Inital import

[l4.git] / l4 / pkg / python / contrib / Doc / whatsnew / 2.1.rst

****************************
  What's New in Python 2.1
****************************

:Author: A.M. Kuchling

.. |release| replace:: 1.01

.. $Id: whatsnew21.tex 50964 2006-07-30 03:03:43Z fred.drake $


Introduction
============

This article explains the new features in Python 2.1.  While there aren't as
many changes in 2.1 as there were in Python 2.0, there are still some pleasant
surprises in store.  2.1 is the first release to be steered through the use of
Python Enhancement Proposals, or PEPs, so most of the sizable changes have
accompanying PEPs that provide more complete documentation and a design
rationale for the change.  This article doesn't attempt to document the new
features completely, but simply provides an overview of the new features for
Python programmers. Refer to the Python 2.1 documentation, or to the specific
PEP, for more details about any new feature that particularly interests you.

One recent goal of the Python development team has been to accelerate the pace
of new releases, with a new release coming every 6 to 9 months. 2.1 is the first
release to come out at this faster pace, with the first alpha appearing in
January, 3 months after the final version of 2.0 was released.

The final release of Python 2.1 was made on April 17, 2001.

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


PEP 227: Nested Scopes
======================

The largest change in Python 2.1 is to Python's scoping rules.  In Python 2.0,
at any given time there are at most three namespaces used to look up variable
names: local, module-level, and the built-in namespace.  This often surprised
people because it didn't match their intuitive expectations.  For example, a
nested recursive function definition doesn't work::

   def f():
       ...
       def g(value):
           ...
           return g(value-1) + 1
       ...

The function :func:`g` will always raise a :exc:`NameError` exception, because
the binding of the name ``g`` isn't in either its local namespace or in the
module-level namespace.  This isn't much of a problem in practice (how often do
you recursively define interior functions like this?), but this also made using
the :keyword:`lambda` statement clumsier, and this was a problem in practice.
In code which uses :keyword:`lambda` you can often find local variables being
copied by passing them as the default values of arguments. ::

   def find(self, name):
       "Return list of any entries equal to 'name'"
       L = filter(lambda x, name=name: x == name,
                  self.list_attribute)
       return L

The readability of Python code written in a strongly functional style suffers
greatly as a result.

The most significant change to Python 2.1 is that static scoping has been added
to the language to fix this problem.  As a first effect, the ``name=name``
default argument is now unnecessary in the above example.  Put simply, when a
given variable name is not assigned a value within a function (by an assignment,
or the :keyword:`def`, :keyword:`class`, or :keyword:`import` statements),
references to the variable will be looked up in the local namespace of the
enclosing scope.  A more detailed explanation of the rules, and a dissection of
the implementation, can be found in the PEP.

This change may cause some compatibility problems for code where the same
variable name is used both at the module level and as a local variable within a
function that contains further function definitions. This seems rather unlikely
though, since such code would have been pretty confusing to read in the first
place.

One side effect of the change is that the ``from module import *`` and
:keyword:`exec` statements have been made illegal inside a function scope under
certain conditions.  The Python reference manual has said all along that ``from
module import *`` is only legal at the top level of a module, but the CPython
interpreter has never enforced this before.  As part of the implementation of
nested scopes, the compiler which turns Python source into bytecodes has to
generate different code to access variables in a containing scope.  ``from
module import *`` and :keyword:`exec` make it impossible for the compiler to
figure this out, because they add names to the local namespace that are
unknowable at compile time. Therefore, if a function contains function
definitions or :keyword:`lambda` expressions with free variables, the compiler
will flag this by raising a :exc:`SyntaxError` exception.

To make the preceding explanation a bit clearer, here's an example::

   x = 1
   def f():
       # The next line is a syntax error
       exec 'x=2'
       def g():
           return x

Line 4 containing the :keyword:`exec` statement is a syntax error, since
:keyword:`exec` would define a new local variable named ``x`` whose value should
be accessed by :func:`g`.

This shouldn't be much of a limitation, since :keyword:`exec` is rarely used in
most Python code (and when it is used, it's often a sign of a poor design
anyway).

Compatibility concerns have led to nested scopes being introduced gradually; in
Python 2.1, they aren't enabled by default, but can be turned on within a module
by using a future statement as described in PEP 236.  (See the following section
for further discussion of PEP 236.)  In Python 2.2, nested scopes will become
the default and there will be no way to turn them off, but users will have had
all of 2.1's lifetime to fix any breakage resulting from their introduction.


