Errors in parsing XML now raise a ``ParseError`` exception, whose
  instances have a ``position`` attribute containing a (*line*,
  *column*) tuple giving the location of the problem.

* ElementTree's code for converting trees to a string has been
  significantly reworked, making it roughly twice as fast in many
  cases.  The ``ElementTree.write()`` and ``Element.write()`` methods
  now have a *method* parameter that can be "xml" (the default),
  "html", or "text".  HTML mode will output empty elements as
  ``<empty></empty>`` instead of ``<empty/>``, and text mode will skip
  over elements and only output the text chunks.  If you set the
  ``tag`` attribute of an element to ``None`` but leave its children
  in place, the element will be omitted when the tree is written out,
  so you don't need to do more extensive rearrangement to remove a
  single element.

  Namespace handling has also been improved.  All ``xmlns:<whatever>``
  declarations are now output on the root element, not scattered
  throughout the resulting XML.  You can set the default namespace for
  a tree by setting the ``default_namespace`` attribute and can
  register new prefixes with ``register_namespace()``.  In XML mode,
  you can use the true/false *xml_declaration* parameter to suppress
  the XML declaration.

* New ``Element`` method: ``extend()`` appends the items from a
  sequence to the element's children.  Elements themselves behave like
  sequences, so it's easy to move children from one element to
  another:

     from xml.etree import ElementTree as ET

     t = ET.XML("""<list>
       <item>1</item> <item>2</item>  <item>3</item>
     </list>""")
     new = ET.XML('<root/>')
     new.extend(t)

     # Outputs <root><item>1</item>...</root>
     print ET.tostring(new)

* New ``Element`` method: ``iter()`` yields the children of the
  element as a generator.  It's also possible to write ``for child in
  elem:`` to loop over an element's children.  The existing method
  ``getiterator()`` is now deprecated, as is ``getchildren()`` which
  constructs and returns a list of children.

* New ``Element`` method: ``itertext()`` yields all chunks of text
  that are descendants of the element.  For example:

     t = ET.XML("""<list>
       <item>1</item> <item>2</item>  <item>3</item>
     </list>""")

     # Outputs ['\n  ', '1', ' ', '2', '  ', '3', '\n']
     print list(t.itertext())

* Deprecated: using an element as a Boolean (i.e., ``if elem:``) would
  return true if the element had any children, or false if there were
  no children.  This behaviour is confusing -- ``None`` is false, but
  so is a childless element? -- so it will now trigger a
  ``FutureWarning``.  In your code, you should be explicit: write
  ``len(elem) != 0`` if you're interested in the number of children,
  or ``elem is not None``.

Fredrik Lundh develops ElementTree and produced the 1.3 version; you
can read his article describing 1.3 at
http://effbot.org/zone/elementtree-13-intro.htm. Florent Xicluna
updated the version included with Python, after discussions on python-
dev and in issue 6472.)


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

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

* The latest release of the GNU Debugger, GDB 7, can be scripted using
  Python. When you begin debugging an executable program P, GDB will
  look for a file named ``P-gdb.py`` and automatically read it.  Dave
  Malcolm contributed a ``python-gdb.py`` that adds a number of
  commands useful when debugging Python itself.  For example,
  ``py-up`` and ``py-down`` go up or down one Python stack frame,
  which usually corresponds to several C stack frames.  ``py-print``
  prints the value of a Python variable, and ``py-bt`` prints the
  Python stack trace.  (Added as a result of issue 8032.)

* If you use the ``.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.)

* ``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
  Kristján Valur Jónsson; issue 4293.)

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

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

* New function: ``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 functions: ``PyLong_AsLongAndOverflow()`` and
  ``PyLong_AsLongLongAndOverflow()``  approximates a Python long
  integer as a C ``long`` or ``long long``. If the number is too large
  to fit into the output type, an *overflow* flag is set and returned
  to the caller. (Contributed by Case Van Horsen; issue 7528 and issue
  7767.)

* New function: stemming from the rewrite of string-to-float
  conversion, a new ``PyOS_string_to_double()`` function was added.
  The old ``PyOS_ascii_strtod()`` and ``PyOS_ascii_atof()`` functions
  are now deprecated.

* New function: ``PySys_SetArgvEx()`` sets the value of ``sys.argv``
  and can optionally update ``sys.path`` to include the directory
  containing the script named by ``sys.argv[0]`` depending on the
  value of an *updatepath* parameter.

  This function was added to close a security hole for applications
  that embed Python.  The old function, ``PySys_SetArgv()``, would
  always update ``sys.path``, and sometimes it would add the current
  directory.  This meant that, if you ran an application embedding
  Python in a directory controlled by someone else, attackers could
  put a Trojan-horse module in the directory (say, a file named
  ``os.py``) that your application would then import and run.

  If you maintain a C/C++ application that embeds Python, check
  whether you're calling ``PySys_SetArgv()`` and carefully consider
  whether the application should be using ``PySys_SetArgvEx()`` with
  *updatepath* set to false.

  Security issue reported as CVE-2008-5983; discussed in issue 5753,
  and fixed by Antoine Pitrou.

