cpython/Doc/whatsnew/2.6.rst

****************************
  What's New in Python 2.6  
****************************

.. XXX add trademark info for Apple, Microsoft, SourceForge.

:Author: A.M. Kuchling
:Release: |release|
:Date: |today|

.. $Id: whatsnew26.tex 55746 2007-06-02 18:33:53Z neal.norwitz $
   Rules for maintenance:
   
   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.
   
   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.
   
   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)
   
   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.
   
   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.
   
   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).
   
   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.
   
   * It's helpful to add the bug/patch number as a comment:
   
   .. Patch 12345
   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer.)
   
   This saves the maintainer the effort of going through the SVN logs
   when researching a change.

This article explains the new features in Python 2.6.  No release date for
Python 2.6 has been set; it will probably be released in mid 2008.

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 2.6. If you want to understand the
complete implementation and design rationale, refer to the PEP for a particular
new feature.

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

.. ========================================================================
.. Large, PEP-level features and changes should be described here.
.. Should there be a new section here for 3k migration?
.. Or perhaps a more general section describing module changes/deprecation?
.. ========================================================================

Python 3.0
================

The development cycle for Python 2.6 also saw the release of the first
alphas of Python 3.0, and the development of 3.0 has influenced 
a number of features in 2.6.

Python 3.0 is a far-ranging redesign of Python that breaks
compatibility with the 2.x series.  This means that existing Python
code will need a certain amount of conversion in order to run on
Python 3.0.  However, not all the changes in 3.0 necessarily break
compatibility.  In cases where new features won't cause existing code
to break, they've been backported to 2.6 and are described in this
document in the appropriate place.  Some of the 3.0-derived features 
are:

* A :meth:`__complex__` method for converting objects to a complex number.
* Alternate syntax for catching exceptions: ``except TypeError as exc``.
* The addition of :func:`functools.reduce` as a synonym for the built-in
  :func:`reduce` function.

A new command-line switch, :option:`-3`, enables warnings
about features that will be removed in Python 3.0.  You can run code
with this switch to see how much work will be necessary to port
code to 3.0.  The value of this switch is available 
to Python code as the boolean variable :data:`sys.py3kwarning`,
and to C extension code as :cdata:`Py_Py3kWarningFlag`.

Python 3.0 adds several new built-in functions and change the
semantics of some existing built-ins.  Entirely new functions such as
:func:`bin` have simply been added to Python 2.6, but existing
built-ins haven't been changed; instead, the :mod:`future_builtins`
module has versions with the new 3.0 semantics.  Code written to be
compatible with 3.0 can do ``from future_builtins import hex, map``
as necessary.

.. seealso::

   The 3xxx series of PEPs, which describes the development process for
   Python 3.0 and various features that have been accepted, rejected,
   or are still under consideration.


Development Changes
==================================================

While 2.6 was being developed, the Python development process 
underwent two significant changes: the developer group 
switched from SourceForge's issue tracker to a customized 
Roundup installation, and the documentation was converted from
LaTeX to reStructured Text.


New Issue Tracker: Roundup
--------------------------------------------------

For a long time, the Python developers have been growing increasingly
annoyed by SourceForge's bug tracker.  SourceForge's hosted solution
doesn't permit much customization; for example, it wasn't possible to
customize the life cycle of issues.

The infrastructure committee of the Python Software Foundation
therefore posted a call for issue trackers, asking volunteers to set
up different products and import some of the bugs and patches from
SourceForge.  Four different trackers were examined: Atlassian's `Jira
<http://www.atlassian.com/software/jira/>`__, 
`Launchpad <http://www.launchpad.net>`__, 
`Roundup <http://roundup.sourceforge.net/>`__, and 
Trac <http://trac.edgewall.org/>`__.  
The committee eventually settled on Jira
and Roundup as the two candidates.  Jira is a commercial product that
offers a no-cost hosted instance to free-software projects; Roundup 
is an open-source project that requires volunteers
to administer it and a server to host it.

After posting a call for volunteers, a new Roundup installation was
set up at http://bugs.python.org.  One installation of Roundup can
host multiple trackers, and this server now also hosts issue trackers
for Jython and for the Python web site.  It will surely find 
other uses in the future.

Hosting is kindly provided by `Upfront Systems <http://www.upfrontsystems.co.za/>`__ of Stellenbosch, South Africa.  Martin von Loewis put a
lot of effort into importing existing bugs and patches from
SourceForge; his scripts for this import operation are at 
http://svn.python.org/view/tracker/importer/.

.. seealso::

  http://bugs.python.org 
    The Python bug tracker.

  http://bugs.jython.org:
    The Jython bug tracker.

  http://roundup.sourceforge.net/
    Roundup downloads and documentation.


New Documentation Format: ReStructured Text Using Sphinx
-----------------------------------------------------------

Since the Python project's inception around 1989, the documentation
had been written using LaTeX.  At that time, most documentation was
printed out for later study, not viewed online. LaTeX was widely used
because it provided attractive printed output while remaining
straightforward to write, once the basic rules of the markup have been
learned.

LaTeX is still used today for writing technical publications destined
for printing, but the landscape for programming tools has shifted.  We
no longer print out reams of documentation; instead, we browse through
it online and HTML has become the most important format to support.
Unfortunately, converting LaTeX to HTML is fairly complicated, and
Fred L. Drake Jr., the Python documentation editor for many years,
spent a lot of time wrestling the conversion process into shape.
Occasionally people would suggest converting the documentation into 
SGML or, later, XML, but performing a good conversion is a major task 
and no one pursued the task to completion.

During the 2.6 development cycle, Georg Brandl put a substantial 
effort into building a new toolchain called Sphinx 
for processing the documentation.
The input format is reStructured Text, 
a markup commonly used in the Python community that supports
custom extensions  and directives.   Sphinx concentrates 
on HTML output, producing attractively styled 
and modern HTML, though printed output is still supported through 
conversion to LaTeX.  Sphinx is a standalone package that
can be used in documenting other projects.

.. seealso::

   :ref:`documenting-index`
       Describes how to write for Python's documentation.

   `Sphinx <http://sphinx.pocoo.org/>`__
     Documentation and code for the Sphinx toolchain.

   `Docutils <http://docutils.sf.net>`__
     The underlying reStructured Text parser and toolset.


PEP 343: The 'with' statement
=============================

The previous version, Python 2.5, added the ':keyword:`with`'
statement an optional feature, to be enabled by a ``from __future__
import with_statement`` directive.  In 2.6 the statement no longer needs to
be specially enabled; this means that :keyword:`with` is now always a
keyword.  The rest of this section is a copy of the corresponding 
section from "What's New in Python 2.5" document; if you read
it back when Python 2.5 came out, you can skip the rest of this
section.

The ':keyword:`with`' statement clarifies code that previously would use
``try...finally`` blocks to ensure that clean-up code is executed.  In this
section, I'll discuss the statement as it will commonly be used.  In the next
section, I'll examine the implementation details and show how to write objects
for use with this statement.

The ':keyword:`with`' statement is a new control-flow structure whose basic
structure is::

   with expression [as variable]:
       with-block

The expression is evaluated, and it should result in an object that supports the
context management protocol (that is, has :meth:`__enter__` and :meth:`__exit__`
methods.

The object's :meth:`__enter__` is called before *with-block* is executed and
therefore can run set-up code. It also may return a value that is bound to the
name *variable*, if given.  (Note carefully that *variable* is *not* assigned
the result of *expression*.)

After execution of the *with-block* is finished, the object's :meth:`__exit__`
method is called, even if the block raised an exception, and can therefore run
clean-up code.

Some standard Python objects now support the context management protocol and can
be used with the ':keyword:`with`' statement. File objects are one example::

   with open('/etc/passwd', 'r') as f:
       for line in f:
           print line
           ... more processing code ...

After this statement has executed, the file object in *f* will have been
automatically closed, even if the :keyword:`for` loop raised an exception part-
way through the block.

.. note::

   In this case, *f* is the same object created by :func:`open`, because
   :meth:`file.__enter__` returns *self*.

The :mod:`threading` module's locks and condition variables  also support the
':keyword:`with`' statement::

   lock = threading.Lock()
   with lock:
       # Critical section of code
       ...

The lock is acquired before the block is executed and always released once  the
block is complete.

The new :func:`localcontext` function in the :mod:`decimal` module makes it easy
to save and restore the current decimal context, which encapsulates the desired
precision and rounding characteristics for computations::

   from decimal import Decimal, Context, localcontext

   # Displays with default precision of 28 digits
   v = Decimal('578')
   print v.sqrt()

   with localcontext(Context(prec=16)):
       # All code in this block uses a precision of 16 digits.
       # The original context is restored on exiting the block.
       print v.sqrt()


.. _new-26-context-managers:

Writing Context Managers
------------------------

Under the hood, the ':keyword:`with`' statement is fairly complicated. Most
people will only use ':keyword:`with`' in company with existing objects and
don't need to know these details, so you can skip the rest of this section if
you like.  Authors of new objects will need to understand the details of the
underlying implementation and should keep reading.

A high-level explanation of the context management protocol is:

* The expression is evaluated and should result in an object called a "context
  manager".  The context manager must have :meth:`__enter__` and :meth:`__exit__`
  methods.

* The context manager's :meth:`__enter__` method is called.  The value returned
  is assigned to *VAR*.  If no ``as VAR`` clause is present, the value is simply
  discarded.

* The code in *BLOCK* is executed.