.. seealso::

   :pep:`227` - Statically Nested Scopes
      Written and implemented by Jeremy Hylton.

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


PEP 236: __future__ Directives
==============================

The reaction to nested scopes was widespread concern about the dangers of
breaking code with the 2.1 release, and it was strong enough to make the
Pythoneers take a more conservative approach.  This approach consists of
introducing a convention for enabling optional functionality in release N that
will become compulsory in release N+1.

The syntax uses a ``from...import`` statement using the reserved module name
:mod:`__future__`.  Nested scopes can be enabled by the following statement::

   from __future__ import nested_scopes

While it looks like a normal :keyword:`import` statement, it's not; there are
strict rules on where such a future statement can be put. They can only be at
the top of a module, and must precede any Python code or regular
:keyword:`import` statements.  This is because such statements can affect how
the Python bytecode compiler parses code and generates bytecode, so they must
precede any statement that will result in bytecodes being produced.


.. seealso::

   :pep:`236` - Back to the :mod:`__future__`
      Written by Tim Peters, and primarily implemented by Jeremy Hylton.

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


PEP 207: Rich Comparisons
=========================

In earlier versions, Python's support for implementing comparisons on user-
defined classes and extension types was quite simple. Classes could implement a
:meth:`__cmp__` method that was given two instances of a class, and could only
return 0 if they were equal or +1 or -1 if they weren't; the method couldn't
raise an exception or return anything other than a Boolean value.  Users of
Numeric Python often found this model too weak and restrictive, because in the
number-crunching programs that numeric Python is used for, it would be more
useful to be able to perform elementwise comparisons of two matrices, returning
a matrix containing the results of a given comparison for each element.  If the
two matrices are of different sizes, then the compare has to be able to raise an
exception to signal the error.

In Python 2.1, rich comparisons were added in order to support this need.
Python classes can now individually overload each of the ``<``, ``<=``, ``>``,
``>=``, ``==``, and ``!=`` operations.  The new magic method names are:

+-----------+----------------+
| Operation | Method name    |
+===========+================+
| ``<``     | :meth:`__lt__` |
+-----------+----------------+
| ``<=``    | :meth:`__le__` |
+-----------+----------------+
| ``>``     | :meth:`__gt__` |
+-----------+----------------+
| ``>=``    | :meth:`__ge__` |
+-----------+----------------+
| ``==``    | :meth:`__eq__` |
+-----------+----------------+
| ``!=``    | :meth:`__ne__` |
+-----------+----------------+

(The magic methods are named after the corresponding Fortran operators ``.LT.``.
``.LE.``, &c.  Numeric programmers are almost certainly quite familiar with
these names and will find them easy to remember.)

Each of these magic methods is of the form ``method(self, other)``, where
``self`` will be the object on the left-hand side of the operator, while
``other`` will be the object on the right-hand side.  For example, the
expression ``A < B`` will cause ``A.__lt__(B)`` to be called.

Each of these magic methods can return anything at all: a Boolean, a matrix, a
list, or any other Python object.  Alternatively they can raise an exception if
the comparison is impossible, inconsistent, or otherwise meaningless.

The built-in :func:`cmp(A,B)` function can use the rich comparison machinery,
and now accepts an optional argument specifying which comparison operation to
use; this is given as one of the strings ``"<"``, ``"<="``, ``">"``, ``">="``,
``"=="``, or ``"!="``.  If called without the optional third argument,
:func:`cmp` will only return -1, 0, or +1 as in previous versions of Python;
otherwise it will call the appropriate method and can return any Python object.

There are also corresponding changes of interest to C programmers; there's a new
slot ``tp_richcmp`` in type objects and an API for performing a given rich
comparison.  I won't cover the C API here, but will refer you to PEP 207, or to
2.1's C API documentation, for the full list of related functions.


.. seealso::

   :pep:`207` - Rich Comparisions
      Written by Guido van Rossum, heavily based on earlier work by David Ascher, and
      implemented by Guido van Rossum.

