Doc/whatsnew/2.3.rst

****************************
  What's New in Python 2.3
****************************

:Author: A.M. Kuchling

.. |release| replace:: 1.01

.. $Id: whatsnew23.tex 54631 2007-03-31 11:58:36Z georg.brandl $

This article explains the new features in Python 2.3.  Python 2.3 was released
on July 29, 2003.

The main themes for Python 2.3 are polishing some of the features added in 2.2,
adding various small but useful enhancements to the core language, and expanding
the standard library.  The new object model introduced in the previous version
has benefited from 18 months of bugfixes and from optimization efforts that have
improved the performance of new-style classes.  A few new built-in functions
have been added such as :func:`sum` and :func:`enumerate`.  The :keyword:`in`
operator can now be used for substring searches (e.g. ``"ab" in "abc"`` returns
:const:`True`).

Some of the many new library features include Boolean, set, heap, and date/time
data types, the ability to import modules from ZIP-format archives, metadata
support for the long-awaited Python catalog, an updated version of IDLE, and
modules for logging messages, wrapping text, parsing CSV files, processing
command-line options, using BerkeleyDB databases...  the list of new and
enhanced modules is lengthy.

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 2.3, such as the Python Library
Reference and the Python Reference Manual.  If you want to understand the
complete implementation and design rationale, refer to the PEP for a particular
new feature.

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


PEP 218: A Standard Set Datatype
================================

The new :mod:`sets` module contains an implementation of a set datatype.  The
:class:`Set` class is for mutable sets, sets that can have members added and
removed.  The :class:`ImmutableSet` class is for sets that can't be modified,
and instances of :class:`ImmutableSet` can therefore be used as dictionary keys.
Sets are built on top of dictionaries, so the elements within a set must be
hashable.

Here's a simple example::

   >>> import sets
   >>> S = sets.Set([1,2,3])
   >>> S
   Set([1, 2, 3])
   >>> 1 in S
   True
   >>> 0 in S
   False
   >>> S.add(5)
   >>> S.remove(3)
   >>> S
   Set([1, 2, 5])
   >>>

The union and intersection of sets can be computed with the :meth:`union` and
:meth:`intersection` methods; an alternative notation uses the bitwise operators
``&`` and ``|``. Mutable sets also have in-place versions of these methods,
:meth:`union_update` and :meth:`intersection_update`. ::

   >>> S1 = sets.Set([1,2,3])
   >>> S2 = sets.Set([4,5,6])
   >>> S1.union(S2)
   Set([1, 2, 3, 4, 5, 6])
   >>> S1 | S2                  # Alternative notation
   Set([1, 2, 3, 4, 5, 6])
   >>> S1.intersection(S2)
   Set([])
   >>> S1 & S2                  # Alternative notation
   Set([])
   >>> S1.union_update(S2)
   >>> S1
   Set([1, 2, 3, 4, 5, 6])
   >>>

It's also possible to take the symmetric difference of two sets.  This is the
set of all elements in the union that aren't in the intersection.  Another way
of putting it is that the symmetric difference contains all elements that are in
exactly one set.  Again, there's an alternative notation (``^``), and an in-
place version with the ungainly name :meth:`symmetric_difference_update`. ::

   >>> S1 = sets.Set([1,2,3,4])
   >>> S2 = sets.Set([3,4,5,6])
   >>> S1.symmetric_difference(S2)
   Set([1, 2, 5, 6])
   >>> S1 ^ S2
   Set([1, 2, 5, 6])
   >>>

There are also :meth:`issubset` and :meth:`issuperset` methods for checking
whether one set is a subset or superset of another::

   >>> S1 = sets.Set([1,2,3])
   >>> S2 = sets.Set([2,3])
   >>> S2.issubset(S1)
   True
   >>> S1.issubset(S2)
   False
   >>> S1.issuperset(S2)
   True
   >>>


.. seealso::

   :pep:`218` - Adding a Built-In Set Object Type
      PEP written by Greg V. Wilson. Implemented by Greg V. Wilson, Alex Martelli, and
      GvR.

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


.. _section-generators:

PEP 255: Simple Generators
==========================

In Python 2.2, generators were added as an optional feature, to be enabled by a
``from __future__ import generators`` directive.  In 2.3 generators no longer
need to be specially enabled, and are now always present; this means that
:keyword:`yield` is now always a keyword.  The rest of this section is a copy of
the description of generators from the "What's New in Python 2.2" document; if
you read it back when Python 2.2 came out, you can skip the rest of this
section.

You're doubtless familiar with how function calls work in Python or C. When you
call a function, it gets a private namespace where its local variables are
created.  When the function reaches a :keyword:`return` statement, the local
variables are destroyed and the resulting value is returned to the caller.  A
later call to the same function will get a fresh new set of local variables.
But, what if the local variables weren't thrown away on exiting a function?
What if you could later resume the function where it left off?  This is what
generators provide; they can be thought of as resumable functions.

Here's the simplest example of a generator function::

   def generate_ints(N):
       for i in range(N):
           yield i

A new keyword, :keyword:`yield`, was introduced for generators.  Any function
containing a :keyword:`yield` statement is a generator function; this is
detected by Python's bytecode compiler which compiles the function specially as
a result.

When you call a generator function, it doesn't return a single value; instead it
returns a generator object that supports the iterator protocol.  On executing
the :keyword:`yield` statement, the generator outputs the value of ``i``,
similar to a :keyword:`return` statement.  The big difference between
:keyword:`yield` and a :keyword:`return` statement is that on reaching a
:keyword:`yield` the generator's state of execution is suspended and local
variables are preserved.  On the next call to the generator's ``.next()``
method, the function will resume executing immediately after the
:keyword:`yield` statement.  (For complicated reasons, the :keyword:`yield`
statement isn't allowed inside the :keyword:`try` block of a :keyword:`try`...\
:keyword:`finally` statement; read :pep:`255` for a full explanation of the
interaction between :keyword:`yield` and exceptions.)

Here's a sample usage of the :func:`generate_ints` generator::

   >>> gen = generate_ints(3)
   >>> gen
   <generator object at 0x8117f90>
   >>> gen.next()
   0
   >>> gen.next()
   1
   >>> gen.next()
   2
   >>> gen.next()
   Traceback (most recent call last):
     File "stdin", line 1, in ?
     File "stdin", line 2, in generate_ints
   StopIteration

You could equally write ``for i in generate_ints(5)``, or ``a,b,c =
generate_ints(3)``.

Inside a generator function, the :keyword:`return` statement can only be used
without a value, and signals the end of the procession of values; afterwards the
generator cannot return any further values. :keyword:`return` with a value, such
as ``return 5``, is a syntax error inside a generator function.  The end of the
generator's results can also be indicated by raising :exc:`StopIteration`
manually, or by just letting the flow of execution fall off the bottom of the
function.

You could achieve the effect of generators manually by writing your own class
and storing all the local variables of the generator as instance variables.  For
example, returning a list of integers could be done by setting ``self.count`` to
0, and having the :meth:`next` method increment ``self.count`` and return it.
However, for a moderately complicated generator, writing a corresponding class
would be much messier. :file:`Lib/test/test_generators.py` contains a number of
more interesting examples.  The simplest one implements an in-order traversal of
a tree using generators recursively. ::

   # A recursive generator that generates Tree leaves in in-order.
   def inorder(t):
       if t:
           for x in inorder(t.left):
               yield x
           yield t.label
           for x in inorder(t.right):
               yield x

Two other examples in :file:`Lib/test/test_generators.py` produce solutions for
the N-Queens problem (placing $N$ queens on an $NxN$ chess board so that no
queen threatens another) and the Knight's Tour (a route that takes a knight to
every square of an $NxN$ chessboard without visiting any square twice).

The idea of generators comes from other programming languages, especially Icon
(http://www.cs.arizona.edu/icon/), where the idea of generators is central.  In
Icon, every expression and function call behaves like a generator.  One example
from "An Overview of the Icon Programming Language" at
http://www.cs.arizona.edu/icon/docs/ipd266.htm gives an idea of what this looks
like::

   sentence := "Store it in the neighboring harbor"
   if (i := find("or", sentence)) > 5 then write(i)

In Icon the :func:`find` function returns the indexes at which the substring
"or" is found: 3, 23, 33.  In the :keyword:`if` statement, ``i`` is first
assigned a value of 3, but 3 is less than 5, so the comparison fails, and Icon
retries it with the second value of 23.  23 is greater than 5, so the comparison
now succeeds, and the code prints the value 23 to the screen.

Python doesn't go nearly as far as Icon in adopting generators as a central
concept.  Generators are considered part of the core Python language, but
learning or using them isn't compulsory; if they don't solve any problems that
you have, feel free to ignore them. One novel feature of Python's interface as
compared to Icon's is that a generator's state is represented as a concrete
object (the iterator) that can be passed around to other functions or stored in
a data structure.


.. seealso::

   :pep:`255` - Simple Generators
      Written by Neil Schemenauer, Tim Peters, Magnus Lie Hetland.  Implemented mostly
      by Neil Schemenauer and Tim Peters, with other fixes from the Python Labs crew.

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


.. _section-encodings:

PEP 263: Source Code Encodings
==============================

Python source files can now be declared as being in different character set
encodings.  Encodings are declared by including a specially formatted comment in
the first or second line of the source file.  For example, a UTF-8 file can be
declared with::

   #!/usr/bin/env python
   # -*- coding: UTF-8 -*-

Without such an encoding declaration, the default encoding used is 7-bit ASCII.
Executing or importing modules that contain string literals with 8-bit
characters and have no encoding declaration will result in a
:exc:`DeprecationWarning` being signalled by Python 2.3; in 2.4 this will be a
syntax error.

The encoding declaration only affects Unicode string literals, which will be
converted to Unicode using the specified encoding.  Note that Python identifiers
are still restricted to ASCII characters, so you can't have variable names that
use characters outside of the usual alphanumerics.