* If *BLOCK* raises an exception, the :meth:`__exit__(type, value, traceback)`
  is called with the exception details, the same values returned by
  :func:`sys.exc_info`.  The method's return value controls whether the exception
  is re-raised: any false value re-raises the exception, and ``True`` will result
  in suppressing it.  You'll only rarely want to suppress the exception, because
  if you do the author of the code containing the ':keyword:`with`' statement will
  never realize anything went wrong.

* If *BLOCK* didn't raise an exception,  the :meth:`__exit__` method is still
  called, but *type*, *value*, and *traceback* are all ``None``.

Let's think through an example.  I won't present detailed code but will only
sketch the methods necessary for a database that supports transactions.

(For people unfamiliar with database terminology: a set of changes to the
database are grouped into a transaction.  Transactions can be either committed,
meaning that all the changes are written into the database, or rolled back,
meaning that the changes are all discarded and the database is unchanged.  See
any database textbook for more information.)

Let's assume there's an object representing a database connection. Our goal will
be to let the user write code like this::

   db_connection = DatabaseConnection()
   with db_connection as cursor:
       cursor.execute('insert into ...')
       cursor.execute('delete from ...')
       # ... more operations ...

The transaction should be committed if the code in the block runs flawlessly or
rolled back if there's an exception. Here's the basic interface for
:class:`DatabaseConnection` that I'll assume::

   class DatabaseConnection:
       # Database interface
       def cursor(self):
           "Returns a cursor object and starts a new transaction"
       def commit(self):
           "Commits current transaction"
       def rollback(self):
           "Rolls back current transaction"

The :meth:`__enter__` method is pretty easy, having only to start a new
transaction.  For this application the resulting cursor object would be a useful
result, so the method will return it.  The user can then add ``as cursor`` to
their ':keyword:`with`' statement to bind the cursor to a variable name. ::

   class DatabaseConnection:
       ...
       def __enter__(self):
           # Code to start a new transaction
           cursor = self.cursor()
           return cursor

The :meth:`__exit__` method is the most complicated because it's where most of
the work has to be done.  The method has to check if an exception occurred.  If
there was no exception, the transaction is committed.  The transaction is rolled
back if there was an exception.

In the code below, execution will just fall off the end of the function,
returning the default value of ``None``.  ``None`` is false, so the exception
will be re-raised automatically.  If you wished, you could be more explicit and
add a :keyword:`return` statement at the marked location. ::

   class DatabaseConnection:
       ...
       def __exit__(self, type, value, tb):
           if tb is None:
               # No exception, so commit
               self.commit()
           else:
               # Exception occurred, so rollback.
               self.rollback()
               # return False


.. _module-contextlib:

The contextlib module
---------------------

The new :mod:`contextlib` module provides some functions and a decorator that
are useful for writing objects for use with the ':keyword:`with`' statement.

The decorator is called :func:`contextmanager`, and lets you write a single
generator function instead of defining a new class.  The generator should yield
exactly one value.  The code up to the :keyword:`yield` will be executed as the
:meth:`__enter__` method, and the value yielded will be the method's return
value that will get bound to the variable in the ':keyword:`with`' statement's
:keyword:`as` clause, if any.  The code after the :keyword:`yield` will be
executed in the :meth:`__exit__` method.  Any exception raised in the block will
be raised by the :keyword:`yield` statement.

Our database example from the previous section could be written  using this
decorator as::

   from contextlib import contextmanager

   @contextmanager
   def db_transaction(connection):
       cursor = connection.cursor()
       try:
           yield cursor
       except:
           connection.rollback()
           raise
       else:
           connection.commit()

   db = DatabaseConnection()
   with db_transaction(db) as cursor:
       ...

The :mod:`contextlib` module also has a :func:`nested(mgr1, mgr2, ...)` function
that combines a number of context managers so you don't need to write nested
':keyword:`with`' statements.  In this example, the single ':keyword:`with`'
statement both starts a database transaction and acquires a thread lock::

   lock = threading.Lock()
   with nested (db_transaction(db), lock) as (cursor, locked):
       ...

Finally, the :func:`closing(object)` function returns *object* so that it can be
bound to a variable, and calls ``object.close`` at the end of the block. ::

   import urllib, sys
   from contextlib import closing

   with closing(urllib.urlopen('http://www.yahoo.com')) as f:
       for line in f:
           sys.stdout.write(line)


.. seealso::

   :pep:`343` - The "with" statement
      PEP written by Guido van Rossum and Nick Coghlan; implemented by Mike Bland,
      Guido van Rossum, and Neal Norwitz.  The PEP shows the code generated for a
      ':keyword:`with`' statement, which can be helpful in learning how the statement
      works.

   The documentation  for the :mod:`contextlib` module.

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

.. _pep-0366:

PEP 366: Explicit Relative Imports From a Main Module
============================================================

Python's :option:`-m` switch allows running a module as a script.
When you ran a module that was located inside a package, relative
imports didn't work correctly.

The fix in Python 2.6 adds a :attr:`__package__` attribute to modules.
When present, relative imports will be relative to the value of this
attribute instead of the :attr:`__name__` attribute.  PEP 302-style
importers can then set :attr:`__package__`.  The :mod:`runpy` module
that implements the :option:`-m` switch now does this, so relative imports
can now be used in scripts running from inside a package.

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

.. ::

    .. _pep-0370:

    PEP 370: XXX
    =====================================================

    When you run Python, the module search page ``sys.modules`` usually
    includes a directory whose path ends in ``"site-packages"``.  This
    directory is intended to hold locally-installed packages available to
    all users on a machine or using a particular site installation.

    Python 2.6 introduces a convention for user-specific site directories.

    .. seealso::

       :pep:`370` - XXX
         PEP written by XXX; implemented by Christian Heimes.

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

.. _pep-3101:

PEP 3101: Advanced String Formatting
=====================================================

In Python 3.0, the `%` operator is supplemented by a more powerful
string formatting method, :meth:`format`.  Support for the
:meth:`format` method has been backported to Python 2.6.

In 2.6, both 8-bit and Unicode strings have a `.format()` method that
treats the string as a template and takes the arguments to be formatted.
The formatting template uses curly brackets (`{`, `}`) as special characters::

     # Substitute positional argument 0 into the string.
     "User ID: {0}".format("root") -> "User ID: root"

     # Use the named keyword arguments
     uid = 'root'
     
     'User ID: {uid}   Last seen: {last_login}'.format(uid='root',
            last_login = '5 Mar 2008 07:20') ->
       'User ID: root   Last seen: 5 Mar 2008 07:20'

Curly brackets can be escaped by doubling them::

     format("Empty dict: {{}}") -> "Empty dict: {}"

Field names can be integers indicating positional arguments, such as 
``{0}``, ``{1}``, etc. or names of keyword arguments.  You can also 
supply compound field names that read attributes or access dictionary keys::

    import sys
    'Platform: {0.platform}\nPython version: {0.version}'.format(sys) ->
        'Platform: darwin\n
         Python version: 2.6a1+ (trunk:61261M, Mar  5 2008, 20:29:41) \n
         [GCC 4.0.1 (Apple Computer, Inc. build 5367)]'

     import mimetypes
     'Content-type: {0[.mp4]}'.format(mimetypes.types_map) ->
       'Content-type: video/mp4'

Note that when using dictionary-style notation such as ``[.mp4]``, you
don't need to put any quotation marks around the string; it will look
up the value using ``.mp4`` as the key.  Strings beginning with a
number will be converted to an integer.  You can't write more
complicated expressions inside a format string.

So far we've shown how to specify which field to substitute into the
resulting string.  The precise formatting used is also controllable by
adding a colon followed by a format specifier.  For example::

     # Field 0: left justify, pad to 15 characters
     # Field 1: right justify, pad to 6 characters
     fmt = '{0:15} ${1:>6}'
     fmt.format('Registration', 35) ->
       'Registration    $    35'
     fmt.format('Tutorial', 50) ->
       'Tutorial        $    50'
     fmt.format('Banquet', 125) ->
       'Banquet         $   125'

Format specifiers can reference other fields through nesting::

    fmt = '{0:{1}}'
    fmt.format('Invoice #1234', 15) ->
      'Invoice #1234  '
    width = 35
    fmt.format('Invoice #1234', width) ->
      'Invoice #1234                      '

The alignment of a field within the desired width can be specified:

================ ============================================
Character        Effect
================ ============================================
< (default)      Left-align
>                Right-align
^                Center
=                (For numeric types only) Pad after the sign.
================ ============================================

Format specifiers can also include a presentation type, which 
controls how the value is formatted.  For example, floating-point numbers
can be formatted as a general number or in exponential notation:

    >>> '{0:g}'.format(3.75)
    '3.75'
    >>> '{0:e}'.format(3.75)
    '3.750000e+00'

A variety of presentation types are available.  Consult the 2.6
documentation for a complete list (XXX add link, once it's in the 2.6
docs), but here's a sample::

        'b' - Binary. Outputs the number in base 2.
        'c' - Character. Converts the integer to the corresponding
              Unicode character before printing.
        'd' - Decimal Integer. Outputs the number in base 10.
        'o' - Octal format. Outputs the number in base 8.
        'x' - Hex format. Outputs the number in base 16, using lower-
              case letters for the digits above 9.
        'e' - Exponent notation. Prints the number in scientific
              notation using the letter 'e' to indicate the exponent.
        'g' - General format. This prints the number as a fixed-point
              number, unless the number is too large, in which case
              it switches to 'e' exponent notation.
        'n' - Number. This is the same as 'g', except that it uses the
              current locale setting to insert the appropriate
              number separator characters.
        '%' - Percentage. Multiplies the number by 100 and displays
              in fixed ('f') format, followed by a percent sign.