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


PEP 230: Warning Framework
==========================

Over its 10 years of existence, Python has accumulated a certain number of
obsolete modules and features along the way.  It's difficult to know when a
feature is safe to remove, since there's no way of knowing how much code uses it
--- perhaps no programs depend on the feature, or perhaps many do.  To enable
removing old features in a more structured way, a warning framework was added.
When the Python developers want to get rid of a feature, it will first trigger a
warning in the next version of Python.  The following Python version can then
drop the feature, and users will have had a full release cycle to remove uses of
the old feature.

Python 2.1 adds the warning framework to be used in this scheme.  It adds a
:mod:`warnings` module that provide functions to issue warnings, and to filter
out warnings that you don't want to be displayed. Third-party modules can also
use this framework to deprecate old features that they no longer wish to
support.

For example, in Python 2.1 the :mod:`regex` module is deprecated, so importing
it causes a warning to be printed::

   >>> import regex
   __main__:1: DeprecationWarning: the regex module
            is deprecated; please use the re module
   >>>

Warnings can be issued by calling the :func:`warnings.warn` function::

   warnings.warn("feature X no longer supported")

The first parameter is the warning message; an additional optional parameters
can be used to specify a particular warning category.

Filters can be added to disable certain warnings; a regular expression pattern
can be applied to the message or to the module name in order to suppress a
warning.  For example, you may have a program that uses the :mod:`regex` module
and not want to spare the time to convert it to use the :mod:`re` module right
now.  The warning can be suppressed by calling ::

   import warnings
   warnings.filterwarnings(action = 'ignore',
                           message='.*regex module is deprecated',
                           category=DeprecationWarning,
                           module = '__main__')

This adds a filter that will apply only to warnings of the class
:class:`DeprecationWarning` triggered in the :mod:`__main__` module, and applies
a regular expression to only match the message about the :mod:`regex` module
being deprecated, and will cause such warnings to be ignored.  Warnings can also
be printed only once, printed every time the offending code is executed, or
turned into exceptions that will cause the program to stop (unless the
exceptions are caught in the usual way, of course).

Functions were also added to Python's C API for issuing warnings; refer to PEP
230 or to Python's API documentation for the details.


.. seealso::

   :pep:`5` - Guidelines for Language Evolution
      Written by Paul Prescod, to specify procedures to be followed when removing old
      features from Python.  The policy described in this PEP hasn't been officially
      adopted, but the eventual policy probably won't be too different from Prescod's
      proposal.

   :pep:`230` - Warning Framework
      Written and implemented by Guido van Rossum.

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


PEP 229: New Build System
=========================

When compiling Python, the user had to go in and edit the :file:`Modules/Setup`
file in order to enable various additional modules; the default set is
relatively small and limited to modules that compile on most Unix platforms.
This means that on Unix platforms with many more features, most notably Linux,
Python installations often don't contain all useful modules they could.

Python 2.0 added the Distutils, a set of modules for distributing and installing
extensions.  In Python 2.1, the Distutils are used to compile much of the
standard library of extension modules, autodetecting which ones are supported on
the current machine.  It's hoped that this will make Python installations easier
and more featureful.

Instead of having to edit the :file:`Modules/Setup` file in order to enable
modules, a :file:`setup.py` script in the top directory of the Python source
distribution is run at build time, and attempts to discover which modules can be
enabled by examining the modules and header files on the system.  If a module is
configured in :file:`Modules/Setup`, the :file:`setup.py` script won't attempt
to compile that module and will defer to the :file:`Modules/Setup` file's
contents.  This provides a way to specific any strange command-line flags or
libraries that are required for a specific platform.

In another far-reaching change to the build mechanism, Neil Schemenauer
restructured things so Python now uses a single makefile that isn't recursive,
instead of makefiles in the top directory and in each of the :file:`Python/`,
:file:`Parser/`, :file:`Objects/`, and :file:`Modules/` subdirectories.  This
makes building Python faster and also makes hacking the Makefiles clearer and
simpler.


.. seealso::

   :pep:`229` - Using Distutils to Build Python
      Written and implemented by A.M. Kuchling.

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


PEP 205: Weak References
========================

Weak references, available through the :mod:`weakref` module, are a minor but
useful new data type in the Python programmer's toolbox.

Storing a reference to an object (say, in a dictionary or a list) has the side
effect of keeping that object alive forever.  There are a few specific cases
where this behaviour is undesirable, object caches being the most common one,
and another being circular references in data structures such as trees.

