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\section{Built-in Module \sectcode{gzip}}
\label{module-gzip}
\bimodindex{gzip}
The data compression provided by the \code{zlib} module is compatible
with that used by the GNU compression program \file{gzip}.
Accordingly, the \code{gzip} module provides the \code{GzipFile} class
to read and write \file{gzip}-format files, automatically compressing
or decompressing the data so it looks like an ordinary file object.
\code{GzipFile} objects simulate most of the methods of a file
object, though it's not possible to use the \code{seek()} and
\code{tell()} methods to access the file randomly.
\setindexsubitem{(in module gzip)}
\begin{funcdesc}{open}{fileobj\optional{\, filename\optional{\, mode\, compresslevel}}}
Returns a new \code{GzipFile} object on top of \var{fileobj}, which
can be a regular file, a \code{StringIO} object, or any object which
simulates a file.
The \file{gzip} file format includes the original filename of the
uncompressed file; when opening a \code{GzipFile} object for
writing, it can be set by the \var{filename} argument. The default
value is an empty string.
\var{mode} can be either \code{'r'} or \code{'w'} depending on
whether the file will be read or written. \var{compresslevel} is an
integer from 1 to 9 controlling the level of compression; 1 is
fastest and produces the least compression, and 9 is slowest and
produces the most compression. The default value of
\var{compresslevel} is 9.
Calling a \code{GzipFile} object's \code{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
\code{StringIO} object opened for writing as \var{fileobj}, and
retrieve the resulting memory buffer using the \code{StringIO}
object's \code{getvalue()} method.
\end{funcdesc}
\begin{seealso}
\seemodule{zlib}{the basic data compression module}
\end{seealso}
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