Classes and types can define a __format__ method to control how they're
formatted.  It receives a single argument, the format specifier::

   def __format__(self, format_spec):
       if isinstance(format_spec, unicode):
           return unicode(str(self))
       else:
           return str(self)

There's also a format() built-in that will format a single value.  It calls
the type's :meth:`__format__` method with the provided specifier::

    >>> format(75.6564, '.2f')
    '75.66'

.. seealso::

   :pep:`3101` - Advanced String Formatting
      PEP written by Talin.

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

.. _pep-3105:

PEP 3105: ``print`` As a Function
=====================================================

The ``print`` statement becomes the :func:`print` function in Python 3.0.
Making :func:`print` a function makes it easier to change 
by doing 'def print(...)' or importing a new function from somewhere else. 

Python 2.6 has a ``__future__`` import that removes ``print`` as language 
syntax, letting you use the functional form instead.  For example::

    from __future__ import print_function
    print('# of entries', len(dictionary), file=sys.stderr)

The signature of the new function is::

    def print(*args, sep=' ', end='\n', file=None)

The parameters are:

 * **args**: positional arguments whose values will be printed out.
 * **sep**: the separator, which will be printed between arguments.
 * **end**: the ending text, which will be printed after all of the 
   arguments have been output.
 * **file**: the file object to which the output will be sent.

.. seealso::

   :pep:`3105` - Make print a function
      PEP written by Georg Brandl.

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

.. _pep-3110:

PEP 3110: Exception-Handling Changes
=====================================================

One error that Python programmers occasionally make 
is the following::

    try:
        ...
    except TypeError, ValueError:
        ...

The author is probably trying to catch both 
:exc:`TypeError` and :exc:`ValueError` exceptions, but this code
actually does something different: it will catch 
:exc:`TypeError` and bind the resulting exception object
to the local name ``"ValueError"``.  The correct code 
would have specified a tuple::

    try:
        ...
    except (TypeError, ValueError):
        ...

This error is possible because the use of the comma here is ambiguous:
does it indicate two different nodes in the parse tree, or a single
node that's a tuple.

Python 3.0 changes the syntax to make this unambiguous by replacing
the comma with the word "as".  To catch an exception and store the 
exception object in the variable ``exc``, you must write::

    try:
        ...
    except TypeError as exc:
        ...

Python 3.0 will only support the use of "as", and therefore interprets
the first example as catching two different exceptions.  Python 2.6
supports both the comma and "as", so existing code will continue to
work.

.. seealso::

   :pep:`3110` - Catching Exceptions in Python 3000
      PEP written and implemented by Collin Winter.

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

.. _pep-3112:

PEP 3112: Byte Literals
=====================================================

Python 3.0 adopts Unicode as the language's fundamental string type, and
denotes 8-bit literals differently, either as ``b'string'`` 
or using a :class:`bytes` constructor.  For future compatibility, 
Python 2.6 adds :class:`bytes` as a synonym for the :class:`str` type,
and it also supports the ``b''`` notation.

.. seealso::

   :pep:`3112` - Bytes literals in Python 3000
      PEP written by Jason Orendorff; backported to 2.6 by Christian Heimes.

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

.. _pep-3116:

PEP 3116: New I/O Library
=====================================================

XXX write this.

.. seealso::

   :pep:`3116` - New I/O
      PEP written by Daniel Stutzbach, Mike Verdone, and Guido van Rossum.
      XXX code written by who?

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

.. _pep-3118:

PEP 3118: Revised Buffer Protocol
=====================================================

The buffer protocol is a C-level API that lets Python types
exchange pointers into their internal representations.  A 
memory-mapped file can be viewed as a buffer of characters, for
example, and this lets another module such as :mod:`re`
treat memory-mapped files as a string of characters to be searched.

The primary users of the buffer protocol are numeric-processing
packages such as NumPy, which can expose the internal representation
of arrays so that callers can write data directly into an array instead
of going through a slower API.  This PEP updates the buffer protocol in light of experience 
from NumPy development, adding a number of new features
such as indicating the shape of an array, 
locking memory .

The most important new C API function is 
``PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags)``, which
takes an object and a set of flags, and fills in the
``Py_buffer`` structure with information 
about the object's memory representation.  Objects
can use this operation to lock memory in place 
while an external caller could be modifying the contents,
so there's a corresponding 
``PyObject_ReleaseBuffer(PyObject *obj, Py_buffer *view)`` to
indicate that the external caller is done.

The **flags** argument to :cfunc:`PyObject_GetBuffer` specifies
constraints upon the memory returned.  Some examples are:

 * :const:`PyBUF_WRITABLE` indicates that the memory must be writable.
 
 * :const:`PyBUF_LOCK` requests a read-only or exclusive lock on the memory.

 * :const:`PyBUF_C_CONTIGUOUS` and :const:`PyBUF_F_CONTIGUOUS`
   requests a C-contiguous (last dimension varies the fastest) or
   Fortran-contiguous (first dimension varies the fastest) layout.

.. XXX this feature is not in 2.6 docs yet

.. seealso::

   :pep:`3118` - Revising the buffer protocol
      PEP written by Travis Oliphant and Carl Banks; implemented by
      Travis Oliphant.
      

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

.. _pep-3119:

PEP 3119: Abstract Base Classes
=====================================================

Some object-oriented languages such as Java support interfaces: declarations
that a class has a given set of methods or supports a given access protocol.
Abstract Base Classes (or ABCs) are an equivalent feature for Python. The ABC
support consists of an :mod:`abc` module containing a metaclass called 
:class:`ABCMeta`, special handling
of this metaclass by the :func:`isinstance` and :func:`issubclass` built-ins,
and a collection of basic ABCs that the Python developers think will be widely
useful.

Let's say you have a particular class and wish to know whether it supports 
dictionary-style access.  The phrase "dictionary-style" is vague, however.
It probably means that accessing items with ``obj[1]`` works.  
Does it imply that setting items with ``obj[2] = value`` works?  
Or that the object will have :meth:`keys`, :meth:`values`, and :meth:`items`
methods?  What about the iterative variants  such as :meth:`iterkeys`?  :meth:`copy`
and :meth:`update`?  Iterating over the object with :func:`iter`?

Python 2.6 includes a number of different ABCs in the :mod:`collections`
module.  :class:`Iterable` indicates that a class defines :meth:`__iter__`,
and :class:`Container` means the class supports  ``x in y`` expressions
by defining a :meth:`__contains__` method.  The basic dictionary interface of
getting items, setting items, and 
:meth:`keys`, :meth:`values`, and :meth:`items`, is defined by the
:class:`MutableMapping` ABC.

You can derive your own classes from a particular ABC
to indicate they support that ABC's interface::

    import collections
  
    class Storage(collections.MutableMapping):
        ...


Alternatively, you could write the class without deriving from 
the desired ABC and instead register the class by
calling the ABC's :meth:`register` method::

    import collections
    
    class Storage:
        ...
	
    collections.MutableMapping.register(Storage)
    
For classes that you write, deriving from the ABC is probably clearer.
The :meth:`register`  method is useful when you've written a new
ABC that can describe an existing type or class, or if you want
to declare that some third-party class implements an ABC.
For example, if you defined a :class:`PrintableType` ABC,
it's legal to do:

  # Register Python's types
  PrintableType.register(int)
  PrintableType.register(float)
  PrintableType.register(str)

Classes should obey the semantics specified by an ABC, but 
Python can't check this; it's up to the class author to  
understand the ABC's requirements and to implement the code accordingly.

To check whether an object supports a particular interface, you can
now write::

    def func(d):
	if not isinstance(d, collections.MutableMapping):
	    raise ValueError("Mapping object expected, not %r" % d)        

(Don't feel that you must now begin writing lots of checks as in the 
above example.  Python has a strong tradition of duck-typing, where 
explicit type-checking isn't done and code simply calls methods on 
an object, trusting that those methods will be there and raising an
exception if they aren't.  Be judicious in checking for ABCs
and only do it where it helps.)

You can write your own ABCs by using ``abc.ABCMeta`` as the
metaclass in a class definition::

  from abc import ABCMeta

  class Drawable():
      __metaclass__ = ABCMeta
  
      def draw(self, x, y, scale=1.0):
	  pass

      def draw_doubled(self, x, y):
	  self.draw(x, y, scale=2.0)

	
  class Square(Drawable):
      def draw(self, x, y, scale):
          ...

	  
In the :class:`Drawable` ABC above, the :meth:`draw_doubled` method
renders the object at twice its size and can be implemented in terms
of other methods described in :class:`Drawable`.  Classes implementing
this ABC therefore don't need to provide their own implementation 
of :meth:`draw_doubled`, though they can do so.  An implementation
of :meth:`draw` is necessary, though; the ABC can't provide 
a useful generic implementation.  You 
can apply the ``@abstractmethod`` decorator to methods such as 
:meth:`draw` that must be implemented; Python will 
then raise an exception for classes that 
don't define the method::

    class Drawable():
	__metaclass__ = ABCMeta
    
	@abstractmethod
	def draw(self, x, y, scale):
	    pass

Note that the exception is only raised when you actually 
try to create an instance of a subclass without the method::

    >>> s=Square()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: Can't instantiate abstract class Square with abstract methods draw
    >>> 

Abstract data attributes can be declared using the ``@abstractproperty`` decorator::

    @abstractproperty
    def readonly(self):
       return self._x

Subclasses must then define a :meth:`readonly` property 

.. seealso::

   :pep:`3119` - Introducing Abstract Base Classes
      PEP written by Guido van Rossum and Talin.
      Implemented by Guido van Rossum.
      Backported to 2.6 by Benjamin Aranguren, with Alex Martelli.