For example, consider a memoizing function that caches the results of another
function :func:`f(x)` by storing the function's argument and its result in a
dictionary::

   _cache = {}
   def memoize(x):
       if _cache.has_key(x):
           return _cache[x]

       retval = f(x)

       # Cache the returned object
       _cache[x] = retval

       return retval

This version works for simple things such as integers, but it has a side effect;
the ``_cache`` dictionary holds a reference to the return values, so they'll
never be deallocated until the Python process exits and cleans up This isn't
very noticeable for integers, but if :func:`f` returns an object, or a data
structure that takes up a lot of memory, this can be a problem.

Weak references provide a way to implement a cache that won't keep objects alive
beyond their time.  If an object is only accessible through weak references, the
object will be deallocated and the weak references will now indicate that the
object it referred to no longer exists.  A weak reference to an object *obj* is
created by calling ``wr = weakref.ref(obj)``.  The object being referred to is
returned by calling the weak reference as if it were a function: ``wr()``.  It
will return the referenced object, or ``None`` if the object no longer exists.

This makes it possible to write a :func:`memoize` function whose cache doesn't
keep objects alive, by storing weak references in the cache. ::

   _cache = {}
   def memoize(x):
       if _cache.has_key(x):
           obj = _cache[x]()
           # If weak reference object still exists,
           # return it
           if obj is not None: return obj

       retval = f(x)

       # Cache a weak reference
       _cache[x] = weakref.ref(retval)

       return retval

The :mod:`weakref` module also allows creating proxy objects which behave like
weak references --- an object referenced only by proxy objects is deallocated --
but instead of requiring an explicit call to retrieve the object, the proxy
transparently forwards all operations to the object as long as the object still
exists.  If the object is deallocated, attempting to use a proxy will cause a
:exc:`weakref.ReferenceError` exception to be raised. ::

   proxy = weakref.proxy(obj)
   proxy.attr   # Equivalent to obj.attr
   proxy.meth() # Equivalent to obj.meth()
   del obj
   proxy.attr   # raises weakref.ReferenceError


.. seealso::

   :pep:`205` - Weak References
      Written and implemented by Fred L. Drake, Jr.

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


PEP 232: Function Attributes
============================

In Python 2.1, functions can now have arbitrary information attached to them.
People were often using docstrings to hold information about functions and
methods, because the ``__doc__`` attribute was the only way of attaching any
information to a function.  For example, in the Zope Web application server,
functions are marked as safe for public access by having a docstring, and in
John Aycock's SPARK parsing framework, docstrings hold parts of the BNF grammar
to be parsed.  This overloading is unfortunate, since docstrings are really
intended to hold a function's documentation; for example, it means you can't
properly document functions intended for private use in Zope.

Arbitrary attributes can now be set and retrieved on functions using the regular
Python syntax::

   def f(): pass

   f.publish = 1
   f.secure = 1
   f.grammar = "A ::= B (C D)*"

The dictionary containing attributes can be accessed as the function's
:attr:`__dict__`. Unlike the :attr:`__dict__` attribute of class instances, in
functions you can actually assign a new dictionary to :attr:`__dict__`, though
the new value is restricted to a regular Python dictionary; you *can't* be
tricky and set it to a :class:`UserDict` instance, or any other random object
that behaves like a mapping.


.. seealso::

   :pep:`232` - Function Attributes
      Written and implemented by Barry Warsaw.

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


PEP 235: Importing Modules on Case-Insensitive Platforms
========================================================

Some operating systems have filesystems that are case-insensitive, MacOS and
Windows being the primary examples; on these systems, it's impossible to
distinguish the filenames ``FILE.PY`` and ``file.py``, even though they do store
the file's name  in its original case (they're case-preserving, too).

In Python 2.1, the :keyword:`import` statement will work to simulate case-
sensitivity on case-insensitive platforms.  Python will now search for the first
case-sensitive match by default, raising an :exc:`ImportError` if no such file
is found, so ``import file`` will not import a module named ``FILE.PY``.  Case-
insensitive matching can be requested by setting the :envvar:`PYTHONCASEOK`
environment variable before starting the Python interpreter.