.. seealso::

   :pep:`263` - Defining Python Source Code Encodings
      Written by Marc-André Lemburg and Martin von Löwis; implemented by Suzuki Hisao
      and Martin von Löwis.

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


PEP 273: Importing Modules from ZIP Archives
============================================

The new :mod:`zipimport` module adds support for importing modules from a ZIP-
format archive.  You don't need to import the module explicitly; it will be
automatically imported if a ZIP archive's filename is added to ``sys.path``.
For example::

   amk@nyman:~/src/python$ unzip -l /tmp/example.zip
   Archive:  /tmp/example.zip
     Length     Date   Time    Name
    --------    ----   ----    ----
        8467  11-26-02 22:30   jwzthreading.py
    --------                   -------
        8467                   1 file
   amk@nyman:~/src/python$ ./python
   Python 2.3 (#1, Aug 1 2003, 19:54:32)
   >>> import sys
   >>> sys.path.insert(0, '/tmp/example.zip')  # Add .zip file to front of path
   >>> import jwzthreading
   >>> jwzthreading.__file__
   '/tmp/example.zip/jwzthreading.py'
   >>>

An entry in ``sys.path`` can now be the filename of a ZIP archive. The ZIP
archive can contain any kind of files, but only files named :file:`\*.py`,
:file:`\*.pyc`, or :file:`\*.pyo` can be imported.  If an archive only contains
:file:`\*.py` files, Python will not attempt to modify the archive by adding the
corresponding :file:`\*.pyc` file, meaning that if a ZIP archive doesn't contain
:file:`\*.pyc` files, importing may be rather slow.

A path within the archive can also be specified to only import from a
subdirectory; for example, the path :file:`/tmp/example.zip/lib/` would only
import from the :file:`lib/` subdirectory within the archive.


.. seealso::

   :pep:`273` - Import Modules from Zip Archives
      Written by James C. Ahlstrom,  who also provided an implementation. Python 2.3
      follows the specification in :pep:`273`,  but uses an implementation written by
      Just van Rossum  that uses the import hooks described in :pep:`302`. See section
      :ref:`section-pep302` for a description of the new import hooks.

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


PEP 277: Unicode file name support for Windows NT
=================================================

On Windows NT, 2000, and XP, the system stores file names as Unicode strings.
Traditionally, Python has represented file names as byte strings, which is
inadequate because it renders some file names inaccessible.

Python now allows using arbitrary Unicode strings (within the limitations of the
file system) for all functions that expect file names, most notably the
:func:`open` built-in function. If a Unicode string is passed to
:func:`os.listdir`, Python now returns a list of Unicode strings.  A new
function, :func:`os.getcwdu`, returns the current directory as a Unicode string.

Byte strings still work as file names, and on Windows Python will transparently
convert them to Unicode using the ``mbcs`` encoding.

Other systems also allow Unicode strings as file names but convert them to byte
strings before passing them to the system, which can cause a :exc:`UnicodeError`
to be raised. Applications can test whether arbitrary Unicode strings are
supported as file names by checking :attr:`os.path.supports_unicode_filenames`,
a Boolean value.

Under MacOS, :func:`os.listdir` may now return Unicode filenames.


.. seealso::

   :pep:`277` - Unicode file name support for Windows NT
      Written by Neil Hodgson; implemented by Neil Hodgson, Martin von Löwis, and Mark
      Hammond.

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


PEP 278: Universal Newline Support
==================================

The three major operating systems used today are Microsoft Windows, Apple's
Macintosh OS, and the various Unix derivatives.  A minor irritation of cross-
platform work  is that these three platforms all use different characters to
mark the ends of lines in text files.  Unix uses the linefeed (ASCII character
10), MacOS uses the carriage return (ASCII character 13), and Windows uses a
two-character sequence of a carriage return plus a newline.

Python's file objects can now support end of line conventions other than the one
followed by the platform on which Python is running. Opening a file with the
mode ``'U'`` or ``'rU'`` will open a file for reading in universal newline mode.
All three line ending conventions will be translated to a ``'\n'`` in the
strings returned by the various file methods such as :meth:`read` and
:meth:`readline`.

Universal newline support is also used when importing modules and when executing
a file with the :func:`execfile` function.  This means that Python modules can
be shared between all three operating systems without needing to convert the
line-endings.

This feature can be disabled when compiling Python by specifying the
:option:`--without-universal-newlines` switch when running Python's
:program:`configure` script.


.. seealso::

   :pep:`278` - Universal Newline Support
      Written and implemented by Jack Jansen.

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


.. _section-enumerate:

PEP 279: enumerate()
====================

A new built-in function, :func:`enumerate`, will make certain loops a bit
clearer.  ``enumerate(thing)``, where *thing* is either an iterator or a
sequence, returns a iterator that will return ``(0, thing[0])``, ``(1,
thing[1])``, ``(2, thing[2])``, and so forth.

A common idiom to change every element of a list looks like this::

   for i in range(len(L)):
       item = L[i]
       # ... compute some result based on item ...
       L[i] = result

This can be rewritten using :func:`enumerate` as::

   for i, item in enumerate(L):
       # ... compute some result based on item ...
       L[i] = result


.. seealso::

   :pep:`279` - The enumerate() built-in function
      Written and implemented by Raymond D. Hettinger.

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


PEP 282: The logging Package
============================

A standard package for writing logs, :mod:`logging`, has been added to Python
2.3.  It provides a powerful and flexible mechanism for generating logging
output which can then be filtered and processed in various ways.  A
configuration file written in a standard format can be used to control the
logging behavior of a program.  Python includes handlers that will write log
records to standard error or to a file or socket, send them to the system log,
or even e-mail them to a particular address; of course, it's also possible to
write your own handler classes.

The :class:`Logger` class is the primary class. Most application code will deal
with one or more :class:`Logger` objects, each one used by a particular
subsystem of the application. Each :class:`Logger` is identified by a name, and
names are organized into a hierarchy using ``.``  as the component separator.
For example, you might have :class:`Logger` instances named ``server``,
``server.auth`` and ``server.network``.  The latter two instances are below
``server`` in the hierarchy.  This means that if you turn up the verbosity for
``server`` or direct ``server`` messages to a different handler, the changes
will also apply to records logged to ``server.auth`` and ``server.network``.
There's also a root :class:`Logger` that's the parent of all other loggers.

For simple uses, the :mod:`logging` package contains some convenience functions
that always use the root log::

   import logging

   logging.debug('Debugging information')
   logging.info('Informational message')
   logging.warning('Warning:config file %s not found', 'server.conf')
   logging.error('Error occurred')
   logging.critical('Critical error -- shutting down')

This produces the following output::

   WARNING:root:Warning:config file server.conf not found
   ERROR:root:Error occurred
   CRITICAL:root:Critical error -- shutting down

In the default configuration, informational and debugging messages are
suppressed and the output is sent to standard error.  You can enable the display
of informational and debugging messages by calling the :meth:`setLevel` method
on the root logger.

Notice the :func:`warning` call's use of string formatting operators; all of the
functions for logging messages take the arguments ``(msg, arg1, arg2, ...)`` and
log the string resulting from ``msg % (arg1, arg2, ...)``.

There's also an :func:`exception` function that records the most recent
traceback.  Any of the other functions will also record the traceback if you
specify a true value for the keyword argument *exc_info*. ::

   def f():
       try:    1/0
       except: logging.exception('Problem recorded')

   f()

This produces the following output::

   ERROR:root:Problem recorded
   Traceback (most recent call last):
     File "t.py", line 6, in f
       1/0
   ZeroDivisionError: integer division or modulo by zero

Slightly more advanced programs will use a logger other than the root logger.
The :func:`getLogger(name)` function is used to get a particular log, creating
it if it doesn't exist yet. :func:`getLogger(None)` returns the root logger. ::

   log = logging.getLogger('server')
    ...
   log.info('Listening on port %i', port)
    ...
   log.critical('Disk full')
    ...

Log records are usually propagated up the hierarchy, so a message logged to
``server.auth`` is also seen by ``server`` and ``root``, but a :class:`Logger`
can prevent this by setting its :attr:`propagate` attribute to :const:`False`.

There are more classes provided by the :mod:`logging` package that can be
customized.  When a :class:`Logger` instance is told to log a message, it
creates a :class:`LogRecord` instance that is sent to any number of different
:class:`Handler` instances.  Loggers and handlers can also have an attached list
of filters, and each filter can cause the :class:`LogRecord` to be ignored or
can modify the record before passing it along.  When they're finally output,
:class:`LogRecord` instances are converted to text by a :class:`Formatter`
class.  All of these classes can be replaced by your own specially-written
classes.

With all of these features the :mod:`logging` package should provide enough
flexibility for even the most complicated applications.  This is only an
incomplete overview of its features, so please see the package's reference
documentation for all of the details.  Reading :pep:`282` will also be helpful.


.. seealso::

   :pep:`282` - A Logging System
      Written by Vinay Sajip and Trent Mick; implemented by Vinay Sajip.

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


.. _section-bool:

PEP 285: A Boolean Type
=======================

A Boolean type was added to Python 2.3.  Two new constants were added to the
:mod:`__builtin__` module, :const:`True` and :const:`False`.  (:const:`True` and
:const:`False` constants were added to the built-ins in Python 2.2.1, but the
2.2.1 versions are simply set to integer values of 1 and 0 and aren't a
different type.)