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

.. _pep-3127:

PEP 3127: Integer Literal Support and Syntax
=====================================================

XXX write this -- this section is currently just brief notes.

Python 3.0 changes the syntax for octal integer literals, and 
adds supports for binary integers: 0o instad of 0,
and 0b for binary.  Python 2.6 doesn't support this, but a bin()
builtin was added.

XXX changes to the hex/oct builtins


New bin() built-in returns the binary form of a number.

.. seealso::

   :pep:`3127` - Integer Literal Support and Syntax
      PEP written by Patrick Maupin.

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

.. _pep-3129:

PEP 3129: Class Decorators
=====================================================

Decorators have been extended from functions to classes.  It's now legal to
write::

  @foo
  @bar
  class A:
    pass

This is equivalent to::

  class A:
    pass

  A = foo(bar(A))

XXX need to find a good motivating example.    

.. seealso::

   :pep:`3129` - Class Decorators
      PEP written by Collin Winter.

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

.. _pep-3141:

PEP 3141: A Type Hierarchy for Numbers
=====================================================

In Python 3.0, several abstract base classes for numeric types,
inspired by Scheme's numeric tower, are being added.
This change was backported to 2.6 as the :mod:`numbers` module.

The most general ABC is :class:`Number`.  It defines no operations at
all, and only exists to allow checking if an object is a number by
doing ``isinstance(obj, Number)``.

Numbers are further divided into :class:`Exact` and :class:`Inexact`.
Exact numbers can represent values precisely and operations never
round off the results or introduce tiny errors that may break the
commutativity and associativity properties; inexact numbers may
perform such rounding or introduce small errors.  Integers, long
integers, and rational numbers are exact, while floating-point 
and complex numbers are inexact.

:class:`Complex` is a subclass of :class:`Number`.  Complex numbers
can undergo the basic operations of addition, subtraction,
multiplication, division, and exponentiation, and you can retrieve the
real and imaginary parts and obtain a number's conjugate.  Python's built-in 
complex type is an implementation of :class:`Complex`.

:class:`Real` further derives from :class:`Complex`, and adds 
operations that only work on real numbers: :func:`floor`, :func:`trunc`, 
rounding, taking the remainder mod N, floor division, 
and comparisons.  

:class:`Rational` numbers derive from :class:`Real`, have
:attr:`numerator` and :attr:`denominator` properties, and can be
converted to floats.  Python 2.6 adds a simple rational-number class,
:class:`Fraction`, in the :mod:`fractions` module.  (It's called 
:class:`Fraction` instead of :class:`Rational` to avoid 
a name clash with :class:`numbers.Rational`.)

:class:`Integral` numbers derive from :class:`Rational`, and
can be shifted left and right with ``<<`` and ``>>``, 
combined using bitwise operations such as ``&`` and ``|``, 
and can be used as array indexes and slice boundaries.

In Python 3.0, the PEP slightly redefines the existing built-ins
:func:`round`, :func:`math.floor`, :func:`math.ceil`, and adds a new
one, :func:`math.trunc`, that's been backported to Python 2.6. 
:func:`math.trunc` rounds toward zero, returning the closest 
:class:`Integral` that's between the function's argument and zero.

.. seealso::

   :pep:`3141` - A Type Hierarchy for Numbers
      PEP written by Jeffrey Yasskin.

   `Scheme's numerical tower <http://www.gnu.org/software/guile/manual/html_node/Numerical-Tower.html#Numerical-Tower>`__, from the Guile manual.

   `Scheme's number datatypes <http://schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-9.html#%_sec_6.2>`__ from the R5RS Scheme specification.
  

The :mod:`fractions` Module
--------------------------------------------------

To fill out the hierarchy of numeric types, a rational-number class
has been added as the :mod:`fractions` module.  Rational numbers are
represented as a fraction, and can exactly represent
numbers such as two-thirds that floating-point numbers can only
approximate.

The :class:`Fraction` constructor takes two :class:`Integral` values
that will be the numerator and denominator of the resulting fraction. ::

    >>> from fractions import Fraction
    >>> a = Fraction(2, 3)
    >>> b = Fraction(2, 5)
    >>> float(a), float(b)
    (0.66666666666666663, 0.40000000000000002)
    >>> a+b
    Fraction(16, 15)
    >>> a/b
    Fraction(5, 3)

To help in converting floating-point numbers to rationals, 
the float type now has a :meth:`as_integer_ratio()` method that returns 
the numerator and denominator for a fraction that evaluates to the same
floating-point value::

    >>> (2.5) .as_integer_ratio()
    (5, 2)
    >>> (3.1415) .as_integer_ratio()
    (7074029114692207L, 2251799813685248L)
    >>> (1./3) .as_integer_ratio()
    (6004799503160661L, 18014398509481984L)

Note that values that can only be approximated by floating-point
numbers, such as 1./3, are not simplified to the number being
approximated; the fraction attempts to match the floating-point value
**exactly**.

The :mod:`fractions` module is based upon an implementation by Sjoerd
Mullender that was in Python's :file:`Demo/classes/` directory for a
long time.  This implementation was significantly updated by Jeffrey
Yasskin.

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

Here are all of the changes that Python 2.6 makes to the core Python language.

* When calling a function using the ``**`` syntax to provide keyword
  arguments, you are no longer required to use a Python dictionary;
  any mapping will now work::

    >>> def f(**kw):
    ...    print sorted(kw)
    ... 
    >>> ud=UserDict.UserDict()
    >>> ud['a'] = 1
    >>> ud['b'] = 'string'
    >>> f(**ud)
    ['a', 'b']

  .. Patch 1686487

* The built-in types now have improved support for extended slicing syntax,
  where various combinations of ``(start, stop, step)`` are supplied.
  Previously, the support was partial and certain corner cases wouldn't work.
  (Implemented by Thomas Wouters.)

  .. Revision 57619

* Properties now have three attributes, :attr:`getter`,
  :attr:`setter` and :attr:`deleter`, that are useful shortcuts for
  adding or modifying a getter, setter or deleter function to an 
  existing property. You would use them like this::

    class C(object):
	@property                                                              
	def x(self): 
	    return self._x                                            

	@x.setter                                                              
	def x(self, value): 
	    self._x = value                                    

	@x.deleter                                                             
	def x(self): 
	    del self._x             

    class D(C):
        @C.x.getter
        def x(self):
            return self._x * 2

        @x.setter
        def x(self, value):
            self._x = value / 2


* C functions and methods that use 
  :cfunc:`PyComplex_AsCComplex` will now accept arguments that 
  have a :meth:`__complex__` method.  In particular, the functions in the 
  :mod:`cmath` module will now accept objects with this method.
  This is a backport of a Python 3.0 change.
  (Contributed by Mark Dickinson.)

  .. Patch #1675423

  A numerical nicety: when creating a complex number from two floats
  on systems that support signed zeros (-0 and +0), the 
  :func:`complex` constructor will now preserve the sign 
  of the zero.

  .. Patch 1507

* More floating-point features were also added.  The :func:`float` function
  will now turn the strings ``+nan`` and ``-nan`` into the corresponding
  IEEE 754 Not A Number values, and ``+inf`` and ``-inf`` into 
  positive or negative infinity.  This works on any platform with 
  IEEE 754 semantics.  (Contributed by Christian Heimes.)

  .. Patch 1635

  Other functions in the :mod:`math` module, :func:`isinf` and
  :func:`isnan`, return true if their floating-point argument is
  infinite or Not A Number.

  .. Patch 1640

  The ``math.copysign(x, y)`` function
  copies the sign bit of an IEEE 754 number, returning the absolute
  value of *x* combined with the sign bit of *y*.  For example,
  ``math.copysign(1, -0.0)`` returns -1.0.  (Contributed by Christian
  Heimes.)

* Changes to the :class:`Exception` interface
  as dictated by :pep:`352` continue to be made.  For 2.6, 
  the :attr:`message` attribute is being deprecated in favor of the
  :attr:`args` attribute.

* The :exc:`GeneratorExit` exception now subclasses 
  :exc:`BaseException` instead of :exc:`Exception`.  This means 
  that an exception handler that does ``except Exception:``
  will not inadvertently catch :exc:`GeneratorExit`. 
  (Contributed by Chad Austin.)

  .. Patch #1537

* Generator objects now have a :attr:`gi_code` attribute that refers to 
  the original code object backing the generator.  
  (Contributed by Collin Winter.)

  .. Patch #1473257

* The :func:`compile` built-in function now accepts keyword arguments
  as well as positional parameters.  (Contributed by Thomas Wouters.)

  .. Patch 1444529

* The :func:`complex` constructor now accepts strings containing 
  parenthesized complex numbers, letting ``complex(repr(cmplx))``
  will now round-trip values.  For example, ``complex('(3+4j)')``
  now returns the value (3+4j).

  .. Patch 1491866

* The string :meth:`translate` method now accepts ``None`` as the 
  translation table parameter, which is treated as the identity 
  transformation.   This makes it easier to carry out operations
  that only delete characters.  (Contributed by Bengt Richter.)