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


PEP 217: Interactive Display Hook
=================================

When using the Python interpreter interactively, the output of commands is
displayed using the built-in :func:`repr` function. In Python 2.1, the variable
:func:`sys.displayhook` can be set to a callable object which will be called
instead of :func:`repr`. For example, you can set it to a special pretty-
printing function::

   >>> # Create a recursive data structure
   ... L = [1,2,3]
   >>> L.append(L)
   >>> L # Show Python's default output
   [1, 2, 3, [...]]
   >>> # Use pprint.pprint() as the display function
   ... import sys, pprint
   >>> sys.displayhook = pprint.pprint
   >>> L
   [1, 2, 3,  <Recursion on list with id=135143996>]
   >>>


.. seealso::

   :pep:`217` - Display Hook for Interactive Use
      Written and implemented by Moshe Zadka.

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


PEP 208: New Coercion Model
===========================

How numeric coercion is done at the C level was significantly modified.  This
will only affect the authors of C extensions to Python, allowing them more
flexibility in writing extension types that support numeric operations.

Extension types can now set the type flag ``Py_TPFLAGS_CHECKTYPES`` in their
``PyTypeObject`` structure to indicate that they support the new coercion model.
In such extension types, the numeric slot functions can no longer assume that
they'll be passed two arguments of the same type; instead they may be passed two
arguments of differing types, and can then perform their own internal coercion.
If the slot function is passed a type it can't handle, it can indicate the
failure by returning a reference to the ``Py_NotImplemented`` singleton value.
The numeric functions of the other type will then be tried, and perhaps they can
handle the operation; if the other type also returns ``Py_NotImplemented``, then
a :exc:`TypeError` will be raised.  Numeric methods written in Python can also
return ``Py_NotImplemented``, causing the interpreter to act as if the method
did not exist (perhaps raising a :exc:`TypeError`, perhaps trying another
object's numeric methods).


.. seealso::

   :pep:`208` - Reworking the Coercion Model
      Written and implemented by Neil Schemenauer, heavily based upon earlier work by
      Marc-André Lemburg.  Read this to understand the fine points of how numeric
      operations will now be processed at the C level.

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


PEP 241: Metadata in Python Packages
====================================

A common complaint from Python users is that there's no single catalog of all
the Python modules in existence.  T. Middleton's Vaults of Parnassus at
http://www.vex.net/parnassus/ are the largest catalog of Python modules, but
registering software at the Vaults is optional, and many people don't bother.

As a first small step toward fixing the problem, Python software packaged using
the Distutils :command:`sdist` command will include a file named
:file:`PKG-INFO` containing information about the package such as its name,
version, and author (metadata, in cataloguing terminology).  PEP 241 contains
the full list of fields that can be present in the :file:`PKG-INFO` file.  As
people began to package their software using Python 2.1, more and more packages
will include metadata, making it possible to build automated cataloguing systems
and experiment with them.  With the result experience, perhaps it'll be possible
to design a really good catalog and then build support for it into Python 2.2.
For example, the Distutils :command:`sdist` and :command:`bdist_\*` commands
could support a :option:`upload` option that would automatically upload your
package to a catalog server.

You can start creating packages containing :file:`PKG-INFO` even if you're not
using Python 2.1, since a new release of the Distutils will be made for users of
earlier Python versions.  Version 1.0.2 of the Distutils includes the changes
described in PEP 241, as well as various bugfixes and enhancements.  It will be
available from  the Distutils SIG at http://www.python.org/sigs/distutils-sig/.


.. seealso::

   :pep:`241` - Metadata for Python Software Packages
      Written and implemented by A.M. Kuchling.

   :pep:`243` - Module Repository Upload Mechanism
      Written by Sean Reifschneider, this draft PEP describes a proposed mechanism for
      uploading  Python packages to a central server.

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


New and Improved Modules
========================

* Ka-Ping Yee contributed two new modules: :mod:`inspect.py`, a module for
  getting information about live Python code, and :mod:`pydoc.py`, a module for
  interactively converting docstrings to HTML or text.  As a bonus,
  :file:`Tools/scripts/pydoc`, which is now automatically installed, uses
  :mod:`pydoc.py` to display documentation given a Python module, package, or
  class name.  For example, ``pydoc xml.dom`` displays the following::

     Python Library Documentation: package xml.dom in xml

     NAME
         xml.dom - W3C Document Object Model implementation for Python.

     FILE
         /usr/local/lib/python2.1/xml/dom/__init__.pyc

     DESCRIPTION
         The Python mapping of the Document Object Model is documented in the
         Python Library Reference in the section on the xml.dom package.

         This package contains the following modules:
           ...

