\section{\module{gzip} ---
         Support for \program{gzip} files}

\declaremodule{standard}{gzip}
\modulesynopsis{Interfaces for \program{gzip} compression and
decompression using file objects.}


The data compression provided by the \code{zlib} module is compatible
with that used by the GNU compression program \program{gzip}.
Accordingly, the \module{gzip} module provides the \class{GzipFile}
class to read and write \program{gzip}-format files, automatically
compressing or decompressing the data so it looks like an ordinary
file object.  Note that additional file formats which can be
decompressed by the \program{gzip} and \program{gunzip} programs, such 
as those produced by \program{compress} and \program{pack}, are not
supported by this module.

The module defines the following items:

\begin{classdesc}{GzipFile}{\optional{filename\optional{, mode\optional{,
                            compresslevel\optional{, fileobj}}}}}
Constructor for the \class{GzipFile} class, which simulates most of
the methods of a file object, with the exception of the \method{readinto()}
and \method{truncate()} methods.  At least one of
\var{fileobj} and \var{filename} must be given a non-trivial value.

The new class instance is based on \var{fileobj}, which can be a
regular file, a \class{StringIO} object, or any other object which
simulates a file.  It defaults to \code{None}, in which case
\var{filename} is opened to provide a file object.

When \var{fileobj} is not \code{None}, the \var{filename} argument is
only used to be included in the \program{gzip} file header, which may
includes the original filename of the uncompressed file.  It defaults
to the filename of \var{fileobj}, if discernible; otherwise, it
defaults to the empty string, and in this case the original filename
is not included in the header.

The \var{mode} argument can be any of \code{'r'}, \code{'rb'},
\code{'a'}, \code{'ab'}, \code{'w'}, or \code{'wb'}, depending on
whether the file will be read or written.  The default is the mode of
\var{fileobj} if discernible; otherwise, the default is \code{'rb'}.
If not given, the 'b' flag will be added to the mode to ensure the
file is opened in binary mode for cross-platform portability.

The \var{compresslevel} argument is an integer from \code{1} to
\code{9} controlling the level of compression; \code{1} is fastest and
produces the least compression, and \code{9} is slowest and produces
the most compression.  The default is \code{9}.

Calling a \class{GzipFile} object's \method{close()} method does not
close \var{fileobj}, since you might wish to append more material
after the compressed data.  This also allows you to pass a
\class{StringIO} object opened for writing as \var{fileobj}, and
retrieve the resulting memory buffer using the \class{StringIO}
object's \method{getvalue()} method.
\end{classdesc}

\begin{funcdesc}{open}{filename\optional{, mode\optional{, compresslevel}}}
This is a shorthand for \code{GzipFile(\var{filename},}
\code{\var{mode},} \code{\var{compresslevel})}.  The \var{filename}
argument is required; \var{mode} defaults to \code{'rb'} and
\var{compresslevel} defaults to \code{9}.
\end{funcdesc}

\begin{seealso}
  \seemodule{zlib}{The basic data compression module needed to support
                   the \program{gzip} file format.}
\end{seealso}