  .. Patch 1193128

* The built-in :func:`dir` function now checks for a :meth:`__dir__`
  method on the objects it receives.  This method must return a list
  of strings containing the names of valid attributes for the object,
  and lets the object control the value that :func:`dir` produces.
  Objects that have :meth:`__getattr__` or :meth:`__getattribute__` 
  methods can use this to advertise pseudo-attributes they will honor.

  .. Patch 1591665

* Instance method objects have new attributes for the object and function
  comprising the method; the new synonym for :attr:`im_self` is
  :attr:`__self__`, and :attr:`im_func` is also available as :attr:`__func__`.
  The old names are still supported in Python 2.6; they're gone in 3.0.

* An obscure change: when you use the the :func:`locals` function inside a
  :keyword:`class` statement, the resulting dictionary no longer returns free
  variables.  (Free variables, in this case, are variables referred to in the
  :keyword:`class` statement  that aren't attributes of the class.)

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


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

* Type objects now have a cache of methods that can reduce
  the amount of work required to find the correct method implementation
  for a particular class; once cached, the interpreter doesn't need to
  traverse base classes to figure out the right method to call.  
  The cache is cleared if a base class or the class itself is modified, 
  so the cache should remain correct even in the face of Python's dynamic 
  nature.
  (Original optimization implemented by Armin Rigo, updated for 
  Python 2.6 by Kevin Jacobs.) 

  .. Patch 1700288

* All of the functions in the :mod:`struct` module have been rewritten in
  C, thanks to work at the Need For Speed sprint.
  (Contributed by Raymond Hettinger.)

* Internally, a bit is now set in type objects to indicate some of the standard
  built-in types.  This speeds up checking if an object is a subclass of one of
  these types.  (Contributed by Neal Norwitz.)

* Unicode strings now uses faster code for detecting
  whitespace and line breaks; this speeds up the :meth:`split` method 
  by about 25% and :meth:`splitlines` by 35%.
  (Contributed by Antoine Pitrou.)  Memory usage is reduced
  by using pymalloc for the Unicode string's data.

* The ``with`` statement now stores the :meth:`__exit__` method on the stack,
  producing a small speedup.  (Implemented by Jeffrey Yasskin.)

* To reduce memory usage, the garbage collector will now clear internal
  free lists when garbage-collecting the highest generation of objects.
  This may return memory to the OS sooner.

The net result of the 2.6 optimizations is that Python 2.6 runs the pystone
benchmark around XX% faster than Python 2.5.

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


New, Improved, and Deprecated Modules
=====================================

As usual, Python's standard library received a number of enhancements and bug
fixes.  Here's a partial list of the most notable changes, sorted alphabetically
by module name. Consult the :file:`Misc/NEWS` file in the source tree for a more
complete list of changes, or look through the CVS logs for all the details.

* The :mod:`bsddb.dbshelve` module now uses the highest pickling protocol
  available, instead of restricting itself to protocol 1.
  (Contributed by W. Barnes.)

  .. Patch 1551443

* A new data type in the :mod:`collections` module: :class:`namedtuple(typename,
  fieldnames)` is a factory function that creates subclasses of the standard tuple
  whose fields are accessible by name as well as index.  For example::

     >>> var_type = collections.namedtuple('variable', 
     ...             'id name type size')
     # Names are separated by spaces or commas.
     # 'id, name, type, size' would also work.
     >>> var_type._fields
     ('id', 'name', 'type', 'size')

     >>> var = var_type(1, 'frequency', 'int', 4)
     >>> print var[0], var.id		# Equivalent
     1 1
     >>> print var[2], var.type          # Equivalent
     int int
     >>> var._asdict()
     {'size': 4, 'type': 'int', 'id': 1, 'name': 'frequency'}
     >>> v2 = var._replace(name='amplitude')
     >>> v2
     variable(id=1, name='amplitude', type='int', size=4)

  Where the new :class:`namedtuple` type proved suitable, the standard
  library has been modified to return them.  For example, 
  the :meth:`Decimal.as_tuple` method now returns a named tuple with 
  :attr:`sign`, :attr:`digits`, and :attr:`exponent` fields.

  (Contributed by Raymond Hettinger.)

* Another change to the :mod:`collections` module is that the 
  :class:`deque` type now supports an optional *maxlen* parameter;
  if supplied, the deque's size will be restricted to no more 
  than *maxlen* items.  Adding more items to a full deque causes
  old items to be discarded.

  ::

    >>> from collections import deque
    >>> dq=deque(maxlen=3)
    >>> dq
    deque([], maxlen=3)
    >>> dq.append(1) ; dq.append(2) ; dq.append(3)
    >>> dq
    deque([1, 2, 3], maxlen=3)
    >>> dq.append(4)
    >>> dq
    deque([2, 3, 4], maxlen=3)

  (Contributed by Raymond Hettinger.)

* The :mod:`ctypes` module now supports a :class:`c_bool` datatype 
  that represents the C99 ``bool`` type.  (Contributed by David Remahl.)

  .. Patch 1649190

  The :mod:`ctypes` string, buffer and array types also have improved
  support for extended slicing syntax,
  where various combinations of ``(start, stop, step)`` are supplied.
  (Implemented by Thomas Wouters.)

  .. Revision 57769

* A new method in the :mod:`curses` module: for a window, :meth:`chgat` changes
  the display characters for a  certain number of characters on a single line.
  (Contributed by Fabian Kreutz.)
  ::

     # Boldface text starting at y=0,x=21 
     # and affecting the rest of the line.
     stdscr.chgat(0,21, curses.A_BOLD)  

  The :class:`Textbox` class in the :mod:`curses.textpad` module
  now supports editing in insert mode as well as overwrite mode.
  Insert mode is enabled by supplying a true value for the *insert_mode*
  parameter when creating the :class:`Textbox` instance.

* The :mod:`datetime` module's :meth:`strftime` methods now support a
  ``%f`` format code that expands to the number of microseconds in the
  object, zero-padded on
  the left to six places.  (Contributed by Skip Montanaro.)

  .. Patch 1158

* The :mod:`decimal` module was updated to version 1.66 of 
  `the General Decimal Specification <http://www2.hursley.ibm.com/decimal/decarith.html>`__.  New features
  include some methods for some basic mathematical functions such as
  :meth:`exp` and :meth:`log10`::

    >>> Decimal(1).exp()
    Decimal("2.718281828459045235360287471")
    >>> Decimal("2.7182818").ln()
    Decimal("0.9999999895305022877376682436")
    >>> Decimal(1000).log10()
    Decimal("3")

  The :meth:`as_tuple` method of :class:`Decimal` objects now returns a 
  named tuple with :attr:`sign`, :attr:`digits`, and :attr:`exponent` fields.
  
  (Implemented by Facundo Batista and Mark Dickinson.  Named tuple
  support added by Raymond Hettinger.)

* The :mod:`difflib` module's :class:`SequenceMatcher` class 
  now returns named tuples representing matches. 
  In addition to behaving like tuples, the returned values
  also have :attr:`a`, :attr:`b`, and :attr:`size` attributes.
  (Contributed by Raymond Hettinger.)

* An optional ``timeout`` parameter was added to the
  :class:`ftplib.FTP` class constructor as well as the :meth:`connect`
  method, specifying a timeout measured in seconds.  (Added by Facundo
  Batista.)  Also, the :class:`FTP` class's 
  :meth:`storbinary` and :meth:`storlines`
  now take an optional *callback* parameter that will be called with 
  each block of data after the data has been sent.
  (Contributed by Phil Schwartz.)

  .. Patch 1221598

* The :func:`reduce` built-in function is also available in the 
  :mod:`functools` module.  In Python 3.0, the built-in is dropped and it's
  only available from :mod:`functools`; currently there are no plans
  to drop the built-in in the 2.x series.  (Patched by 
  Christian Heimes.)

  .. Patch 1739906

* The :func:`glob.glob` function can now return Unicode filenames if 
  a Unicode path was used and Unicode filenames are matched within the directory.

  .. Patch #1001604

* The :mod:`gopherlib` module has been removed.

* A new function in the :mod:`heapq` module: ``merge(iter1, iter2, ...)`` 
  takes any number of iterables that return data *in sorted
  order*, and returns a new iterator that returns the contents of all
  the iterators, also in sorted order.  For example::

     heapq.merge([1, 3, 5, 9], [2, 8, 16]) ->
       [1, 2, 3, 5, 8, 9, 16]

  Another new function, ``heappushpop(heap, item)``,
  pushes *item* onto *heap*, then pops off and returns the smallest item. 
  This is more efficient than making a call to :func:`heappush` and then
  :func:`heappop`.

  (Contributed by Raymond Hettinger.)

* An optional ``timeout`` parameter was added to the
  :class:`httplib.HTTPConnection` and :class:`HTTPSConnection` 
  class constructors, specifying a timeout measured in seconds.
  (Added by Facundo Batista.)

* Most of the :mod:`inspect` module's functions, such as 
  :func:`getmoduleinfo` and :func:`getargs`, now return named tuples.  
  In addition to behaving like tuples, the elements of the  return value
  can also be accessed as attributes.
  (Contributed by Raymond Hettinger.)

  Some new functions in the module include 
  :func:`isgenerator`, :func:`isgeneratorfunction`, 
  and :func:`isabstract`.

* The :mod:`itertools` module gained several new functions.