  :file:`pydoc` also includes a Tk-based interactive help browser.   :file:`pydoc`
  quickly becomes addictive; try it out!

* Two different modules for unit testing were added to the standard library.
  The :mod:`doctest` module, contributed by Tim Peters, provides a testing
  framework based on running embedded examples in docstrings and comparing the
  results against the expected output.  PyUnit, contributed by Steve Purcell, is a
  unit testing framework inspired by JUnit, which was in turn an adaptation of
  Kent Beck's Smalltalk testing framework.  See http://pyunit.sourceforge.net/ for
  more information about PyUnit.

* The :mod:`difflib` module contains a class, :class:`SequenceMatcher`, which
  compares two sequences and computes the changes required to transform one
  sequence into the other.  For example, this module can be used to write a tool
  similar to the Unix :program:`diff` program, and in fact the sample program
  :file:`Tools/scripts/ndiff.py` demonstrates how to write such a script.

* :mod:`curses.panel`, a wrapper for the panel library, part of ncurses and of
  SYSV curses, was contributed by Thomas Gellekum.  The panel library provides
  windows with the additional feature of depth. Windows can be moved higher or
  lower in the depth ordering, and the panel library figures out where panels
  overlap and which sections are visible.

* The PyXML package has gone through a few releases since Python 2.0, and Python
  2.1 includes an updated version of the :mod:`xml` package.  Some of the
  noteworthy changes include support for Expat 1.2 and later versions, the ability
  for Expat parsers to handle files in any encoding supported by Python, and
  various bugfixes for SAX, DOM, and the :mod:`minidom` module.

* Ping also contributed another hook for handling uncaught exceptions.
  :func:`sys.excepthook` can be set to a callable object.  When an exception isn't
  caught by any :keyword:`try`...\ :keyword:`except` blocks, the exception will be
  passed to :func:`sys.excepthook`, which can then do whatever it likes.  At the
  Ninth Python Conference, Ping demonstrated an application for this hook:
  printing an extended traceback that not only lists the stack frames, but also
  lists the function arguments and the local variables for each frame.

* Various functions in the :mod:`time` module, such as :func:`asctime` and
  :func:`localtime`, require a floating point argument containing the time in
  seconds since the epoch.  The most common use of these functions is to work with
  the current time, so the floating point argument has been made optional; when a
  value isn't provided, the current time will be used.  For example, log file
  entries usually need a string containing the current time; in Python 2.1,
  ``time.asctime()`` can be used, instead of the lengthier
  ``time.asctime(time.localtime(time.time()))`` that was previously required.

  This change was proposed and implemented by Thomas Wouters.

* The :mod:`ftplib` module now defaults to retrieving files in passive mode,
  because passive mode is more likely to work from behind a firewall.  This
  request came from the Debian bug tracking system, since other Debian packages
  use :mod:`ftplib` to retrieve files and then don't work from behind a firewall.
  It's deemed unlikely that this will cause problems for anyone, because Netscape
  defaults to passive mode and few people complain, but if passive mode is
  unsuitable for your application or network setup, call :meth:`set_pasv(0)` on
  FTP objects to disable passive mode.

* Support for raw socket access has been added to the :mod:`socket` module,
  contributed by Grant Edwards.

* The :mod:`pstats` module now contains a simple interactive statistics browser
  for displaying timing profiles for Python programs, invoked when the module is
  run as a script.  Contributed by  Eric S. Raymond.

* A new implementation-dependent function, :func:`sys._getframe([depth])`, has
  been added to return a given frame object from the current call stack.
  :func:`sys._getframe` returns the frame at the top of the call stack;  if the
  optional integer argument *depth* is supplied, the function returns the frame
  that is *depth* calls below the top of the stack.  For example,
  ``sys._getframe(1)`` returns the caller's frame object.

  This function is only present in CPython, not in Jython or the .NET
  implementation.  Use it for debugging, and resist the temptation to put it into
  production code.

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


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

There were relatively few smaller changes made in Python 2.1 due to the shorter
release cycle.  A search through the CVS change logs turns up 117 patches
applied, and 136 bugs fixed; both figures are likely to be underestimates.  Some
of the more notable changes are:

* A specialized object allocator is now optionally available, that should be
  faster than the system :func:`malloc` and have less memory overhead.  The
  allocator uses C's :func:`malloc` function to get large pools of memory, and
  then fulfills smaller memory requests from these pools.  It can be enabled by
  providing the :option:`--with-pymalloc` option to the :program:`configure`
  script; see :file:`Objects/obmalloc.c` for the implementation details.