The type object for this new type is named :class:`bool`; the constructor for it
takes any Python value and converts it to :const:`True` or :const:`False`. ::

   >>> bool(1)
   True
   >>> bool(0)
   False
   >>> bool([])
   False
   >>> bool( (1,) )
   True

Most of the standard library modules and built-in functions have been changed to
return Booleans. ::

   >>> obj = []
   >>> hasattr(obj, 'append')
   True
   >>> isinstance(obj, list)
   True
   >>> isinstance(obj, tuple)
   False

Python's Booleans were added with the primary goal of making code clearer.  For
example, if you're reading a function and encounter the statement ``return 1``,
you might wonder whether the ``1`` represents a Boolean truth value, an index,
or a coefficient that multiplies some other quantity.  If the statement is
``return True``, however, the meaning of the return value is quite clear.

Python's Booleans were *not* added for the sake of strict type-checking.  A very
strict language such as Pascal would also prevent you performing arithmetic with
Booleans, and would require that the expression in an :keyword:`if` statement
always evaluate to a Boolean result.  Python is not this strict and never will
be, as :pep:`285` explicitly says.  This means you can still use any expression
in an :keyword:`if` statement, even ones that evaluate to a list or tuple or
some random object.  The Boolean type is a subclass of the :class:`int` class so
that arithmetic using a Boolean still works. ::

   >>> True + 1
   2
   >>> False + 1
   1
   >>> False * 75
   0
   >>> True * 75
   75

To sum up :const:`True` and :const:`False` in a sentence: they're alternative
ways to spell the integer values 1 and 0, with the single difference that
:func:`str` and :func:`repr` return the strings ``'True'`` and ``'False'``
instead of ``'1'`` and ``'0'``.


.. seealso::

   :pep:`285` - Adding a bool type
      Written and implemented by GvR.

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


PEP 293: Codec Error Handling Callbacks
=======================================

When encoding a Unicode string into a byte string, unencodable characters may be
encountered.  So far, Python has allowed specifying the error processing as
either "strict" (raising :exc:`UnicodeError`), "ignore" (skipping the
character), or "replace" (using a question mark in the output string), with
"strict" being the default behavior. It may be desirable to specify alternative
processing of such errors, such as inserting an XML character reference or HTML
entity reference into the converted string.

Python now has a flexible framework to add different processing strategies.  New
error handlers can be added with :func:`codecs.register_error`, and codecs then
can access the error handler with :func:`codecs.lookup_error`. An equivalent C
API has been added for codecs written in C. The error handler gets the necessary
state information such as the string being converted, the position in the string
where the error was detected, and the target encoding.  The handler can then
either raise an exception or return a replacement string.

Two additional error handlers have been implemented using this framework:
"backslashreplace" uses Python backslash quoting to represent unencodable
characters and "xmlcharrefreplace" emits XML character references.


.. seealso::

   :pep:`293` - Codec Error Handling Callbacks
      Written and implemented by Walter Dörwald.

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


.. _section-pep301:

PEP 301: Package Index and Metadata for Distutils
=================================================

Support for the long-requested Python catalog makes its first appearance in 2.3.

The heart of the catalog is the new Distutils :command:`register` command.
Running ``python setup.py register`` will collect the metadata describing a
package, such as its name, version, maintainer, description, &c., and send it to
a central catalog server.  The resulting catalog is available from
http://www.python.org/pypi.

To make the catalog a bit more useful, a new optional *classifiers* keyword
argument has been added to the Distutils :func:`setup` function.  A list of
`Trove <http://catb.org/~esr/trove/>`_-style strings can be supplied to help
classify the software.

Here's an example :file:`setup.py` with classifiers, written to be compatible
with older versions of the Distutils::

   from distutils import core
   kw = {'name': "Quixote",
         'version': "0.5.1",
         'description': "A highly Pythonic Web application framework",
         # ...
         }

   if (hasattr(core, 'setup_keywords') and
       'classifiers' in core.setup_keywords):
       kw['classifiers'] = \
           ['Topic :: Internet :: WWW/HTTP :: Dynamic Content',
            'Environment :: No Input/Output (Daemon)',
            'Intended Audience :: Developers'],

   core.setup(**kw)

The full list of classifiers can be obtained by running  ``python setup.py
register --list-classifiers``.


.. seealso::

   :pep:`301` - Package Index and Metadata for Distutils
      Written and implemented by Richard Jones.

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


.. _section-pep302:

PEP 302: New Import Hooks
=========================

While it's been possible to write custom import hooks ever since the
:mod:`ihooks` module was introduced in Python 1.3, no one has ever been really
happy with it because writing new import hooks is difficult and messy.  There
have been various proposed alternatives such as the :mod:`imputil` and :mod:`iu`
modules, but none of them has ever gained much acceptance, and none of them were
easily usable from C code.

:pep:`302` borrows ideas from its predecessors, especially from Gordon
McMillan's :mod:`iu` module.  Three new items  are added to the :mod:`sys`
module:

* ``sys.path_hooks`` is a list of callable objects; most  often they'll be
  classes.  Each callable takes a string containing a path and either returns an
  importer object that will handle imports from this path or raises an
  :exc:`ImportError` exception if it can't handle this path.

* ``sys.path_importer_cache`` caches importer objects for each path, so
  ``sys.path_hooks`` will only need to be traversed once for each path.

* ``sys.meta_path`` is a list of importer objects that will be traversed before
  ``sys.path`` is checked.  This list is initially empty, but user code can add
  objects to it.  Additional built-in and frozen modules can be imported by an
  object added to this list.

Importer objects must have a single method, :meth:`find_module(fullname,
path=None)`.  *fullname* will be a module or package name, e.g. ``string`` or
``distutils.core``.  :meth:`find_module` must return a loader object that has a
single method, :meth:`load_module(fullname)`, that creates and returns the
corresponding module object.

Pseudo-code for Python's new import logic, therefore, looks something like this
(simplified a bit; see :pep:`302` for the full details)::

   for mp in sys.meta_path:
       loader = mp(fullname)
       if loader is not None:
           <module> = loader.load_module(fullname)

   for path in sys.path:
       for hook in sys.path_hooks:
           try:
               importer = hook(path)
           except ImportError:
               # ImportError, so try the other path hooks
               pass
           else:
               loader = importer.find_module(fullname)
               <module> = loader.load_module(fullname)

   # Not found!
   raise ImportError


.. seealso::

   :pep:`302` - New Import Hooks
      Written by Just van Rossum and Paul Moore. Implemented by Just van Rossum.

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


.. _section-pep305:

PEP 305: Comma-separated Files
==============================

Comma-separated files are a format frequently used for exporting data from
databases and spreadsheets.  Python 2.3 adds a parser for comma-separated files.

Comma-separated format is deceptively simple at first glance::

   Costs,150,200,3.95

Read a line and call ``line.split(',')``: what could be simpler? But toss in
string data that can contain commas, and things get more complicated::

   "Costs",150,200,3.95,"Includes taxes, shipping, and sundry items"

A big ugly regular expression can parse this, but using the new  :mod:`csv`
package is much simpler::

   import csv

   input = open('datafile', 'rb')
   reader = csv.reader(input)
   for line in reader:
       print line

The :func:`reader` function takes a number of different options. The field
separator isn't limited to the comma and can be changed to any character, and so
can the quoting and line-ending characters.

Different dialects of comma-separated files can be defined and registered;
currently there are two dialects, both used by Microsoft Excel. A separate
:class:`csv.writer` class will generate comma-separated files from a succession
of tuples or lists, quoting strings that contain the delimiter.


.. seealso::

   :pep:`305` - CSV File API
      Written and implemented  by Kevin Altis, Dave Cole, Andrew McNamara, Skip
      Montanaro, Cliff Wells.

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


.. _section-pep307:

PEP 307: Pickle Enhancements
============================

The :mod:`pickle` and :mod:`cPickle` modules received some attention during the
2.3 development cycle.  In 2.2, new-style classes could be pickled without
difficulty, but they weren't pickled very compactly; :pep:`307` quotes a trivial
example where a new-style class results in a pickled string three times longer
than that for a classic class.

The solution was to invent a new pickle protocol.  The :func:`pickle.dumps`
function has supported a text-or-binary flag  for a long time.  In 2.3, this
flag is redefined from a Boolean to an integer: 0 is the old text-mode pickle
format, 1 is the old binary format, and now 2 is a new 2.3-specific format.  A
new constant, :const:`pickle.HIGHEST_PROTOCOL`, can be used to select the
fanciest protocol available.

Unpickling is no longer considered a safe operation.  2.2's :mod:`pickle`
provided hooks for trying to prevent unsafe classes from being unpickled
(specifically, a :attr:`__safe_for_unpickling__` attribute), but none of this
code was ever audited and therefore it's all been ripped out in 2.3.  You should
not unpickle untrusted data in any version of Python.

To reduce the pickling overhead for new-style classes, a new interface for
customizing pickling was added using three special methods:
:meth:`__getstate__`, :meth:`__setstate__`, and :meth:`__getnewargs__`.  Consult
:pep:`307` for the full semantics  of these methods.

As a way to compress pickles yet further, it's now possible to use integer codes
instead of long strings to identify pickled classes. The Python Software
Foundation will maintain a list of standardized codes; there's also a range of
codes for private use.  Currently no codes have been specified.


.. seealso::

   :pep:`307` - Extensions to the pickle protocol
      Written and implemented  by Guido van Rossum and Tim Peters.

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


.. _section-slices:

Extended Slices
===============

Ever since Python 1.4, the slicing syntax has supported an optional third "step"
or "stride" argument.  For example, these are all legal Python syntax:
``L[1:10:2]``, ``L[:-1:1]``, ``L[::-1]``.  This was added to Python at the
request of the developers of Numerical Python, which uses the third argument
extensively.  However, Python's built-in list, tuple, and string sequence types
have never supported this feature, raising a :exc:`TypeError` if you tried it.
Michael Hudson contributed a patch to fix this shortcoming.