  ``izip_longest(iter1, iter2, ...[, fillvalue])`` makes tuples from
  each of the elements; if some of the iterables are shorter than
  others, the missing values are set to *fillvalue*.  For example::

     itertools.izip_longest([1,2,3], [1,2,3,4,5]) ->
       [(1, 1), (2, 2), (3, 3), (None, 4), (None, 5)]

  ``product(iter1, iter2, ..., [repeat=N])`` returns the Cartesian product
  of the supplied iterables, a set of tuples containing
  every possible combination of the elements returned from each iterable. ::

     itertools.product([1,2,3], [4,5,6]) ->
       [(1, 4), (1, 5), (1, 6), 
	(2, 4), (2, 5), (2, 6), 
	(3, 4), (3, 5), (3, 6)]

  The optional *repeat* keyword argument is used for taking the
  product of an iterable or a set of iterables with themselves, 
  repeated *N* times.  With a single iterable argument, *N*-tuples
  are returned::

     itertools.product([1,2], repeat=3)) ->
       [(1, 1, 1), (1, 1, 2), (1, 2, 1), (1, 2, 2), 
        (2, 1, 1), (2, 1, 2), (2, 2, 1), (2, 2, 2)]

  With two iterables, *2N*-tuples are returned. ::

     itertools(product([1,2], [3,4], repeat=2) ->
       [(1, 3, 1, 3), (1, 3, 1, 4), (1, 3, 2, 3), (1, 3, 2, 4), 
        (1, 4, 1, 3), (1, 4, 1, 4), (1, 4, 2, 3), (1, 4, 2, 4), 
        (2, 3, 1, 3), (2, 3, 1, 4), (2, 3, 2, 3), (2, 3, 2, 4), 
        (2, 4, 1, 3), (2, 4, 1, 4), (2, 4, 2, 3), (2, 4, 2, 4)]

  ``combinations(iterable, r)`` returns sub-sequences of length *r* from
  the elements of *iterable*. ::

    itertools.combinations('123', 2) ->
      [('1', '2'), ('1', '3'), ('2', '3')]

    itertools.combinations('123', 3) ->
      [('1', '2', '3')]

    itertools.combinations('1234', 3) ->
      [('1', '2', '3'), ('1', '2', '4'), ('1', '3', '4'), 
       ('2', '3', '4')]

  ``permutations(iter[, r])`` returns all the permutations of length *r* of
  the iterable's elements.  If *r* is not specified, it will default to the 
  number of elements produced by the iterable.

    itertools.permutations([1,2,3,4], 2) ->
      [(1, 2), (1, 3), (1, 4), 
       (2, 1), (2, 3), (2, 4), 
       (3, 1), (3, 2), (3, 4), 
       (4, 1), (4, 2), (4, 3)]

  ``itertools.chain(*iterables)` is an existing function in
  :mod:`itertools` that gained a new constructor in Python 2.6.
  ``itertools.chain.from_iterable(iterable)`` takes a single 
  iterable that should return other iterables.  :func:`chain` will
  then return all the elements of the first iterable, then
  all the elements of the second, and so on. ::

    chain.from_iterable([[1,2,3], [4,5,6]]) ->
       [1, 2, 3, 4, 5, 6]
  
  (All contributed by Raymond Hettinger.)

* The :mod:`logging` module's :class:`FileHandler` class 
  and its subclasses :class:`WatchedFileHandler`, :class:`RotatingFileHandler`,
  and :class:`TimedRotatingFileHandler` now 
  have an optional *delay* parameter to its constructor.  If *delay* 
  is true, opening of the log file is deferred until the first
  :meth:`emit` call is made.  (Contributed by Vinay Sajip.)

* The :mod:`macfs` module has been removed.  This in turn required the
  :func:`macostools.touched` function to be removed because it depended on the
  :mod:`macfs` module.

  .. Patch #1490190

* :class:`mmap` objects now have a :meth:`rfind` method that finds
  a substring, beginning at the end of the string and searching
  backwards.  The :meth:`find` method
  also gained an *end* parameter containing the index at which to stop
  the forward search.
  (Contributed by John Lenton.)

* (3.0-warning mode) The :mod:`new` module has been removed from
  Python 3.0.  Importing it therefore triggers a warning message.

* The :mod:`operator` module gained a 
  :func:`methodcaller` function that takes a name and an optional 
  set of arguments, returning a callable that will call 
  the named function on any arguments passed to it.  For example::

    >>> # Equivalent to lambda s: s.replace('old', 'new')
    >>> replacer = operator.methodcaller('replace', 'old', 'new')
    >>> replacer('old wine in old bottles')
    'new wine in new bottles'
   
  (Contributed by Georg Brandl, after a suggestion by Gregory Petrosyan.)

  The :func:`attrgetter` function now accepts dotted names and performs
  the corresponding attribute lookups::

    >>> inst_name = operator.attrgetter('__class__.__name__')
    >>> inst_name('')
    'str'
    >>> inst_name(help)
    '_Helper'

  (Contributed by Georg Brandl, after a suggestion by Barry Warsaw.)

* New functions in the :mod:`os` module include 
  ``fchmod(fd, mode)``,   ``fchown(fd, uid, gid)``,  
  and ``lchmod(path, mode)``, on operating systems that support these
  functions. :func:`fchmod` and :func:`fchown` let you change the mode
  and ownership of an opened file, and :func:`lchmod` changes the mode
  of a symlink.

  (Contributed by Georg Brandl and Christian Heimes.)

* The :func:`os.walk` function now has a ``followlinks`` parameter. If
  set to True, it will follow symlinks pointing to directories and
  visit the directory's contents.  For backward compatibility, the
  parameter's default value is false.  Note that the function can fall
  into an infinite recursion if there's a symlink that points to a
  parent directory.
       
  .. Patch 1273829

* The ``os.environ`` object's :meth:`clear` method will now unset the 
  environment variables using :func:`os.unsetenv` in addition to clearing
  the object's keys.  (Contributed by Martin Horcicka.)

  .. Patch #1181 

* In the :mod:`os.path` module, the :func:`splitext` function
  has been changed to not split on leading period characters.
  This produces better results when operating on Unix's dot-files.
  For example, ``os.path.splitext('.ipython')``
  now returns ``('.ipython', '')`` instead of ``('', '.ipython')``.

  .. Bug #115886

  A new function, :func:`relpath(path, start)` returns a relative path
  from the ``start`` path, if it's supplied, or from the current
  working directory to the destination ``path``.  (Contributed by
  Richard Barran.)

  .. Patch 1339796

  On Windows, :func:`os.path.expandvars` will now expand environment variables 
  in the form "%var%", and "~user" will be expanded into the 
  user's home directory path.  (Contributed by Josiah Carlson.)

  .. Patch 957650

* The Python debugger provided by the :mod:`pdb` module 
  gained a new command: "run" restarts the Python program being debugged,
  and can optionally take new command-line arguments for the program.
  (Contributed by Rocky Bernstein.)

  .. Patch #1393667

* The :mod:`pickletools` module now has an :func:`optimize` function 
  that takes a string containing a pickle and removes some unused 
  opcodes, returning a shorter pickle that contains the same data structure.
  (Contributed by Raymond Hettinger.)

* New functions in the :mod:`posix` module: :func:`chflags` and :func:`lchflags`
  are wrappers for the corresponding system calls (where they're available).
  Constants for the flag values are defined in the :mod:`stat` module; some
  possible values include :const:`UF_IMMUTABLE` to signal the file may not be
  changed and :const:`UF_APPEND` to indicate that data can only be appended to the
  file.  (Contributed by M. Levinson.)

  ``os.closerange(*low*, *high*)`` efficiently closes all file descriptors
  from *low* to *high*, ignoring any errors and not including *high* itself.
  This function is now used by the :mod:`subprocess` module to make starting
  processes faster.  (Contributed by Georg Brandl.)

  .. Patch #1663329  

* The :mod:`pyexpat` module's :class:`Parser` objects now allow setting
  their :attr:`buffer_size` attribute to change the size of the buffer 
  used to hold character data.
  (Contributed by Achim Gaedke.)

  .. Patch 1137

* The :mod:`Queue` module now provides queue classes that retrieve entries
  in different orders.  The :class:`PriorityQueue` class stores 
  queued items in a heap and retrieves them in priority order, 
  and :class:`LifoQueue` retrieves the most recently added entries first,
  meaning that it behaves like a stack.
  (Contributed by Raymond Hettinger.)

* The :mod:`random` module's :class:`Random` objects can
  now be pickled on a 32-bit system and unpickled on a 64-bit
  system, and vice versa.  Unfortunately, this change also means
  that Python 2.6's :class:`Random` objects can't be unpickled correctly
  on earlier versions of Python.
  (Contributed by Shawn Ligocki.)

  .. Issue 1727780

  The new ``triangular(low, high, mode)`` function returns random
  numbers following a triangular distribution.   The returned values
  are between *low* and *high*, not including *high* itself, and 
  with *mode* as the mode, the most frequently occurring value 
  in the distribution.  (Contributed by Wladmir van der Laan and
  Raymond Hettinger.)

  .. Patch 1681432

* Long regular expression searches carried out by the  :mod:`re`
  module will now check for signals being delivered, so especially
  long searches can now be interrupted.
  (Contributed by Josh Hoyt and Ralf Schmitt.)

  .. Patch 846388

* The :mod:`rgbimg` module has been removed.

* The :mod:`sched` module's :class:`scheduler` instances now 
  have a read-only :attr:`queue` attribute that returns the 
  contents of the scheduler's queue, represented as a list of
  named tuples with the fields ``(time, priority, action, argument)``.
  (Contributed by Raymond Hettinger.)

