****************************

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` expression 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

``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 ``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 ``exec`` statement is a syntax error, since

``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 ``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:`~object.__lt__` |

+-----------+------------------------+

| ``<=`` | :meth:`~object.__le__` |

+-----------+------------------------+

| ``>`` | :meth:`~object.__gt__` |

+-----------+------------------------+

| ``>=`` | :meth:`~object.__ge__` |

+-----------+------------------------+

| ``==`` | :meth:`~object.__eq__` |

+-----------+------------------------+

| ``!=`` | :meth:`~object.__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 ``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 Comparisons

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 ``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 :attr:`~function.__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:`~function.__dict__`. Unlike the :attr:`~type.__dict__` attribute of class instances, in

functions you can actually assign a new dictionary to :attr:`~function.__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

``www.vex.net/parnassus/`` (retired in February 2009, `available in the

Internet Archive Wayback Machine

<https://web.archive.org/web/20090130140102/http://www.vex.net/parnassus/>`_)

was the largest catalog of Python modules, but

registering software at the Vaults is optional, and many people did not 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 an ``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 https://www.python.org/community/sigs/current/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 https://pyunit.sourceforge.net/ for

more information about PyUnit.

* The :mod:`difflib` module contains a class, :class:`~difflib.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:`~time.asctime` and

:func:`~time.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 ``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, ``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 :c:func:`malloc` and have less memory overhead. The

allocator uses C's :c: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 :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 will get you a core dump. For

example, if memory was allocated using :c:macro:`PyMem_New`, it has to be freed

using :c:func:`PyMem_Del`, 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.

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 :c: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 https://mail.python.org/pipermail/python-dev/2001-January/.

* A new method, :meth:`~dict.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

:c: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.