For example, you can now easily extract the elements of a list that have even
indexes::

   >>> L = range(10)
   >>> L[::2]
   [0, 2, 4, 6, 8]

Negative values also work to make a copy of the same list in reverse order::

   >>> L[::-1]
   [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

This also works for tuples, arrays, and strings::

   >>> s='abcd'
   >>> s[::2]
   'ac'
   >>> s[::-1]
   'dcba'

If you have a mutable sequence such as a list or an array you can assign to or
delete an extended slice, but there are some differences between assignment to
extended and regular slices.  Assignment to a regular slice can be used to
change the length of the sequence::

   >>> a = range(3)
   >>> a
   [0, 1, 2]
   >>> a[1:3] = [4, 5, 6]
   >>> a
   [0, 4, 5, 6]

Extended slices aren't this flexible.  When assigning to an extended slice, the
list on the right hand side of the statement must contain the same number of
items as the slice it is replacing::

   >>> a = range(4)
   >>> a
   [0, 1, 2, 3]
   >>> a[::2]
   [0, 2]
   >>> a[::2] = [0, -1]
   >>> a
   [0, 1, -1, 3]
   >>> a[::2] = [0,1,2]
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ValueError: attempt to assign sequence of size 3 to extended slice of size 2

Deletion is more straightforward::

   >>> a = range(4)
   >>> a
   [0, 1, 2, 3]
   >>> a[::2]
   [0, 2]
   >>> del a[::2]
   >>> a
   [1, 3]

One can also now pass slice objects to the :meth:`__getitem__` methods of the
built-in sequences::

   >>> range(10).__getitem__(slice(0, 5, 2))
   [0, 2, 4]

Or use slice objects directly in subscripts::

   >>> range(10)[slice(0, 5, 2)]
   [0, 2, 4]

To simplify implementing sequences that support extended slicing, slice objects
now have a method :meth:`indices(length)` which, given the length of a sequence,
returns a ``(start, stop, step)`` tuple that can be passed directly to
:func:`range`. :meth:`indices` handles omitted and out-of-bounds indices in a
manner consistent with regular slices (and this innocuous phrase hides a welter
of confusing details!).  The method is intended to be used like this::

   class FakeSeq:
       ...
       def calc_item(self, i):
           ...
       def __getitem__(self, item):
           if isinstance(item, slice):
               indices = item.indices(len(self))
               return FakeSeq([self.calc_item(i) for i in range(*indices)])
           else:
               return self.calc_item(i)

From this example you can also see that the built-in :class:`slice` object is
now the type object for the slice type, and is no longer a function.  This is
consistent with Python 2.2, where :class:`int`, :class:`str`, etc., underwent
the same change.

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


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

Here are all of the changes that Python 2.3 makes to the core Python language.

* The :keyword:`yield` statement is now always a keyword, as described in
  section :ref:`section-generators` of this document.

* A new built-in function :func:`enumerate` was added, as described in section
  :ref:`section-enumerate` of this document.

* Two new constants, :const:`True` and :const:`False` were added along with the
  built-in :class:`bool` type, as described in section :ref:`section-bool` of this
  document.

* The :func:`int` type constructor will now return a long integer instead of
  raising an :exc:`OverflowError` when a string or floating-point number is too
  large to fit into an integer.  This can lead to the paradoxical result that
  ``isinstance(int(expression), int)`` is false, but that seems unlikely to cause
  problems in practice.

* Built-in types now support the extended slicing syntax, as described in
  section :ref:`section-slices` of this document.

* A new built-in function, :func:`sum(iterable, start=0)`,  adds up the numeric
  items in the iterable object and returns their sum.  :func:`sum` only accepts
  numbers, meaning that you can't use it to concatenate a bunch of strings.
  (Contributed by Alex Martelli.)

* ``list.insert(pos, value)`` used to  insert *value* at the front of the list
  when *pos* was negative.  The behaviour has now been changed to be consistent
  with slice indexing, so when *pos* is -1 the value will be inserted before the
  last element, and so forth.

* ``list.index(value)``, which searches for *value*  within the list and returns
  its index, now takes optional  *start* and *stop* arguments to limit the search
  to  only part of the list.

* Dictionaries have a new method, :meth:`pop(key[, *default*])`, that returns
  the value corresponding to *key* and removes that key/value pair from the
  dictionary.  If the requested key isn't present in the dictionary, *default* is
  returned if it's specified and :exc:`KeyError` raised if it isn't. ::

     >>> d = {1:2}
     >>> d
     {1: 2}
     >>> d.pop(4)
     Traceback (most recent call last):
       File "stdin", line 1, in ?
     KeyError: 4
     >>> d.pop(1)
     2
     >>> d.pop(1)
     Traceback (most recent call last):
       File "stdin", line 1, in ?
     KeyError: 'pop(): dictionary is empty'
     >>> d
     {}
     >>>

  There's also a new class method,  :meth:`dict.fromkeys(iterable, value)`, that
  creates a dictionary with keys taken from the supplied iterator *iterable* and
  all values set to *value*, defaulting to ``None``.

  (Patches contributed by Raymond Hettinger.)

  Also, the :func:`dict` constructor now accepts keyword arguments to simplify
  creating small dictionaries::

     >>> dict(red=1, blue=2, green=3, black=4)
     {'blue': 2, 'black': 4, 'green': 3, 'red': 1}

  (Contributed by Just van Rossum.)

* The :keyword:`assert` statement no longer checks the ``__debug__`` flag, so
  you can no longer disable assertions by assigning to ``__debug__``. Running
  Python with the :option:`-O` switch will still generate code that doesn't
  execute any assertions.

* Most type objects are now callable, so you can use them to create new objects
  such as functions, classes, and modules.  (This means that the :mod:`new` module
  can be deprecated in a future Python version, because you can now use the type
  objects available in the :mod:`types` module.) For example, you can create a new
  module object with the following code:

  ::

     >>> import types
     >>> m = types.ModuleType('abc','docstring')
     >>> m
     <module 'abc' (built-in)>
     >>> m.__doc__
     'docstring'

* A new warning, :exc:`PendingDeprecationWarning` was added to indicate features
  which are in the process of being deprecated.  The warning will *not* be printed
  by default.  To check for use of features that will be deprecated in the future,
  supply :option:`-Walways::PendingDeprecationWarning::` on the command line or
  use :func:`warnings.filterwarnings`.

* The process of deprecating string-based exceptions, as in ``raise "Error
  occurred"``, has begun.  Raising a string will now trigger
  :exc:`PendingDeprecationWarning`.

* Using ``None`` as a variable name will now result in a :exc:`SyntaxWarning`
  warning.  In a future version of Python, ``None`` may finally become a keyword.

* The :meth:`xreadlines` method of file objects, introduced in Python 2.1, is no
  longer necessary because files now behave as their own iterator.
  :meth:`xreadlines` was originally introduced as a faster way to loop over all
  the lines in a file, but now you can simply write ``for line in file_obj``.
  File objects also have a new read-only :attr:`encoding` attribute that gives the
  encoding used by the file; Unicode strings written to the file will be
  automatically  converted to bytes using the given encoding.

* The method resolution order used by new-style classes has changed, though
  you'll only notice the difference if you have a really complicated inheritance
  hierarchy.  Classic classes are unaffected by this change.  Python 2.2
  originally used a topological sort of a class's ancestors, but 2.3 now uses the
  C3 algorithm as described in the paper `"A Monotonic Superclass Linearization
  for Dylan" <http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>`_. To
  understand the motivation for this change,  read Michele Simionato's article
  `"Python 2.3 Method Resolution Order" <http://www.python.org/2.3/mro.html>`_, or
  read the thread on python-dev starting with the message at
  http://mail.python.org/pipermail/python-dev/2002-October/029035.html. Samuele
  Pedroni first pointed out the problem and also implemented the fix by coding the
  C3 algorithm.

* Python runs multithreaded programs by switching between threads after
  executing N bytecodes.  The default value for N has been increased from 10 to
  100 bytecodes, speeding up single-threaded applications by reducing the
  switching overhead.  Some multithreaded applications may suffer slower response
  time, but that's easily fixed by setting the limit back to a lower number using
  :func:`sys.setcheckinterval(N)`. The limit can be retrieved with the new
  :func:`sys.getcheckinterval` function.

* One minor but far-reaching change is that the names of extension types defined
  by the modules included with Python now contain the module and a ``'.'`` in
  front of the type name.  For example, in Python 2.2, if you created a socket and
  printed its :attr:`__class__`, you'd get this output::

     >>> s = socket.socket()
     >>> s.__class__
     <type 'socket'>

  In 2.3, you get this::

     >>> s.__class__
     <type '_socket.socket'>

* One of the noted incompatibilities between old- and new-style classes has been
  removed: you can now assign to the :attr:`__name__` and :attr:`__bases__`
  attributes of new-style classes.  There are some restrictions on what can be
  assigned to :attr:`__bases__` along the lines of those relating to assigning to
  an instance's :attr:`__class__` attribute.

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


String Changes
--------------

* The :keyword:`in` operator now works differently for strings. Previously, when
  evaluating ``X in Y`` where *X* and *Y* are strings, *X* could only be a single
  character. That's now changed; *X* can be a string of any length, and ``X in Y``
  will return :const:`True` if *X* is a substring of *Y*.  If *X* is the empty
  string, the result is always :const:`True`. ::

     >>> 'ab' in 'abcd'
     True
     >>> 'ad' in 'abcd'
     False
     >>> '' in 'abcd'
     True

  Note that this doesn't tell you where the substring starts; if you need that
  information, use the :meth:`find` string method.

* The :meth:`strip`, :meth:`lstrip`, and :meth:`rstrip` string methods now have
  an optional argument for specifying the characters to strip.  The default is
  still to remove all whitespace characters::

     >>> '   abc '.strip()
     'abc'
     >>> '><><abc<><><>'.strip('<>')
     'abc'
     >>> '><><abc<><><>\n'.strip('<>')
     'abc<><><>\n'
     >>> u'\u4000\u4001abc\u4000'.strip(u'\u4000')
     u'\u4001abc'
     >>>

  (Suggested by Simon Brunning and implemented by Walter Dörwald.)

