\documentclass{howto}
\usepackage{distutils}
% $Id$

\title{What's New in Python 2.4}
\release{0.0}
\author{A.M.\ Kuchling}
\authoraddress{\email{amk@amk.ca}}

\begin{document}
\maketitle
\tableofcontents

This article explains the new features in Python 2.4.  No release date
for Python 2.4 has been set; expect that this will happen in 2004.

While Python 2.3 was primarily a library development release, Python
2.4 may extend the core language and interpreter in
as-yet-undetermined ways.

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 2.4.
% add hyperlink when the documentation becomes available online.
If you want to understand the complete implementation and design
rationale, refer to the PEP for a particular new feature.


%======================================================================
\section{PEP 322: Reverse Iteration}

A new built-in function, \function{reversed(seq)}, takes a sequence
and returns an iterator that returns the elements of the sequence 
in reverse order.  

\begin{verbatim}
>>> for i in reversed([1,2,3]):
...    print i
... 
3
2
1
\end{verbatim}

Note that \function{reversed()} only accepts sequences, not arbitrary
iterators.  If you want to reverse an iterator, convert it to 
a list or tuple with \function{list()} or \function{tuple()}.

\begin{verbatim}
>>> input = open('/etc/passwd', 'r')
>>> for line in reversed(list(input)):
...   print line
... 
root:*:0:0:System Administrator:/var/root:/bin/tcsh
  ...
\end{verbatim}

\begin{seealso}
\seepep{322}{Reverse Iteration}{Written and implemented by Raymond Hettinger.}

\end{seealso}


%======================================================================
\section{Other Language Changes}

Here are all of the changes that Python 2.4 makes to the core Python
language.

\begin{itemize}
\item The \method{sort()} method of lists gained three keyword
arguments, \var{cmp}, \var{key}, and \var{reverse}.  These arguments
make some common usages of \method{sort()} simpler.  All are optional.

\var{cmp} is the same as the previous single argument to
\method{sort()}; if provided, the value should be a comparison
function that takes two arguments and returns -1, 0, or +1 depending
on how the arguments compare.  

\var{key} should be a single-argument function that takes a list
element and returns a comparison key for the element.  The list is
then sorted using the comparison keys.  The following example sorts a list
case-insensitively:

\begin{verbatim}
>>> L = ['A', 'b', 'c', 'D']
>>> L.sort()                 # Case-sensitive sort
>>> L
['A', 'D', 'b', 'c']
>>> L.sort(key=lambda x: x.lower())
>>> L
['A', 'b', 'c', 'D']
>>> L.sort(cmp=lambda x,y: cmp(x.lower(), y.lower()))
>>> L
['A', 'b', 'c', 'D']
\end{verbatim}

The last example, which uses the \var{cmp} parameter, is the old way
to perform a case-insensitive sort.  It works, but is slower than
using a \var{key} parameter.  Using \var{key} results in calling the
\method{lower()} method once for each element in the list while using
\var{cmp} will call the method twice for each comparison.

The \var{reverse} parameter should have a Boolean value.  If the value is
\constant{True}, the list will be sorted into reverse order.  Instead
of \code{L.sort() ; L.reverse()}, you can now write
\code{L.sort(reverse=True)}.

\item The list type gained a \method{sorted(iterable)} method that
returns the elements of the iterable as a sorted list.  It also accepts 
the \var{cmp}, \var{key}, and \var{reverse} keyword arguments, same as 
the \method{sort()} method.  An  example usage:

\begin{verbatim}
>>> L = [9,7,8,3,2,4,1,6,5]
>>> list.sorted(L)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> L
[9, 7, 8, 3, 2, 4, 1, 6, 5]
>>> 
\end{verbatim}

Note that the original list is unchanged; the list returned by
\method{sorted()} is a newly-created one.

\item The \function{zip()} built-in function and \function{itertools.izip()} now return  an empty list 
  instead of raising a \exception{TypeError} exception if called
  with no arguments.

\end{itemize}


%======================================================================
\subsection{Optimizations}

\begin{itemize}

\item Optimizations should be described here.

\end{itemize}

The net result of the 2.4 optimizations is that Python 2.4 runs the
pystone benchmark around XX\% faster than Python 2.3 and YY\% faster
than Python 2.2.


%======================================================================
\section{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.

\begin{itemize}

\item The \module{curses} modules now supports the ncurses extension 
   \function{use_default_colors()}.   On platforms where the terminal 
   supports transparency, this makes it possible to use a transparent background.
   (Contributed by J\"org Lehmann.)

\item The \module{heapq} module is no longer implemented in Python,
      having been converted into C.

\item The \module{random} module has a new method called \method{getrandbits(N)} 
   which returns an N-bit long integer.

\item The regular expression language accepted by the \module{re} module
   was extended with simple conditional expressions, written as
   \code{(?(\var{group})\var{A}|\var{B})}.  \var{group} is either a
   numeric group ID or a group name defined with \code{(?P<group>...)} 
   earlier in the expression.  If the specified group matched, the
   regular expression pattern \var{A} will be tested against the string; if
   the group didn't match, the pattern \var{B} will be used instead.
   
\end{itemize}


%======================================================================
% whole new modules get described in \subsections here


% ======================================================================
\section{Build and C API Changes}

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

\begin{itemize}

  \item Three new convenience macros were added for common return
  values from extension functions: \csimplemacro{Py_RETURN_NONE},
  \csimplemacro{Py_RETURN_TRUE}, and \csimplemacro{Py_RETURN_FALSE}.

  \item A new function, \cfunction{PyTuple_Pack(N, obj1, obj2, ...,
  objN)}, constructs tuples from a variable length argument list of
  Python objects.

\end{itemize}


%======================================================================
\subsection{Port-Specific Changes}

Platform-specific changes go here.


%======================================================================
\section{Other Changes and Fixes \label{section-other}}

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 XXX patches applied and YYY bugs fixed between
Python 2.3 and 2.4.  Both figures are likely to be underestimates.

Some of the more notable changes are:

\begin{itemize}

\item Details go here.

\end{itemize}


%======================================================================
\section{Porting to Python 2.4}

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

\begin{itemize}

\item The \function{zip()} built-in function and \function{itertools.izip()} now return  an empty list 
  instead of raising a \exception{TypeError} exception if called
  with no arguments.

\item \function{dircache.listdir()} now passes exceptions to the caller
      instead of returning empty lists.

\end{itemize}


%======================================================================
\section{Acknowledgements \label{acks}}

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

\end{document}