  .. Patch 1861

* The :mod:`select` module now has wrapper functions
  for the Linux :cfunc:`epoll` and BSD :cfunc:`kqueue` system calls.
  Also, a :meth:`modify` method was added to the existing :class:`poll`
  objects; ``pollobj.modify(fd, eventmask)`` takes a file descriptor
  or file object and an event mask, 
  
  (Contributed by Christian Heimes.)

  .. Patch 1657

* The :mod:`sets` module has been deprecated; it's better to 
  use the built-in :class:`set` and :class:`frozenset` types.

* Integrating signal handling with GUI handling event loops 
  like those used by Tkinter or GTk+ has long been a problem; most
  software ends up polling, waking up every fraction of a second.
  The :mod:`signal` module can now make this more efficient.
  Calling ``signal.set_wakeup_fd(fd)`` sets a file descriptor
  to be used; when a signal is received, a byte is written to that 
  file descriptor.  There's also a C-level function,
  :cfunc:`PySignal_SetWakeupFd`, for setting the descriptor.

  Event loops will use this by opening a pipe to create two descriptors,
  one for reading and one for writing.  The writable descriptor
  will be passed to :func:`set_wakeup_fd`, and the readable descriptor
  will be added to the list of descriptors monitored by the event loop via
  :cfunc:`select` or :cfunc:`poll`.
  On receiving a signal, a byte will be written and the main event loop 
  will be woken up, without the need to poll.

  (Contributed by Adam Olsen.)

  .. Patch 1583

  The :func:`siginterrupt` function is now available from Python code,
  and allows changing whether signals can interrupt system calls or not.
  (Contributed by Ralf Schmitt.)

  The :func:`setitimer` and :func:`getitimer` functions have also been
  added on systems that support these system calls.  :func:`setitimer`
  allows setting interval timers that will cause a signal to be
  delivered to the process after a specified time, measured in
  wall-clock time, consumed process time, or combined process+system
  time.  (Contributed by Guilherme Polo.)

  .. Patch 2240


* The :mod:`smtplib` module now supports SMTP over SSL thanks to the
  addition of the :class:`SMTP_SSL` class. This class supports an
  interface identical to the existing :class:`SMTP` class.   Both 
  class constructors also have an optional ``timeout`` parameter
  that specifies a timeout for the initial connection attempt, measured in
  seconds.

  An implementation of the LMTP protocol (:rfc:`2033`) was also added to
  the module.  LMTP is used in place of SMTP when transferring e-mail
  between agents that don't manage a mail queue.

  (SMTP over SSL contributed by Monty Taylor; timeout parameter
  added by Facundo Batista; LMTP implemented by Leif
  Hedstrom.)

  .. Patch #957003

* In the :mod:`smtplib` module, SMTP.starttls() now complies with :rfc:`3207`
  and forgets any knowledge obtained from the server not obtained from
  the TLS negotiation itself.  (Patch contributed by Bill Fenner.)

  .. Issue 829951

* The :mod:`socket` module now supports TIPC (http://tipc.sf.net),
  a high-performance non-IP-based protocol designed for use in clustered
  environments.  TIPC addresses are 4- or 5-tuples.
  (Contributed by Alberto Bertogli.)

  .. Patch #1646

* The base classes in the :mod:`SocketServer` module now support
  calling a :meth:`handle_timeout` method after a span of inactivity 
  specified by the server's :attr:`timeout` attribute.  (Contributed 
  by Michael Pomraning.)

  .. Patch #742598

* The :mod:`struct` module now supports the C99 :ctype:`_Bool` type,
  using the format character ``'?'``. 
  (Contributed by David Remahl.)
 
* A new variable in the :mod:`sys` module,
  :attr:`float_info`, is an object
  containing information about the platform's floating-point support
  derived from the :file:`float.h` file.  Attributes of this object
  include 
  :attr:`mant_dig` (number of digits in the mantissa), :attr:`epsilon`
  (smallest difference between 1.0 and the next largest value
  representable), and several others.  (Contributed by Christian Heimes.)

  .. Patch 1534

  Another new variable, :attr:`dont_write_bytecode`, controls whether Python
  writes any :file:`.pyc` or :file:`.pyo` files on importing a module.
  If this variable is true, the compiled files are not written.  The
  variable is initially set on start-up by supplying the :option:`-B`
  switch to the Python interpreter, or by setting the
  :envvar:`PYTHONDONTWRITEBYTECODE` environment variable before
  running the interpreter.  Python code can subsequently 
  change the value of this variable to control whether bytecode files
  are written or not.
  (Contributed by Neal Norwitz and Georg Brandl.)

  Information about the command-line arguments supplied to the Python 
  interpreter are available as attributes of a ``sys.flags`` named 
  tuple.  For example, the :attr:`verbose` attribute is true if Python 
  was executed in verbose mode, :attr:`debug` is true in debugging mode, etc.
  These attributes are all read-only.
  (Contributed by Christian Heimes.)

  It's now possible to determine the current profiler and tracer functions
  by calling :func:`sys.getprofile` and :func:`sys.gettrace`.  
  (Contributed by Georg Brandl.)

  .. Patch #1648

* The :mod:`tarfile` module now supports POSIX.1-2001 (pax) and
  POSIX.1-1988 (ustar) format tarfiles, in addition to the GNU tar
  format that was already supported.  The default format 
  is GNU tar; specify the ``format`` parameter to open a file
  using a different format::

    tar = tarfile.open("output.tar", "w", format=tarfile.PAX_FORMAT)

  The new ``errors`` parameter lets you specify an error handling
  scheme for character conversions: the three standard ways Python can
  handle errors ``'strict'``, ``'ignore'``, ``'replace'`` , or the
  special value ``'utf-8'``, which replaces bad characters with their
  UTF-8 representation.  Character conversions occur because the PAX
  format supports Unicode filenames, defaulting to UTF-8 encoding.

  The :meth:`TarFile.add` method now accepts a ``exclude`` argument that's
  a function that can be used to exclude certain filenames from
  an archive. 
  The function must take a filename and return true if the file 
  should be excluded or false if it should be archived.
  The function is applied to both the name initially passed to :meth:`add`
  and to the names of files in recursively-added directories.
  
  (All changes contributed by Lars Gustäbel).

* An optional ``timeout`` parameter was added to the
  :class:`telnetlib.Telnet` class constructor, specifying a timeout
  measured in seconds.  (Added by Facundo Batista.)

* The :class:`tempfile.NamedTemporaryFile` class usually deletes 
  the temporary file it created when the file is closed.  This 
  behaviour can now be changed by passing ``delete=False`` to the 
  constructor.  (Contributed by Damien Miller.)

  .. Patch #1537850

  A new class, :class:`SpooledTemporaryFile`, behaves like 
  a temporary file but stores its data in memory until a maximum size is 
  exceeded.  On reaching that limit, the contents will be written to 
  an on-disk temporary file.  (Contributed by Dustin J. Mitchell.)

  The :class:`NamedTemporaryFile` and :class:`SpooledTemporaryFile` classes
  both work as context managers, so you can write 
  ``with tempfile.NamedTemporaryFile() as tmp: ...``.
  (Contributed by Alexander Belopolsky.)

  .. Issue #2021

* The :mod:`test.test_support` module now contains a
  :func:`EnvironmentVarGuard`
  context manager that  supports temporarily changing environment variables and
  automatically restores them to their old values. 

  Another context manager, :class:`TransientResource`, can surround calls
  to resources that may or may not be available; it will catch and
  ignore a specified list of exceptions.  For example,
  a network test may ignore certain failures when connecting to an
  external web site::

      with test_support.TransientResource(IOError, errno=errno.ETIMEDOUT):
          f = urllib.urlopen('https://sf.net')                         
          ...

  (Contributed by Brett Cannon.)

* The :mod:`textwrap` module can now preserve existing whitespace 
  at the beginnings and ends of the newly-created lines
  by specifying ``drop_whitespace=False``
  as an argument::

    >>> S = """This  sentence  has a bunch   of    extra   whitespace."""
    >>> print textwrap.fill(S, width=15)
    This  sentence
    has a bunch
    of    extra
    whitespace.
    >>> print textwrap.fill(S, drop_whitespace=False, width=15)
    This  sentence
      has a bunch
       of    extra
       whitespace.
    >>> 

  (Contributed by Dwayne Bailey.)

  .. Patch #1581073

* The :mod:`timeit` module now accepts callables as well as strings 
  for the statement being timed and for the setup code.
  Two convenience functions were added for creating 
  :class:`Timer` instances: 
  ``repeat(stmt, setup, time, repeat, number)`` and 
  ``timeit(stmt, setup, time, number)`` create an instance and call
  the corresponding method. (Contributed by Erik Demaine.)

  .. Patch #1533909

* An optional ``timeout`` parameter was added to the
  :func:`urllib.urlopen` function and the
  :class:`urllib.ftpwrapper` class constructor, as well as the 
  :func:`urllib2.urlopen` function.  The parameter specifies a timeout
  measured in seconds.   For example::

     >>> u = urllib2.urlopen("http://slow.example.com", timeout=3)
     Traceback (most recent call last):
       ...
     urllib2.URLError: <urlopen error timed out>
     >>>   

  (Added by Facundo Batista.) 

* The XML-RPC classes :class:`SimpleXMLRPCServer` and :class:`DocXMLRPCServer`
  classes can now be prevented from immediately opening and binding to
  their socket by passing True as the ``bind_and_activate``
  constructor parameter.  This can be used to modify the instance's
  :attr:`allow_reuse_address` attribute before calling the 
  :meth:`server_bind` and :meth:`server_activate` methods to 
  open the socket and begin listening for connections.
  (Contributed by Peter Parente.)