* The :meth:`startswith` and :meth:`endswith` string methods now accept negative
  numbers for the *start* and *end* parameters.

* Another new string method is :meth:`zfill`, originally a function in the
  :mod:`string` module.  :meth:`zfill` pads a numeric string with zeros on the
  left until it's the specified width. Note that the ``%`` operator is still more
  flexible and powerful than :meth:`zfill`. ::

     >>> '45'.zfill(4)
     '0045'
     >>> '12345'.zfill(4)
     '12345'
     >>> 'goofy'.zfill(6)
     '0goofy'

  (Contributed by Walter Dörwald.)

* A new type object, :class:`basestring`, has been added. Both 8-bit strings and
  Unicode strings inherit from this type, so ``isinstance(obj, basestring)`` will
  return :const:`True` for either kind of string.  It's a completely abstract
  type, so you can't create :class:`basestring` instances.

* Interned strings are no longer immortal and will now be garbage-collected in
  the usual way when the only reference to them is from the internal dictionary of
  interned strings.  (Implemented by Oren Tirosh.)

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


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

* The creation of new-style class instances has been made much faster; they're
  now faster than classic classes!

* The :meth:`sort` method of list objects has been extensively rewritten by Tim
  Peters, and the implementation is significantly faster.

* Multiplication of large long integers is now much faster thanks to an
  implementation of Karatsuba multiplication, an algorithm that scales better than
  the O(n\*n) required for the grade-school multiplication algorithm.  (Original
  patch by Christopher A. Craig, and significantly reworked by Tim Peters.)

* The ``SET_LINENO`` opcode is now gone.  This may provide a small speed
  increase, depending on your compiler's idiosyncrasies. See section
  :ref:`23section-other` for a longer explanation. (Removed by Michael Hudson.)

* :func:`xrange` objects now have their own iterator, making ``for i in
  xrange(n)`` slightly faster than ``for i in range(n)``.  (Patch by Raymond
  Hettinger.)

* A number of small rearrangements have been made in various hotspots to improve
  performance, such as inlining a function or removing some code.  (Implemented
  mostly by GvR, but lots of people have contributed single changes.)

The net result of the 2.3 optimizations is that Python 2.3 runs the  pystone
benchmark around 25% faster than Python 2.2.

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


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:`array` module now supports arrays of Unicode characters using the
  ``'u'`` format character.  Arrays also now support using the ``+=`` assignment
  operator to add another array's contents, and the ``*=`` assignment operator to
  repeat an array. (Contributed by Jason Orendorff.)

* The :mod:`bsddb` module has been replaced by version 4.1.6 of the `PyBSDDB
  <http://pybsddb.sourceforge.net>`_ package, providing a more complete interface
  to the transactional features of the BerkeleyDB library.

  The old version of the module has been renamed to  :mod:`bsddb185` and is no
  longer built automatically; you'll  have to edit :file:`Modules/Setup` to enable
  it.  Note that the new :mod:`bsddb` package is intended to be compatible with
  the  old module, so be sure to file bugs if you discover any incompatibilities.
  When upgrading to Python 2.3, if the new interpreter is compiled with a new
  version of  the underlying BerkeleyDB library, you will almost certainly have to
  convert your database files to the new version.  You can do this fairly easily
  with the new scripts :file:`db2pickle.py` and :file:`pickle2db.py` which you
  will find in the distribution's :file:`Tools/scripts` directory.  If you've
  already been using the PyBSDDB package and importing it as :mod:`bsddb3`, you
  will have to change your ``import`` statements to import it as :mod:`bsddb`.

* The new :mod:`bz2` module is an interface to the bz2 data compression library.
  bz2-compressed data is usually smaller than  corresponding :mod:`zlib`\
  -compressed data. (Contributed by Gustavo Niemeyer.)

* A set of standard date/time types has been added in the new :mod:`datetime`
  module.  See the following section for more details.

* The Distutils :class:`Extension` class now supports an extra constructor
  argument named *depends* for listing additional source files that an extension
  depends on.  This lets Distutils recompile the module if any of the dependency
  files are modified.  For example, if :file:`sampmodule.c` includes the header
  file :file:`sample.h`, you would create the :class:`Extension` object like
  this::

     ext = Extension("samp",
                     sources=["sampmodule.c"],
                     depends=["sample.h"])

  Modifying :file:`sample.h` would then cause the module to be recompiled.
  (Contributed by Jeremy Hylton.)

* Other minor changes to Distutils: it now checks for the :envvar:`CC`,
  :envvar:`CFLAGS`, :envvar:`CPP`, :envvar:`LDFLAGS`, and :envvar:`CPPFLAGS`
  environment variables, using them to override the settings in Python's
  configuration (contributed by Robert Weber).

* Previously the :mod:`doctest` module would only search the docstrings of
  public methods and functions for test cases, but it now also examines private
  ones as well.  The :func:`DocTestSuite(` function creates a
  :class:`unittest.TestSuite` object from a set of :mod:`doctest` tests.

* The new :func:`gc.get_referents(object)` function returns a list of all the
  objects referenced by *object*.

* The :mod:`getopt` module gained a new function, :func:`gnu_getopt`, that
  supports the same arguments as the existing :func:`getopt` function but uses
  GNU-style scanning mode. The existing :func:`getopt` stops processing options as
  soon as a non-option argument is encountered, but in GNU-style mode processing
  continues, meaning that options and arguments can be mixed.  For example::

     >>> getopt.getopt(['-f', 'filename', 'output', '-v'], 'f:v')
     ([('-f', 'filename')], ['output', '-v'])
     >>> getopt.gnu_getopt(['-f', 'filename', 'output', '-v'], 'f:v')
     ([('-f', 'filename'), ('-v', '')], ['output'])

  (Contributed by Peter Åstrand.)

* The :mod:`grp`, :mod:`pwd`, and :mod:`resource` modules now return enhanced
  tuples::

     >>> import grp
     >>> g = grp.getgrnam('amk')
     >>> g.gr_name, g.gr_gid
     ('amk', 500)

* The :mod:`gzip` module can now handle files exceeding 2 GiB.

* The new :mod:`heapq` module contains an implementation of a heap queue
  algorithm.  A heap is an array-like data structure that keeps items in a
  partially sorted order such that, for every index *k*, ``heap[k] <=
  heap[2*k+1]`` and ``heap[k] <= heap[2*k+2]``.  This makes it quick to remove the
  smallest item, and inserting a new item while maintaining the heap property is
  O(lg n).  (See http://www.nist.gov/dads/HTML/priorityque.html for more
  information about the priority queue data structure.)

  The :mod:`heapq` module provides :func:`heappush` and :func:`heappop` functions
  for adding and removing items while maintaining the heap property on top of some
  other mutable Python sequence type.  Here's an example that uses a Python list::

     >>> import heapq
     >>> heap = []
     >>> for item in [3, 7, 5, 11, 1]:
     ...    heapq.heappush(heap, item)
     ...
     >>> heap
     [1, 3, 5, 11, 7]
     >>> heapq.heappop(heap)
     1
     >>> heapq.heappop(heap)
     3
     >>> heap
     [5, 7, 11]

  (Contributed by Kevin O'Connor.)

* The IDLE integrated development environment has been updated using the code
  from the IDLEfork project (http://idlefork.sf.net).  The most notable feature is
  that the code being developed is now executed in a subprocess, meaning that
  there's no longer any need for manual ``reload()`` operations. IDLE's core code
  has been incorporated into the standard library as the :mod:`idlelib` package.

* The :mod:`imaplib` module now supports IMAP over SSL. (Contributed by Piers
  Lauder and Tino Lange.)

* The :mod:`itertools` contains a number of useful functions for use with
  iterators, inspired by various functions provided by the ML and Haskell
  languages.  For example, ``itertools.ifilter(predicate, iterator)`` returns all
  elements in the iterator for which the function :func:`predicate` returns
  :const:`True`, and ``itertools.repeat(obj, N)`` returns ``obj`` *N* times.
  There are a number of other functions in the module; see the package's reference
  documentation for details.
  (Contributed by Raymond Hettinger.)

* Two new functions in the :mod:`math` module, :func:`degrees(rads)` and
  :func:`radians(degs)`, convert between radians and degrees.  Other functions in
  the :mod:`math` module such as :func:`math.sin` and :func:`math.cos` have always
  required input values measured in radians.  Also, an optional *base* argument
  was added to :func:`math.log` to make it easier to compute logarithms for bases
  other than ``e`` and ``10``.  (Contributed by Raymond Hettinger.)

* Several new POSIX functions (:func:`getpgid`, :func:`killpg`, :func:`lchown`,
  :func:`loadavg`, :func:`major`, :func:`makedev`, :func:`minor`, and
  :func:`mknod`) were added to the :mod:`posix` module that underlies the
  :mod:`os` module. (Contributed by Gustavo Niemeyer, Geert Jansen, and Denis S.
  Otkidach.)

* In the :mod:`os` module, the :func:`\*stat` family of functions can now report
  fractions of a second in a timestamp.  Such time stamps are represented as
  floats, similar to the value returned by :func:`time.time`.

  During testing, it was found that some applications will break if time stamps
  are floats.  For compatibility, when using the tuple interface of the
  :class:`stat_result` time stamps will be represented as integers. When using
  named fields (a feature first introduced in Python 2.2), time stamps are still
  represented as integers, unless :func:`os.stat_float_times` is invoked to enable
  float return values::

     >>> os.stat("/tmp").st_mtime
     1034791200
     >>> os.stat_float_times(True)
     >>> os.stat("/tmp").st_mtime
     1034791200.6335014

  In Python 2.4, the default will change to always returning floats.