* New macros: the Python header files now define the following macros:
  ``Py_ISALNUM``, ``Py_ISALPHA``, ``Py_ISDIGIT``, ``Py_ISLOWER``,
  ``Py_ISSPACE``, ``Py_ISUPPER``, ``Py_ISXDIGIT``, ``Py_TOLOWER``, and
  ``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.)

* Removed function: ``PyEval_CallObject`` is now only available as a
  macro.  A function version was being kept around to preserve ABI
  linking compatibility, but that was in 1997; it can certainly be
  deleted by now.  (Removed by Antoine Pitrou; issue 8276.)

* New format codes: the ``PyFormat_FromString()``,
  ``PyFormat_FromStringV()``, and ``PyErr_Format()`` functions now
  accept ``%lld`` and ``%llu`` format codes for displaying C's ``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
  ``os.fork()`` might fail because the child is created with only a
  single thread running, the thread performing the ``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 acquires the import lock before performing an
  ``os.fork()``, and will also clean up any locks created using the
  ``threading`` module.  C extension modules that have internal locks,
  or that call ``fork()`` themselves, will not benefit from this
  clean-up.

  (Fixed by Thomas Wouters; issue 1590864.)

* The ``Py_Finalize()`` function now calls the internal
  ``threading._shutdown()`` function; this prevents some exceptions
  from being raised when an interpreter shuts down. (Patch by Adam
  Olsen; issue 1722344.)

* When using the ``PyMemberDef`` structure to define attributes of a
  type, Python will no longer let you try to delete or set a
  ``T_STRING_INPLACE`` attribute.

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

* New configure option: the *--with-system-expat* switch allows
  building the ``pyexpat`` module to use the system Expat library.
  (Contributed by Arfrever Frehtes Taifersar Arahesis; issue 7609.)

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

* New configure option: you can now supply an empty string to *--with-
  dbmliborder=* in order to disable all of the various DBM modules.
  (Added by Arfrever Frehtes Taifersar Arahesis; issue 6491.)

* 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.)

  **configure** also now sets a ``LDCXXSHARED`` Makefile variable for
  supporting C++ linking.  (Contributed by Arfrever Frehtes Taifersar
  Arahesis; issue 1222585.)

* 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.)


Capsules
--------

Python 3.1 adds a new C datatype, ``PyCapsule``, for providing a C API
to an extension module.  A capsule is essentially the holder of a C
``void *`` pointer, and is made available as a module attribute; for
example, the ``socket`` module's API is exposed as ``socket.CAPI``,
and ``unicodedata`` exposes ``ucnhash_CAPI``.  Other extensions can
import the module, access its dictionary to get the capsule object,
and then get the ``void *`` pointer, which will usually point to an
array of pointers to the module's various API functions.

There is an existing data type already used for this, ``PyCObject``,
but it doesn't provide type safety.  Evil code written in pure Python
could cause a segmentation fault by taking a ``PyCObject`` from module
A and somehow substituting it for the ``PyCObject`` in module B.
Capsules know their own name, and getting the pointer requires
providing the name:

   void *vtable;

   if (!PyCapsule_IsValid(capsule, "mymodule.CAPI") {
           PyErr_SetString(PyExc_ValueError, "argument type invalid");
           return NULL;
   }

   vtable = PyCapsule_GetPointer(capsule, "mymodule.CAPI");

You are assured that ``vtable`` points to whatever you're expecting.
If a different capsule was passed in, ``PyCapsule_IsValid()`` would
detect the mismatched name and return false.  Refer to *Providing a C
API for an Extension Module* for more information on using these
objects.

Python 2.7 now uses capsules internally to provide various extension-
module APIs, but the ``PyCObject_AsVoidPtr()`` was modified to handle
capsules, preserving compile-time compatibility with the ``CObject``
interface.  Use of ``PyCObject_AsVoidPtr()`` will signal a
``PendingDeprecationWarning``, which is silent by default.

Implemented in Python 3.1 and backported to 2.7 by Larry Hastings;
discussed in issue 5630.


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 ``_winreg`` module for accessing the registry now implements the
  ``CreateKeyEx()`` and ``DeleteKeyEx()`` functions, extended versions
  of previously-supported functions that take several extra arguments.
  The ``DisableReflectionKey()``, ``EnableReflectionKey()``, and
  ``QueryReflectionKey()`` were also tested and documented.
  (Implemented by Brian Curtin: issue 7347.)

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

* The ``os.kill()`` function now works on Windows.  The signal value
  can be the constants ``CTRL_C_EVENT``, ``CTRL_BREAK_EVENT``, or any
  integer.  The first two constants will send Control-C and Control-
  Break keystroke events to subprocesses; any other value will use the
  ``TerminateProcess()`` API.  (Contributed by Miki Tebeka; issue
  1220212.)

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

* The ``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.)


Port-Specific Changes: FreeBSD
------------------------------

* FreeBSD 7.1's ``SO_SETFIB`` constant, used with
  ``getsockopt()``/``setsockopt()`` to select an alternate routing
  table, is now available in the ``socket`` module.  (Added by Kyle
  VanderBeek; issue 8235.)