  .. Patch 1599845

  :class:`SimpleXMLRPCServer` also has a :attr:`_send_traceback_header`
  attribute; if true, the exception and formatted traceback are returned 
  as HTTP headers "X-Exception" and "X-Traceback".  This feature is 
  for debugging purposes only and should not be used on production servers
  because the tracebacks could possibly reveal passwords or other sensitive
  information.  (Contributed by Alan McIntyre as part of his 
  project for Google's Summer of Code 2007.)

* The :mod:`zipfile` module's :class:`ZipFile` class now has 
  :meth:`extract` and :meth:`extractall` methods that will unpack 
  a single file or all the files in the archive to the current directory, or 
  to a specified directory::

    z = zipfile.ZipFile('python-251.zip')

    # Unpack a single file, writing it relative to the /tmp directory.
    z.extract('Python/sysmodule.c', '/tmp')

    # Unpack all the files in the archive.
    z.extractall()

  (Contributed by Alan McIntyre.)

  .. Patch 467924

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

Improved SSL Support
--------------------------------------------------

Bill Janssen made extensive improvements to Python 2.6's support for
SSL.

XXX use ssl.sslsocket - subclass of socket.socket.

XXX Can specify if certificate is required, and obtain certificate info
by calling getpeercert method.

XXX sslwrap() behaves like socket.ssl

XXX Certain features require the OpenSSL package to be installed, notably
  the 'openssl' binary.

.. seealso::

   SSL module documentation.


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

plistlib: A Property-List Parser
--------------------------------------------------

A commonly-used format on MacOS X is the ``.plist`` format, 
which stores basic data types (numbers, strings, lists, 
and dictionaries) and serializes them into an XML-based format.
(It's a lot like the XML-RPC serialization of data types.)

Despite being primarily used on MacOS X, the format 
has nothing Mac-specific about it and the Python implementation works
on any platform that Python supports, so the :mod:`plistlib` module
has been promoted to the standard library.

Using the module is simple::

    import sys
    import plistlib
    import datetime

    # Create data structure
    data_struct = dict(lastAccessed=datetime.datetime.now(),
		       version=1,
		       categories=('Personal', 'Shared', 'Private'))

    # Create string containing XML.
    plist_str = plistlib.writePlistToString(data_struct)
    new_struct = plistlib.readPlistFromString(plist_str)
    print data_struct
    print new_struct

    # Write data structure to a file and read it back.
    plistlib.writePlist(data_struct, '/tmp/customizations.plist')
    new_struct = plistlib.readPlist('/tmp/customizations.plist')

    # read/writePlist accepts file-like objects as well as paths.
    plistlib.writePlist(data_struct, sys.stdout)
   

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


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

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

* Python 2.6 can be built with Microsoft Visual Studio 2008.
  See the :file:`PCbuild9` directory for the build files.
  (Implemented by Christian Heimes.)

* Python now can only be compiled with C89 compilers (after 19
  years!).  This means that the Python source tree can now drop its
  own implementations of :cfunc:`memmove` and :cfunc:`strerror`, which
  are in the C89 standard library.

* The BerkeleyDB module now has a C API object, available as 
  ``bsddb.db.api``.   This object can be used by other C extensions
  that wish to use the :mod:`bsddb` module for their own purposes.
  (Contributed by Duncan Grisby.)

  .. Patch 1551895

* Several functions return information about the platform's 
  floating-point support.  :cfunc:`PyFloat_GetMax` returns
  the maximum representable floating point value,
  and :cfunc:`PyFloat_GetMin` returns the minimum 
  positive value.  :cfunc:`PyFloat_GetInfo` returns a dictionary 
  containing more information from the :file:`float.h` file, such as
  ``"mant_dig"`` (number of digits in the mantissa), ``"epsilon"``
  (smallest difference between 1.0 and the next largest value
  representable), and several others.
  (Contributed by Christian Heimes.)

  .. Issue 1534

* Python's C API now includes two functions for case-insensitive string
  comparisons, ``PyOS_stricmp(char*, char*)``
  and ``PyOS_strnicmp(char*, char*, Py_ssize_t)``.
  (Contributed by Christian Heimes.)

  .. Issue 1635

* Some macros were renamed in both 3.0 and 2.6 to make it clearer that
  they are macros,
  not functions.  :cmacro:`Py_Size()` became :cmacro:`Py_SIZE()`,
  :cmacro:`Py_Type()` became :cmacro:`Py_TYPE()`, and
  :cmacro:`Py_Refcnt()` became :cmacro:`Py_REFCNT()`.  
  The mixed-case macros are still available
  in Python 2.6 for backward compatibility.

  .. Issue 1629

* Distutils now places C extensions it builds in a 
  different directory when running on a debug version of Python.
  (Contributed by Collin Winter.)

  .. Patch 1530959

* Several basic data types, such as integers and strings, maintain
  internal free lists of objects that can be re-used.  The data
  structures for these free lists now follow a naming convention: the
  variable is always named ``free_list``, the counter is always named
  ``numfree``, and a macro :cmacro:`Py<typename>_MAXFREELIST` is
  always defined.

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


Port-Specific Changes: Windows
-----------------------------------

* The :mod:`msvcrt` module now supports 
  both the normal and wide char variants of the console I/O
  API.  The :func:`getwch` function reads a keypress and returns a Unicode 
  value, as does the :func:`getwche` function.  The :func:`putwch` function
  takes a Unicode character and writes it to the console.
  (Contributed by Christian Heimes.)

* :func:`os.path.expandvars` will now expand environment variables 
  in the form "%var%", and "~user" will be expanded into the 
  user's home directory path.  (Contributed by Josiah Carlson.)

* The :mod:`socket` module's socket objects now have an 
  :meth:`ioctl` method that provides a limited interface to the 
  :cfunc:`WSAIoctl` system interface.

* The :mod:`_winreg` module now has a function, 
  :func:`ExpandEnvironmentStrings`, 
  that expands environment variable references such as ``%NAME%``
  in an input string.  The handle objects provided by this
  module now support the context protocol, so they can be used 
  in :keyword:`with` statements. (Contributed by Christian Heimes.)

* The new default compiler on Windows is Visual Studio 2008 (VS 9.0). The 
  build directories for Visual Studio 2003 (VS7.1) and 2005 (VS8.0)
  were moved into the PC/ directory. The new PCbuild directory supports
  cross compilation for X64, debug builds and Profile Guided Optimization
  (PGO). PGO builds are roughly 10% faster than normal builds.
  (Contributed by Christian Heimes with help from Amaury Forgeot d'Arc and
  Martin von Loewis.)

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


.. _section-other:

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

As usual, there were a bunch of other improvements and bugfixes
scattered throughout the source tree.  A search through the change
logs finds there were XXX patches applied and YYY bugs fixed between
Python 2.5 and 2.6.  Both figures are likely to be underestimates.

Some of the more notable changes are:

* It's now possible to prevent Python from writing any :file:`.pyc` 
  or :file:`.pyo` files by either supplying the :option:`-B` switch
  or setting the :envvar:`PYTHONDONTWRITEBYTECODE` environment variable
  to any non-empty string when running the Python interpreter.  These
  are also used to set the :data:`sys.dont_write_bytecode` attribute;
  Python code can change this variable to control whether bytecode
  files are subsequently written.
  (Contributed by Neal Norwitz and Georg Brandl.)

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


Porting to Python 2.6
=====================

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

* The :meth:`__init__` method of :class:`collections.deque`
  now clears any existing contents of the deque
  before adding elements from the iterable.  This change makes the
  behavior match that of ``list.__init__()``.  

* The :class:`Decimal` constructor now accepts leading and trailing 
  whitespace when passed a string.  Previously it would raise an
  :exc:`InvalidOperation` exception.  On the other hand, the
  :meth:`create_decimal` method of :class:`Context` objects now
  explicitly disallows extra whitespace, raising a 
  :exc:`ConversionSyntax` exception.

* Due to an implementation accident, if you passed a file path to 
  the built-in  :func:`__import__` function, it would actually import
  the specified file.  This was never intended to work, however, and 
  the implementation now explicitly checks for this case and raises 
  an :exc:`ImportError`.

* The :mod:`socket` module exception :exc:`socket.error` now inherits
  from :exc:`IOError`.  Previously it wasn't a subclass of
  :exc:`StandardError` but now it is, through :exc:`IOError`.
  (Implemented by Gregory P. Smith.)

  .. Issue 1706815

* The :mod:`xmlrpclib` module no longer automatically converts
  :class:`datetime.date` and :class:`datetime.time` to the 
  :class:`xmlrpclib.DateTime` type; the conversion semantics were
  not necessarily correct for all applications.  Code using
  :mod:`xmlrpclib` should convert :class:`date` and :class:`time` 
  instances.

  .. Issue 1330538

* (3.0-warning mode) The :class:`Exception` class now warns 
  when accessed using slicing or index access; having 
  :class:`Exception` behave like a tuple is being phased out.

* (3.0-warning mode) inequality comparisons between two dictionaries
  or two objects that don't implement comparison methods are reported
  as warnings.  ``dict1 == dict2`` still works, but ``dict1 < dict2``
  is being phased out.
  
  Comparisons between cells, which are an implementation detail of Python's
  scoping rules, also cause warnings because such comparisons are forbidden
  entirely in 3.0.

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


.. _acks:

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

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