  Application developers should enable this feature only if all their libraries
  work properly when confronted with floating point time stamps, or if they use
  the tuple API. If used, the feature should be activated on an application level
  instead of trying to enable it on a per-use basis.

* The :mod:`optparse` module contains a new parser for command-line arguments
  that can convert option values to a particular Python type  and will
  automatically generate a usage message.  See the following section for  more
  details.

* The old and never-documented :mod:`linuxaudiodev` module has been deprecated,
  and a new version named :mod:`ossaudiodev` has been added.  The module was
  renamed because the OSS sound drivers can be used on platforms other than Linux,
  and the interface has also been tidied and brought up to date in various ways.
  (Contributed by Greg Ward and Nicholas FitzRoy-Dale.)

* The new :mod:`platform` module contains a number of functions that try to
  determine various properties of the platform you're running on.  There are
  functions for getting the architecture, CPU type, the Windows OS version, and
  even the Linux distribution version. (Contributed by Marc-André Lemburg.)

* The parser objects provided by the :mod:`pyexpat` module can now optionally
  buffer character data, resulting in fewer calls to your character data handler
  and therefore faster performance.  Setting the parser object's
  :attr:`buffer_text` attribute to :const:`True` will enable buffering.

* The :func:`sample(population, k)` function was added to the :mod:`random`
  module.  *population* is a sequence or :class:`xrange` object containing the
  elements of a population, and :func:`sample` chooses *k* elements from the
  population without replacing chosen elements.  *k* can be any value up to
  ``len(population)``. For example::

     >>> days = ['Mo', 'Tu', 'We', 'Th', 'Fr', 'St', 'Sn']
     >>> random.sample(days, 3)      # Choose 3 elements
     ['St', 'Sn', 'Th']
     >>> random.sample(days, 7)      # Choose 7 elements
     ['Tu', 'Th', 'Mo', 'We', 'St', 'Fr', 'Sn']
     >>> random.sample(days, 7)      # Choose 7 again
     ['We', 'Mo', 'Sn', 'Fr', 'Tu', 'St', 'Th']
     >>> random.sample(days, 8)      # Can't choose eight
     Traceback (most recent call last):
       File "<stdin>", line 1, in ?
       File "random.py", line 414, in sample
           raise ValueError, "sample larger than population"
     ValueError: sample larger than population
     >>> random.sample(xrange(1,10000,2), 10)   # Choose ten odd nos. under 10000
     [3407, 3805, 1505, 7023, 2401, 2267, 9733, 3151, 8083, 9195]

  The :mod:`random` module now uses a new algorithm, the Mersenne Twister,
  implemented in C.  It's faster and more extensively studied than the previous
  algorithm.

  (All changes contributed by Raymond Hettinger.)

* The :mod:`readline` module also gained a number of new functions:
  :func:`get_history_item`, :func:`get_current_history_length`, and
  :func:`redisplay`.

* The :mod:`rexec` and :mod:`Bastion` modules have been declared dead, and
  attempts to import them will fail with a :exc:`RuntimeError`.  New-style classes
  provide new ways to break out of the restricted execution environment provided
  by :mod:`rexec`, and no one has interest in fixing them or time to do so.  If
  you have applications using :mod:`rexec`, rewrite them to use something else.

  (Sticking with Python 2.2 or 2.1 will not make your applications any safer
  because there are known bugs in the :mod:`rexec` module in those versions.  To
  repeat: if you're using :mod:`rexec`, stop using it immediately.)

* The :mod:`rotor` module has been deprecated because the  algorithm it uses for
  encryption is not believed to be secure.  If you need encryption, use one of the
  several AES Python modules that are available separately.

* The :mod:`shutil` module gained a :func:`move(src, dest)` function that
  recursively moves a file or directory to a new location.

* Support for more advanced POSIX signal handling was added to the :mod:`signal`
  but then removed again as it proved impossible to make it work reliably across
  platforms.

* The :mod:`socket` module now supports timeouts.  You can call the
  :meth:`settimeout(t)` method on a socket object to set a timeout of *t* seconds.
  Subsequent socket operations that take longer than *t* seconds to complete will
  abort and raise a :exc:`socket.timeout` exception.

  The original timeout implementation was by Tim O'Malley.  Michael Gilfix
  integrated it into the Python :mod:`socket` module and shepherded it through a
  lengthy review.  After the code was checked in, Guido van Rossum rewrote parts
  of it.  (This is a good example of a collaborative development process in
  action.)

* On Windows, the :mod:`socket` module now ships with Secure  Sockets Layer
  (SSL) support.

* The value of the C :const:`PYTHON_API_VERSION` macro is now exposed at the
  Python level as ``sys.api_version``.  The current exception can be cleared by
  calling the new :func:`sys.exc_clear` function.

* The new :mod:`tarfile` module  allows reading from and writing to
  :program:`tar`\ -format archive files. (Contributed by Lars Gustäbel.)

* The new :mod:`textwrap` module contains functions for wrapping strings
  containing paragraphs of text.  The :func:`wrap(text, width)` function takes a
  string and returns a list containing the text split into lines of no more than
  the chosen width.  The :func:`fill(text, width)` function returns a single
  string, reformatted to fit into lines no longer than the chosen width. (As you
  can guess, :func:`fill` is built on top of :func:`wrap`.  For example::

     >>> import textwrap
     >>> paragraph = "Not a whit, we defy augury: ... more text ..."
     >>> textwrap.wrap(paragraph, 60)
     ["Not a whit, we defy augury: there's a special providence in",
      "the fall of a sparrow. If it be now, 'tis not to come; if it",
      ...]
     >>> print textwrap.fill(paragraph, 35)
     Not a whit, we defy augury: there's
     a special providence in the fall of
     a sparrow. If it be now, 'tis not
     to come; if it be not to come, it
     will be now; if it be not now, yet
     it will come: the readiness is all.
     >>>

  The module also contains a :class:`TextWrapper` class that actually implements
  the text wrapping strategy.   Both the :class:`TextWrapper` class and the
  :func:`wrap` and :func:`fill` functions support a number of additional keyword
  arguments for fine-tuning the formatting; consult the module's documentation
  for details. (Contributed by Greg Ward.)

* The :mod:`thread` and :mod:`threading` modules now have companion modules,
  :mod:`dummy_thread` and :mod:`dummy_threading`, that provide a do-nothing
  implementation of the :mod:`thread` module's interface for platforms where
  threads are not supported.  The intention is to simplify thread-aware modules
  (ones that *don't* rely on threads to run) by putting the following code at the
  top::

     try:
         import threading as _threading
     except ImportError:
         import dummy_threading as _threading

  In this example, :mod:`_threading` is used as the module name to make it clear
  that the module being used is not necessarily the actual :mod:`threading`
  module. Code can call functions and use classes in :mod:`_threading` whether or
  not threads are supported, avoiding an :keyword:`if` statement and making the
  code slightly clearer.  This module will not magically make multithreaded code
  run without threads; code that waits for another thread to return or to do
  something will simply hang forever.

* The :mod:`time` module's :func:`strptime` function has long been an annoyance
  because it uses the platform C library's :func:`strptime` implementation, and
  different platforms sometimes have odd bugs.  Brett Cannon contributed a
  portable implementation that's written in pure Python and should behave
  identically on all platforms.

* The new :mod:`timeit` module helps measure how long snippets of Python code
  take to execute.  The :file:`timeit.py` file can be run directly from the
  command line, or the module's :class:`Timer` class can be imported and used
  directly.  Here's a short example that figures out whether it's faster to
  convert an 8-bit string to Unicode by appending an empty Unicode string to it or
  by using the :func:`unicode` function::

     import timeit

     timer1 = timeit.Timer('unicode("abc")')
     timer2 = timeit.Timer('"abc" + u""')

     # Run three trials
     print timer1.repeat(repeat=3, number=100000)
     print timer2.repeat(repeat=3, number=100000)

     # On my laptop this outputs:
     # [0.36831796169281006, 0.37441694736480713, 0.35304892063140869]
     # [0.17574405670166016, 0.18193507194519043, 0.17565798759460449]

* The :mod:`Tix` module has received various bug fixes and updates for the
  current version of the Tix package.

* The :mod:`Tkinter` module now works with a thread-enabled  version of Tcl.
  Tcl's threading model requires that widgets only be accessed from the thread in
  which they're created; accesses from another thread can cause Tcl to panic.  For
  certain Tcl interfaces, :mod:`Tkinter` will now automatically avoid this  when a
  widget is accessed from a different thread by marshalling a command, passing it
  to the correct thread, and waiting for the results.  Other interfaces can't be
  handled automatically but :mod:`Tkinter` will now raise an exception on such an
  access so that you can at least find out about the problem.  See
  http://mail.python.org/pipermail/python-dev/2002-December/031107.html for a more
  detailed explanation of this change.  (Implemented by Martin von Löwis.)

* Calling Tcl methods through :mod:`_tkinter` no longer  returns only strings.
  Instead, if Tcl returns other objects those objects are converted to their
  Python equivalent, if one exists, or wrapped with a :class:`_tkinter.Tcl_Obj`
  object if no Python equivalent exists. This behavior can be controlled through
  the :meth:`wantobjects` method of :class:`tkapp` objects.

  When using :mod:`_tkinter` through the :mod:`Tkinter` module (as most Tkinter
  applications will), this feature is always activated. It should not cause
  compatibility problems, since Tkinter would always convert string results to
  Python types where possible.

  If any incompatibilities are found, the old behavior can be restored by setting
  the :attr:`wantobjects` variable in the :mod:`Tkinter` module to false before
  creating the first :class:`tkapp` object. ::

     import Tkinter
     Tkinter.wantobjects = 0

  Any breakage caused by this change should be reported as a bug.

* The :mod:`UserDict` module has a new :class:`DictMixin` class which defines
  all dictionary methods for classes that already have a minimum mapping
  interface.  This greatly simplifies writing classes that need to be
  substitutable for dictionaries, such as the classes in  the :mod:`shelve`
  module.

