mirror of https://github.com/python/cpython
155 lines
6.9 KiB
TeX
155 lines
6.9 KiB
TeX
\section{\module{zlib} ---
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Compression compatible with \program{gzip}}
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\declaremodule{builtin}{zlib}
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\modulesynopsis{Low-level interface to compression and decompression
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routines compatible with \program{gzip}.}
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For applications that require data compression, the functions in this
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module allow compression and decompression, using the zlib library.
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The zlib library has its own home page at
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\url{http://www.info-zip.org/pub/infozip/zlib/}. Version 1.1.3 is the
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most recent version as of September 2000; use a later version if one
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is available. There are known incompatibilities between the Python
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module and earlier versions of the zlib library.
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The available exception and functions in this module are:
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\begin{excdesc}{error}
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Exception raised on compression and decompression errors.
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\end{excdesc}
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\begin{funcdesc}{adler32}{string\optional{, value}}
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Computes a Adler-32 checksum of \var{string}. (An Adler-32
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checksum is almost as reliable as a CRC32 but can be computed much
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more quickly.) If \var{value} is present, it is used as the
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starting value of the checksum; otherwise, a fixed default value is
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used. This allows computing a running checksum over the
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concatenation of several input strings. The algorithm is not
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cryptographically strong, and should not be used for
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authentication or digital signatures.
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\end{funcdesc}
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\begin{funcdesc}{compress}{string\optional{, level}}
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Compresses the data in \var{string}, returning a string contained
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compressed data. \var{level} is an integer from \code{1} to
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\code{9} controlling the level of compression; \code{1} is fastest
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and produces the least compression, \code{9} is slowest and produces
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the most. The default value is \code{6}. Raises the
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\exception{error} exception if any error occurs.
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\end{funcdesc}
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\begin{funcdesc}{compressobj}{\optional{level}}
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Returns a compression object, to be used for compressing data streams
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that won't fit into memory at once. \var{level} is an integer from
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\code{1} to \code{9} controlling the level of compression; \code{1} is
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fastest and produces the least compression, \code{9} is slowest and
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produces the most. The default value is \code{6}.
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\end{funcdesc}
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\begin{funcdesc}{crc32}{string\optional{, value}}
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Computes a CRC (Cyclic Redundancy Check)%
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\index{Cyclic Redundancy Check}
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\index{checksum!Cyclic Redundancy Check}
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checksum of \var{string}. If
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\var{value} is present, it is used as the starting value of the
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checksum; otherwise, a fixed default value is used. This allows
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computing a running checksum over the concatenation of several
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input strings. The algorithm is not cryptographically strong, and
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should not be used for authentication or digital signatures.
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\end{funcdesc}
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\begin{funcdesc}{decompress}{string\optional{, wbits\optional{, bufsize}}}
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Decompresses the data in \var{string}, returning a string containing
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the uncompressed data. The \var{wbits} parameter controls the size of
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the window buffer. If \var{bufsize} is given, it is used as the
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initial size of the output buffer. Raises the \exception{error}
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exception if any error occurs.
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The absolute value of \var{wbits} is the base two logarithm of the
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size of the history buffer (the ``window size'') used when compressing
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data. Its absolute value should be between 8 and 15 for the most
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recent versions of the zlib library, larger values resulting in better
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compression at the expense of greater memory usage. The default value
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is 15. When \var{wbits} is negative, the standard
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\program{gzip} header is suppressed; this is an undocumented feature
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of the zlib library, used for compatibility with \program{unzip}'s
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compression file format.
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\var{bufsize} is the initial size of the buffer used to hold
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decompressed data. If more space is required, the buffer size will be
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increased as needed, so you don't have to get this value exactly
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right; tuning it will only save a few calls to \cfunction{malloc()}. The
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default size is 16384.
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\end{funcdesc}
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\begin{funcdesc}{decompressobj}{\optional{wbits}}
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Returns a decompression object, to be used for decompressing data
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streams that won't fit into memory at once. The \var{wbits}
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parameter controls the size of the window buffer.
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\end{funcdesc}
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Compression objects support the following methods:
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\begin{methoddesc}[Compress]{compress}{string}
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Compress \var{string}, returning a string containing compressed data
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for at least part of the data in \var{string}. This data should be
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concatenated to the output produced by any preceding calls to the
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\method{compress()} method. Some input may be kept in internal buffers
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for later processing.
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\end{methoddesc}
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\begin{methoddesc}[Compress]{flush}{\optional{mode}}
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All pending input is processed, and a string containing the remaining
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compressed output is returned. \var{mode} can be selected from the
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constants \constant{Z_SYNC_FLUSH}, \constant{Z_FULL_FLUSH}, or
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\constant{Z_FINISH}, defaulting to \constant{Z_FINISH}. \constant{Z_SYNC_FLUSH} and
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\constant{Z_FULL_FLUSH} allow compressing further strings of data and
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are used to allow partial error recovery on decompression, while
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\constant{Z_FINISH} finishes the compressed stream and
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prevents compressing any more data. After calling
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\method{flush()} with \var{mode} set to \constant{Z_FINISH}, the
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\method{compress()} method cannot be called again; the only realistic
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action is to delete the object.
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\end{methoddesc}
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Decompression objects support the following methods, and a single attribute:
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\begin{memberdesc}{unused_data}
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A string which contains any unused data from the last string fed to
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this decompression object. If the whole string turned out to contain
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compressed data, this is \code{""}, the empty string.
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The only way to determine where a string of compressed data ends is by
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actually decompressing it. This means that when compressed data is
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contained part of a larger file, you can only find the end of it by
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reading data and feeding it into a decompression object's
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\method{decompress} method until the \member{unused_data} attribute is
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no longer the empty string.
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\end{memberdesc}
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\begin{methoddesc}[Decompress]{decompress}{string}
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Decompress \var{string}, returning a string containing the
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uncompressed data corresponding to at least part of the data in
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\var{string}. This data should be concatenated to the output produced
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by any preceding calls to the
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\method{decompress()} method. Some of the input data may be preserved
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in internal buffers for later processing.
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\end{methoddesc}
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\begin{methoddesc}[Decompress]{flush}{}
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All pending input is processed, and a string containing the remaining
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uncompressed output is returned. After calling \method{flush()}, the
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\method{decompress()} method cannot be called again; the only realistic
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action is to delete the object.
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\end{methoddesc}
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\begin{seealso}
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\seemodule{gzip}{Reading and writing \program{gzip}-format files.}
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\seeurl{http://www.info-zip.org/pub/infozip/zlib/}{The
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zlib library home page.}
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\end{seealso}
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