  Authors of C extension modules should test their code with the object allocator
  enabled, because some incorrect code may break, causing core dumps at runtime.
  There are a bunch of memory allocation functions in Python's C API that have
  previously been just aliases for the C library's :func:`malloc` and
  :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 :func:`malloc` and :func:`free` any more, and
  calling the wrong function to free memory will get you a core dump.  For
  example, if memory was allocated using :func:`PyMem_New`, it has to be freed
  using :func:`PyMem_Del`, not :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.

  The object allocator was contributed by Vladimir Marangozov.

* The speed of line-oriented file I/O has been improved because people often
  complain about its lack of speed, and because it's often been used as a naïve
  benchmark.  The :meth:`readline` method of file objects has therefore been
  rewritten to be much faster.  The exact amount of the speedup will vary from
  platform to platform depending on how slow the C library's :func:`getc` was, but
  is around 66%, and potentially much faster on some particular operating systems.
  Tim Peters did much of the benchmarking and coding for this change, motivated by
  a discussion in comp.lang.python.

  A new module and method for file objects was also added, contributed by Jeff
  Epler. The new method, :meth:`xreadlines`, is similar to the existing
  :func:`xrange` built-in.  :func:`xreadlines` returns an opaque sequence object
  that only supports being iterated over, reading a line on every iteration but
  not reading the entire file into memory as the existing :meth:`readlines` method
  does. You'd use it like this::

     for line in sys.stdin.xreadlines():
         # ... do something for each line ...
         ...

  For a fuller discussion of the line I/O changes, see the python-dev summary for
  January 1-15, 2001 at http://www.python.org/dev/summary/2001-01-1.html.

* A new method, :meth:`popitem`, was added to dictionaries to enable
  destructively iterating through the contents of a dictionary; this can be faster
  for large dictionaries because there's no need to construct a list containing
  all the keys or values. ``D.popitem()`` removes a random ``(key, value)`` pair
  from the dictionary ``D`` and returns it as a 2-tuple.  This was implemented
  mostly by Tim Peters and Guido van Rossum, after a suggestion and preliminary
  patch by Moshe Zadka.

* Modules can now control which names are imported when ``from module import *``
  is used, by defining an ``__all__`` attribute containing a list of names that
  will be imported.  One common complaint is that if the module imports other
  modules such as :mod:`sys` or :mod:`string`, ``from module import *`` will add
  them to the importing module's namespace.  To fix this, simply list the public
  names in ``__all__``::

     # List public names
     __all__ = ['Database', 'open']

  A stricter version of this patch was first suggested and implemented by Ben
  Wolfson, but after some python-dev discussion, a weaker final version was
  checked in.

* Applying :func:`repr` to strings previously used octal escapes for
  non-printable characters; for example, a newline was ``'\012'``.  This was a
  vestigial trace of Python's C ancestry, but today octal is of very little
  practical use.  Ka-Ping Yee suggested using hex escapes instead of octal ones,
  and using the ``\n``, ``\t``, ``\r`` escapes for the appropriate characters,
  and implemented this new formatting.

* Syntax errors detected at compile-time can now raise exceptions containing the
  filename and line number of the error, a pleasant side effect of the compiler
  reorganization done by Jeremy Hylton.

* C extensions which import other modules have been changed to use
  :func:`PyImport_ImportModule`, which means that they will use any import hooks
  that have been installed.  This is also encouraged for third-party extensions
  that need to import some other module from C code.

* The size of the Unicode character database was shrunk by another 340K thanks
  to Fredrik Lundh.

* Some new ports were contributed: MacOS X (by Steven Majewski), Cygwin (by
  Jason Tishler); RISCOS (by Dietmar Schwertberger); Unixware 7  (by Billy G.
  Allie).

And there's the usual list of minor bugfixes, minor memory leaks, docstring
edits, and other tweaks, too lengthy to be worth itemizing; see the CVS logs for
the full details if you want them.

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


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

The author would like to thank the following people for offering suggestions on
various drafts of this article: Graeme Cross, David Goodger, Jay Graves, Michael
Hudson, Marc-André Lemburg, Fredrik Lundh, Neil Schemenauer, Thomas Wouters.