  Adding the mix-in as a superclass provides the full dictionary interface
  whenever the class defines :meth:`__getitem__`, :meth:`__setitem__`,
  :meth:`__delitem__`, and :meth:`keys`. For example::

     >>> import UserDict
     >>> class SeqDict(UserDict.DictMixin):
     ...     """Dictionary lookalike implemented with lists."""
     ...     def __init__(self):
     ...         self.keylist = []
     ...         self.valuelist = []
     ...     def __getitem__(self, key):
     ...         try:
     ...             i = self.keylist.index(key)
     ...         except ValueError:
     ...             raise KeyError
     ...         return self.valuelist[i]
     ...     def __setitem__(self, key, value):
     ...         try:
     ...             i = self.keylist.index(key)
     ...             self.valuelist[i] = value
     ...         except ValueError:
     ...             self.keylist.append(key)
     ...             self.valuelist.append(value)
     ...     def __delitem__(self, key):
     ...         try:
     ...             i = self.keylist.index(key)
     ...         except ValueError:
     ...             raise KeyError
     ...         self.keylist.pop(i)
     ...         self.valuelist.pop(i)
     ...     def keys(self):
     ...         return list(self.keylist)
     ...
     >>> s = SeqDict()
     >>> dir(s)      # See that other dictionary methods are implemented
     ['__cmp__', '__contains__', '__delitem__', '__doc__', '__getitem__',
      '__init__', '__iter__', '__len__', '__module__', '__repr__',
      '__setitem__', 'clear', 'get', 'has_key', 'items', 'iteritems',
      'iterkeys', 'itervalues', 'keylist', 'keys', 'pop', 'popitem',
      'setdefault', 'update', 'valuelist', 'values']

  (Contributed by Raymond Hettinger.)

* The DOM implementation in :mod:`xml.dom.minidom` can now generate XML output
  in a particular encoding by providing an optional encoding argument to the
  :meth:`toxml` and :meth:`toprettyxml` methods of DOM nodes.

* The :mod:`xmlrpclib` module now supports an XML-RPC extension for handling nil
  data values such as Python's ``None``.  Nil values are always supported on
  unmarshalling an XML-RPC response.  To generate requests containing ``None``,
  you must supply a true value for the *allow_none* parameter when creating a
  :class:`Marshaller` instance.

* The new :mod:`DocXMLRPCServer` module allows writing self-documenting XML-RPC
  servers. Run it in demo mode (as a program) to see it in action.   Pointing the
  Web browser to the RPC server produces pydoc-style documentation; pointing
  xmlrpclib to the server allows invoking the actual methods. (Contributed by
  Brian Quinlan.)

* Support for internationalized domain names (RFCs 3454, 3490, 3491, and 3492)
  has been added. The "idna" encoding can be used to convert between a Unicode
  domain name and the ASCII-compatible encoding (ACE) of that name. ::

     >{}>{}> u"www.Alliancefrançaise.nu".encode("idna")
     'www.xn--alliancefranaise-npb.nu'

  The :mod:`socket` module has also been extended to transparently convert
  Unicode hostnames to the ACE version before passing them to the C library.
  Modules that deal with hostnames such as :mod:`httplib` and :mod:`ftplib`)
  also support Unicode host names; :mod:`httplib` also sends HTTP ``Host``
  headers using the ACE version of the domain name.  :mod:`urllib` supports
  Unicode URLs with non-ASCII host names as long as the ``path`` part of the URL
  is ASCII only.

  To implement this change, the :mod:`stringprep` module, the  ``mkstringprep``
  tool and the ``punycode`` encoding have been added.

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


Date/Time Type
--------------

Date and time types suitable for expressing timestamps were added as the
:mod:`datetime` module.  The types don't support different calendars or many
fancy features, and just stick to the basics of representing time.

The three primary types are: :class:`date`, representing a day, month, and year;
:class:`time`, consisting of hour, minute, and second; and :class:`datetime`,
which contains all the attributes of both :class:`date` and :class:`time`.
There's also a :class:`timedelta` class representing differences between two
points in time, and time zone logic is implemented by classes inheriting from
the abstract :class:`tzinfo` class.

You can create instances of :class:`date` and :class:`time` by either supplying
keyword arguments to the appropriate constructor, e.g.
``datetime.date(year=1972, month=10, day=15)``, or by using one of a number of
class methods.  For example, the :meth:`date.today` class method returns the
current local date.

Once created, instances of the date/time classes are all immutable. There are a
number of methods for producing formatted strings from objects::

   >>> import datetime
   >>> now = datetime.datetime.now()
   >>> now.isoformat()
   '2002-12-30T21:27:03.994956'
   >>> now.ctime()  # Only available on date, datetime
   'Mon Dec 30 21:27:03 2002'
   >>> now.strftime('%Y %d %b')
   '2002 30 Dec'

The :meth:`replace` method allows modifying one or more fields  of a
:class:`date` or :class:`datetime` instance, returning a new instance::

   >>> d = datetime.datetime.now()
   >>> d
   datetime.datetime(2002, 12, 30, 22, 15, 38, 827738)
   >>> d.replace(year=2001, hour = 12)
   datetime.datetime(2001, 12, 30, 12, 15, 38, 827738)
   >>>

Instances can be compared, hashed, and converted to strings (the result is the
same as that of :meth:`isoformat`).  :class:`date` and :class:`datetime`
instances can be subtracted from each other, and added to :class:`timedelta`
instances.  The largest missing feature is that there's no standard library
support for parsing strings and getting back a :class:`date` or
:class:`datetime`.

For more information, refer to the module's reference documentation.
(Contributed by Tim Peters.)

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


The optparse Module
-------------------

The :mod:`getopt` module provides simple parsing of command-line arguments.  The
new :mod:`optparse` module (originally named Optik) provides more elaborate
command-line parsing that follows the Unix conventions, automatically creates
the output for :option:`--help`, and can perform different actions for different
options.

You start by creating an instance of :class:`OptionParser` and telling it what
your program's options are. ::

   import sys
   from optparse import OptionParser

   op = OptionParser()
   op.add_option('-i', '--input',
                 action='store', type='string', dest='input',
                 help='set input filename')
   op.add_option('-l', '--length',
                 action='store', type='int', dest='length',
                 help='set maximum length of output')

Parsing a command line is then done by calling the :meth:`parse_args` method. ::

   options, args = op.parse_args(sys.argv[1:])
   print options
   print args

This returns an object containing all of the option values, and a list of
strings containing the remaining arguments.

Invoking the script with the various arguments now works as you'd expect it to.
Note that the length argument is automatically converted to an integer. ::

   $ ./python opt.py -i data arg1
   <Values at 0x400cad4c: {'input': 'data', 'length': None}>
   ['arg1']
   $ ./python opt.py --input=data --length=4
   <Values at 0x400cad2c: {'input': 'data', 'length': 4}>
   []
   $

The help message is automatically generated for you::

   $ ./python opt.py --help
   usage: opt.py [options]

   options:
     -h, --help            show this help message and exit
     -iINPUT, --input=INPUT
                           set input filename
     -lLENGTH, --length=LENGTH
                           set maximum length of output
   $

See the module's documentation for more details.


Optik was written by Greg Ward, with suggestions from the readers of the Getopt
SIG.

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


.. _section-pymalloc:

Pymalloc: A Specialized Object Allocator
========================================

Pymalloc, a specialized object allocator written by Vladimir Marangozov, was a
feature added to Python 2.1.  Pymalloc is intended to be faster than the system
:c:func:`malloc` and to have less memory overhead for allocation patterns typical
of Python programs. The allocator uses C's :c:func:`malloc` function to get large
pools of memory and then fulfills smaller memory requests from these pools.

In 2.1 and 2.2, pymalloc was an experimental feature and wasn't enabled by
default; you had to explicitly enable it when compiling Python by providing the
:option:`--with-pymalloc` option to the :program:`configure` script.  In 2.3,
pymalloc has had further enhancements and is now enabled by default; you'll have
to supply :option:`--without-pymalloc` to disable it.

This change is transparent to code written in Python; however, pymalloc may
expose bugs in C extensions.  Authors of C extension modules should test their
code with pymalloc enabled, because some incorrect code may cause core dumps at
runtime.

There's one particularly common error that causes problems.  There are a number
of memory allocation functions in Python's C API that have previously just been
aliases for the C library's :c:func:`malloc` and :c:func:`free`, meaning that if
you accidentally called mismatched functions the error wouldn't be noticeable.
When the object allocator is enabled, these functions aren't aliases of
:c:func:`malloc` and :c:func:`free` any more, and calling the wrong function to
free memory may get you a core dump.  For example, if memory was allocated using
:c:func:`PyObject_Malloc`, it has to be freed using :c:func:`PyObject_Free`, not
:c:func:`free`.  A few modules included with Python fell afoul of this and had to
be fixed; doubtless there are more third-party modules that will have the same
problem.

As part of this change, the confusing multiple interfaces for allocating memory
have been consolidated down into two API families. Memory allocated with one
family must not be manipulated with functions from the other family.  There is
one family for allocating chunks of memory and another family of functions
specifically for allocating Python objects.

* To allocate and free an undistinguished chunk of memory use the "raw memory"
  family: :c:func:`PyMem_Malloc`, :c:func:`PyMem_Realloc`, and :c:func:`PyMem_Free`.

* The "object memory" family is the interface to the pymalloc facility described
  above and is biased towards a large number of "small" allocations:
  :c:func:`PyObject_Malloc`, :c:func:`PyObject_Realloc`, and :c:func:`PyObject_Free`.

* To allocate and free Python objects, use the "object" family
  :c:func:`PyObject_New`, :c:func:`PyObject_NewVar`, and :c:func:`PyObject_Del`.