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

* Two benchmark scripts, ``iobench`` and ``ccbench``, were added to
  the ``Tools`` directory.  ``iobench`` measures the speed of the
  built-in file I/O objects returned by ``open()`` while performing
  various operations, and ``ccbench`` is a concurrency benchmark that
  tries to measure computing throughput, thread switching latency, and
  IO processing bandwidth when performing several tasks using a
  varying number of threads.

* The ``Tools/i18n/msgfmt.py`` script now understands plural forms in
  ``.po`` files.  (Fixed by Martin von Löwis; issue 5464.)

* 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
  reports the seed that was used (Added by Collin Winter.)

* Another ``regrtest.py`` switch is *-j*, which 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 *-R* switch which is known to
  produce long runtimes. (Added by Antoine Pitrou, issue 6152.)  This
  can also be used with a new *-F* switch that runs selected tests in
  a loop until they fail.  (Added by Antoine Pitrou; issue 7312.)

* When executed as a script, the ``py_compile.py`` module now accepts
  ``'-'`` as an argument, which will read standard input for the list
  of filenames to be compiled.  (Contributed by Piotr Ożarowski; issue
  8233.)


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

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

* The ``range()`` function processes its arguments more consistently;
  it will now call ``__int__()`` on non-float, non-integer arguments
  that are supplied to it.  (Fixed by Alexander Belopolsky; issue
  1533.)

* The string ``format()`` method changed the default precision used
  for floating-point and complex numbers from 6 decimal places to 12,
  which matches the precision used by ``str()``. (Changed by Eric
  Smith; issue 5920.)

* 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.)

* Due to a bug in Python 2.6, the *exc_value* parameter to
  ``__exit__()`` methods was often the string representation of the
  exception, not an instance.  This was fixed in 2.7, so *exc_value*
  will be an instance as expected.  (Fixed by Florent Xicluna; issue
  7853.)

* When a restricted set of attributes were set using ``__slots__``,
  deleting an unset attribute would not raise ``AttributeError`` as
  you would expect.  Fixed by Benjamin Peterson; issue 7604.)

In the standard library:

* Operations with ``datetime`` instances that resulted in a year
  falling outside the supported range didn't always raise
  ``OverflowError``.  Such errors are now checked more carefully and
  will now raise the exception. (Reported by Mark Leander, patch by
  Anand B. Pillai and Alexander Belopolsky; issue 7150.)

* When using ``Decimal`` instances with a string's ``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.)

  Comparisons involving a signaling NaN value (or ``sNAN``) now signal
  ``InvalidOperation`` instead of silently returning a true or false
  value depending on the comparison operator.  Quiet NaN values (or
  ``NaN``) are now hashable.  (Fixed by Mark Dickinson; issue 7279.)

* The ElementTree library, ``xml.etree``, no longer escapes ampersands
  and angle brackets when outputting an XML processing instruction
  (which looks like *<?xml-stylesheet href="#style1"?>*) or comment
  (which looks like *<!-- comment -->*). (Patch by Neil Muller; issue
  2746.)

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

* The ``syslog`` module will now use the value of ``sys.argv[0]`` as
  the identifier instead of the previous default value of
  ``'python'``. (Changed by Sean Reifschneider; issue 8451.)

* The ``tarfile`` module's default error handling has changed, to no
  longer suppress fatal errors.  The default error level was
  previously 0, which meant that errors would only result in a message
  being written to the debug log, but because the debug log is not
  activated by default, these errors go unnoticed.  The default error
  level is now 1, which raises an exception if there's an error.
  (Changed by Lars Gustäbel; issue 7357.)

* The ``urlparse`` module's ``urlsplit()`` now handles unknown URL
  schemes in a fashion compliant with **RFC 3986**: if the URL is of
  the form ``"<something>://..."``, the text before the ``://`` is
  treated as the scheme, even if it's a made-up scheme that the module
  doesn't know about.  This change may break code that worked around
  the old behaviour.  For example, Python 2.6.4 or 2.5 will return the
  following:

  >>> import urlparse
  >>> urlparse.urlsplit('invented://host/filename?query')
  ('invented', '', '//host/filename?query', '', '')

  Python 2.7 (and Python 2.6.5) will return:

  >>> import urlparse
  >>> urlparse.urlsplit('invented://host/filename?query')
  ('invented', 'host', '/filename?query', '', '')

  (Python 2.7 actually produces slightly different output, since it
  returns a named tuple instead of a standard tuple.)

For C extensions:

* C extensions that use integer format codes with the ``PyArg_Parse*``
  family of functions will now raise a ``TypeError`` exception instead
  of triggering a ``DeprecationWarning`` (issue 5080).

* Use the new ``PyOS_string_to_double()`` function instead of the old
  ``PyOS_ascii_strtod()`` and ``PyOS_ascii_atof()`` functions, which
  are now deprecated.

For applications that embed Python:

* The ``PySys_SetArgvEx()`` function was added, letting applications
  close a security hole when the existing ``PySys_SetArgv()`` function
  was used.  Check whether you're calling ``PySys_SetArgv()`` and
  carefully consider whether the application should be using
  ``PySys_SetArgvEx()`` with *updatepath* set to false.


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

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