Thanks to lots of work by Tim Peters, pymalloc in 2.3 also provides debugging
features to catch memory overwrites and doubled frees in both extension modules
and in the interpreter itself.  To enable this support, compile a debugging
version of the Python interpreter by running :program:`configure` with
:option:`--with-pydebug`.

To aid extension writers, a header file :file:`Misc/pymemcompat.h` is
distributed with the source to Python 2.3 that allows Python extensions to use
the 2.3 interfaces to memory allocation while compiling against any version of
Python since 1.5.2.  You would copy the file from Python's source distribution
and bundle it with the source of your extension.


.. seealso::

   http://svn.python.org/view/python/trunk/Objects/obmalloc.c
      For the full details of the pymalloc implementation, see the comments at
      the top of the file :file:`Objects/obmalloc.c` in the Python source code.
      The above link points to the file within the python.org SVN browser.

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


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

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

* The cycle detection implementation used by the garbage collection has proven
  to be stable, so it's now been made mandatory.  You can no longer compile Python
  without it, and the :option:`--with-cycle-gc` switch to :program:`configure` has
  been removed.

* Python can now optionally be built as a shared library
  (:file:`libpython2.3.so`) by supplying :option:`--enable-shared` when running
  Python's :program:`configure` script.  (Contributed by Ondrej Palkovsky.)

* The :c:macro:`DL_EXPORT` and :c:macro:`DL_IMPORT` macros are now deprecated.
  Initialization functions for Python extension modules should now be declared
  using the new macro :c:macro:`PyMODINIT_FUNC`, while the Python core will
  generally use the :c:macro:`PyAPI_FUNC` and :c:macro:`PyAPI_DATA` macros.

* The interpreter can be compiled without any docstrings for the built-in
  functions and modules by supplying :option:`--without-doc-strings` to the
  :program:`configure` script. This makes the Python executable about 10% smaller,
  but will also mean that you can't get help for Python's built-ins.  (Contributed
  by Gustavo Niemeyer.)

* The :c:func:`PyArg_NoArgs` macro is now deprecated, and code that uses it
  should be changed.  For Python 2.2 and later, the method definition table can
  specify the :const:`METH_NOARGS` flag, signalling that there are no arguments,
  and the argument checking can then be removed.  If compatibility with pre-2.2
  versions of Python is important, the code could use ``PyArg_ParseTuple(args,
  "")`` instead, but this will be slower than using :const:`METH_NOARGS`.

* :c:func:`PyArg_ParseTuple` accepts new format characters for various sizes of
  unsigned integers: ``B`` for :c:type:`unsigned char`, ``H`` for :c:type:`unsigned
  short int`,  ``I`` for :c:type:`unsigned int`,  and ``K`` for :c:type:`unsigned
  long long`.

* A new function, :c:func:`PyObject_DelItemString(mapping, char \*key)` was added
  as shorthand for ``PyObject_DelItem(mapping, PyString_New(key))``.

* File objects now manage their internal string buffer differently, increasing
  it exponentially when needed.  This results in the benchmark tests in
  :file:`Lib/test/test_bufio.py` speeding up considerably (from 57 seconds to 1.7
  seconds, according to one measurement).

* It's now possible to define class and static methods for a C extension type by
  setting either the :const:`METH_CLASS` or :const:`METH_STATIC` flags in a
  method's :c:type:`PyMethodDef` structure.

* Python now includes a copy of the Expat XML parser's source code, removing any
  dependence on a system version or local installation of Expat.

* If you dynamically allocate type objects in your extension, you should be
  aware of a change in the rules relating to the :attr:`__module__` and
  :attr:`__name__` attributes.  In summary, you will want to ensure the type's
  dictionary contains a ``'__module__'`` key; making the module name the part of
  the type name leading up to the final period will no longer have the desired
  effect.  For more detail, read the API reference documentation or the  source.

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


Port-Specific Changes
---------------------

Support for a port to IBM's OS/2 using the EMX runtime environment was merged
into the main Python source tree.  EMX is a POSIX emulation layer over the OS/2
system APIs.  The Python port for EMX tries to support all the POSIX-like
capability exposed by the EMX runtime, and mostly succeeds; :func:`fork` and
:func:`fcntl` are restricted by the limitations of the underlying emulation
layer.  The standard OS/2 port, which uses IBM's Visual Age compiler, also
gained support for case-sensitive import semantics as part of the integration of
the EMX port into CVS.  (Contributed by Andrew MacIntyre.)

On MacOS, most toolbox modules have been weaklinked to improve backward
compatibility.  This means that modules will no longer fail to load if a single
routine is missing on the current OS version. Instead calling the missing
routine will raise an exception. (Contributed by Jack Jansen.)

The RPM spec files, found in the :file:`Misc/RPM/` directory in the Python
source distribution, were updated for 2.3.  (Contributed by Sean Reifschneider.)

Other new platforms now supported by Python include AtheOS
(http://www.atheos.cx/), GNU/Hurd, and OpenVMS.

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


.. _23section-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 CVS change logs finds there
were 523 patches applied and 514 bugs fixed between Python 2.2 and 2.3.  Both
figures are likely to be underestimates.

Some of the more notable changes are:

* If the :envvar:`PYTHONINSPECT` environment variable is set, the Python
  interpreter will enter the interactive prompt after running a Python program, as
  if Python had been invoked with the :option:`-i` option. The environment
  variable can be set before running the Python interpreter, or it can be set by
  the Python program as part of its execution.

* The :file:`regrtest.py` script now provides a way to allow "all resources
  except *foo*."  A resource name passed to the :option:`-u` option can now be
  prefixed with a hyphen (``'-'``) to mean "remove this resource."  For example,
  the option '``-uall,-bsddb``' could be used to enable the use of all resources
  except ``bsddb``.

* The tools used to build the documentation now work under Cygwin as well as
  Unix.

* The ``SET_LINENO`` opcode has been removed.  Back in the mists of time, this
  opcode was needed to produce line numbers in tracebacks and support trace
  functions (for, e.g., :mod:`pdb`). Since Python 1.5, the line numbers in
  tracebacks have been computed using a different mechanism that works with
  "python -O".  For Python 2.3 Michael Hudson implemented a similar scheme to
  determine when to call the trace function, removing the need for ``SET_LINENO``
  entirely.

  It would be difficult to detect any resulting difference from Python code, apart
  from a slight speed up when Python is run without :option:`-O`.

  C extensions that access the :attr:`f_lineno` field of frame objects should
  instead call ``PyCode_Addr2Line(f->f_code, f->f_lasti)``. This will have the
  added effect of making the code work as desired under "python -O" in earlier
  versions of Python.

  A nifty new feature is that trace functions can now assign to the
  :attr:`f_lineno` attribute of frame objects, changing the line that will be
  executed next.  A ``jump`` command has been added to the :mod:`pdb` debugger
  taking advantage of this new feature. (Implemented by Richie Hindle.)

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


Porting to Python 2.3
=====================

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

* :keyword:`yield` is now always a keyword; if it's used as a variable name in
  your code, a different name must be chosen.

* For strings *X* and *Y*, ``X in Y`` now works if *X* is more than one
  character long.

* The :func:`int` type constructor will now return a long integer instead of
  raising an :exc:`OverflowError` when a string or floating-point number is too
  large to fit into an integer.

* If you have Unicode strings that contain 8-bit characters, you must declare
  the file's encoding (UTF-8, Latin-1, or whatever) by adding a comment to the top
  of the file.  See section :ref:`section-encodings` for more information.

* Calling Tcl methods through :mod:`_tkinter` no longer  returns only strings.
  Instead, if Tcl returns other objects those objects are converted to their
  Python equivalent, if one exists, or wrapped with a :class:`_tkinter.Tcl_Obj`
  object if no Python equivalent exists.

* Large octal and hex literals such as ``0xffffffff`` now trigger a
  :exc:`FutureWarning`. Currently they're stored as 32-bit numbers and result in a
  negative value, but in Python 2.4 they'll become positive long integers.

  There are a few ways to fix this warning.  If you really need a positive number,
  just add an ``L`` to the end of the literal.  If you're trying to get a 32-bit
  integer with low bits set and have previously used an expression such as ``~(1
  << 31)``, it's probably clearest to start with all bits set and clear the
  desired upper bits. For example, to clear just the top bit (bit 31), you could
  write ``0xffffffffL &~(1L<<31)``.

* You can no longer disable assertions by assigning to ``__debug__``.

* The Distutils :func:`setup` function has gained various new keyword arguments
  such as *depends*.  Old versions of the Distutils will abort if passed unknown
  keywords.  A solution is to check for the presence of the new
  :func:`get_distutil_options` function in your :file:`setup.py` and only uses the
  new keywords with a version of the Distutils that supports them::

     from distutils import core

     kw = {'sources': 'foo.c', ...}
     if hasattr(core, 'get_distutil_options'):
         kw['depends'] = ['foo.h']
     ext = Extension(**kw)

* Using ``None`` as a variable name will now result in a :exc:`SyntaxWarning`
  warning.

* Names of extension types defined by the modules included with Python now
  contain the module and a ``'.'`` in front of the type name.

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


.. _23acks:

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

The author would like to thank the following people for offering suggestions,
corrections and assistance with various drafts of this article: Jeff Bauer,
Simon Brunning, Brett Cannon, Michael Chermside, Andrew Dalke, Scott David
Daniels, Fred L. Drake, Jr., David Fraser,  Kelly Gerber, Raymond Hettinger,
Michael Hudson, Chris Lambert, Detlef Lannert, Martin von Löwis, Andrew
MacIntyre, Lalo Martins, Chad Netzer, Gustavo Niemeyer, Neal Norwitz, Hans
Nowak, Chris Reedy, Francesco Ricciardi, Vinay Sajip, Neil Schemenauer, Roman
Suzi, Jason Tishler, Just van Rossum.