
``zlib`` --- Compression compatible with **gzip**
*************************************************

For applications that require data compression, the functions in this
module allow compression and decompression, using the zlib library.
The zlib library has its own home page at http://www.zlib.net.   There
are known incompatibilities between the Python module and versions of
the zlib library earlier than 1.1.3; 1.1.3 has a security
vulnerability, so we recommend using 1.1.4 or later.

zlib's functions have many options and often need to be used in a
particular order.  This documentation doesn't attempt to cover all of
the permutations; consult the zlib manual at
http://www.zlib.net/manual.html for authoritative information.

For reading and writing ``.gz`` files see the ``gzip`` module. For
other archive formats, see the ``bz2``, ``zipfile``, and ``tarfile``
modules.

The available exception and functions in this module are:

exception zlib.error

   Exception raised on compression and decompression errors.

zlib.adler32(string[, value])

   Computes a Adler-32 checksum of *string*.  (An Adler-32 checksum is
   almost as reliable as a CRC32 but can be computed much more
   quickly.)  If *value* is present, it is used as the starting value
   of the checksum; otherwise, a fixed default value is used.  This
   allows computing a running checksum over the concatenation of
   several input strings.  The algorithm is not cryptographically
   strong, and should not be used for authentication or digital
   signatures.  Since the algorithm is designed for use as a checksum
   algorithm, it is not suitable for use as a general hash algorithm.

   This function always returns an integer object.

   Changed in version 2.6: For consistent cross-platform behavior we
   always return a signed integer. ie: Results in the
   (2**31)...(2**32-1) range will be negative.

zlib.compress(string[, level])

   Compresses the data in *string*, returning a string contained
   compressed data. *level* is an integer from ``1`` to ``9``
   controlling the level of compression; ``1`` is fastest and produces
   the least compression, ``9`` is slowest and produces the most.  The
   default value is ``6``.  Raises the ``error`` exception if any
   error occurs.

zlib.compressobj([level])

   Returns a compression object, to be used for compressing data
   streams that won't fit into memory at once.  *level* is an integer
   from ``1`` to ``9`` controlling the level of compression; ``1`` is
   fastest and produces the least compression, ``9`` is slowest and
   produces the most.  The default value is ``6``.

zlib.crc32(string[, value])

   Computes a CRC (Cyclic Redundancy Check)  checksum of *string*. If
   *value* is present, it is used as the starting value of the
   checksum; otherwise, a fixed default value is used.  This allows
   computing a running checksum over the concatenation of several
   input strings.  The algorithm is not cryptographically strong, and
   should not be used for authentication or digital signatures.  Since
   the algorithm is designed for use as a checksum algorithm, it is
   not suitable for use as a general hash algorithm.

   This function always returns an integer object.

   Changed in version 2.6: For consistent cross-platform behavior we
   always return a signed integer. ie: Results in the
   (2**31)...(2**32-1) range will be negative.

zlib.decompress(string[, wbits[, bufsize]])

   Decompresses the data in *string*, returning a string containing
   the uncompressed data.  The *wbits* parameter controls the size of
   the window buffer.  If *bufsize* is given, it is used as the
   initial size of the output buffer.  Raises the ``error`` exception
   if any error occurs.

   The absolute value of *wbits* is the base two logarithm of the size
   of the history buffer (the "window size") used when compressing
   data.  Its absolute value should be between 8 and 15 for the most
   recent versions of the zlib library, larger values resulting in
   better compression at the expense of greater memory usage.  The
   default value is 15.  When *wbits* is negative, the standard
   **gzip** header is suppressed; this is an undocumented feature of
   the zlib library, used for compatibility with **unzip**'s
   compression file format.

   *bufsize* is the initial size of the buffer used to hold
   decompressed data.  If more space is required, the buffer size will
   be increased as needed, so you don't have to get this value exactly
   right; tuning it will only save a few calls to ``malloc``.  The
   default size is 16384.

zlib.decompressobj([wbits])

   Returns a decompression object, to be used for decompressing data
   streams that won't fit into memory at once.  The *wbits* parameter
   controls the size of the window buffer.

Compression objects support the following methods:

Compress.compress(string)

   Compress *string*, returning a string containing compressed data
   for at least part of the data in *string*.  This data should be
   concatenated to the output produced by any preceding calls to the
   ``compress()`` method.  Some input may be kept in internal buffers
   for later processing.

Compress.flush([mode])

   All pending input is processed, and a string containing the
   remaining compressed output is returned.  *mode* can be selected
   from the constants ``Z_SYNC_FLUSH``,  ``Z_FULL_FLUSH``,  or
   ``Z_FINISH``, defaulting to ``Z_FINISH``.  ``Z_SYNC_FLUSH`` and
   ``Z_FULL_FLUSH`` allow compressing further strings of data, while
   ``Z_FINISH`` finishes the compressed stream and  prevents
   compressing any more data.  After calling ``flush()`` with *mode*
   set to ``Z_FINISH``, the ``compress()`` method cannot be called
   again; the only realistic action is to delete the object.

Compress.copy()

   Returns a copy of the compression object.  This can be used to
   efficiently compress a set of data that share a common initial
   prefix.

   New in version 2.5.

Decompression objects support the following methods, and two
attributes:

Decompress.unused_data

   A string which contains any bytes past the end of the compressed
   data. That is, this remains ``""`` until the last byte that
   contains compression data is available.  If the whole string turned
   out to contain compressed data, this is ``""``, the empty string.

   The only way to determine where a string of compressed data ends is
   by actually decompressing it.  This means that when compressed data
   is contained part of a larger file, you can only find the end of it
   by reading data and feeding it followed by some non-empty string
   into a decompression object's ``decompress()`` method until the
   ``unused_data`` attribute is no longer the empty string.

Decompress.unconsumed_tail

   A string that contains any data that was not consumed by the last
   ``decompress()`` call because it exceeded the limit for the
   uncompressed data buffer.  This data has not yet been seen by the
   zlib machinery, so you must feed it (possibly with further data
   concatenated to it) back to a subsequent ``decompress()`` method
   call in order to get correct output.

Decompress.decompress(string[, max_length])

   Decompress *string*, returning a string containing the uncompressed
   data corresponding to at least part of the data in *string*.  This
   data should be concatenated to the output produced by any preceding
   calls to the ``decompress()`` method.  Some of the input data may
   be preserved in internal buffers for later processing.

   If the optional parameter *max_length* is supplied then the return
   value will be no longer than *max_length*. This may mean that not
   all of the compressed input can be processed; and unconsumed data
   will be stored in the attribute ``unconsumed_tail``. This string
   must be passed to a subsequent call to ``decompress()`` if
   decompression is to continue.  If *max_length* is not supplied then
   the whole input is decompressed, and ``unconsumed_tail`` is an
   empty string.

Decompress.flush([length])

   All pending input is processed, and a string containing the
   remaining uncompressed output is returned.  After calling
   ``flush()``, the ``decompress()`` method cannot be called again;
   the only realistic action is to delete the object.

   The optional parameter *length* sets the initial size of the output
   buffer.

Decompress.copy()

   Returns a copy of the decompression object.  This can be used to
   save the state of the decompressor midway through the data stream
   in order to speed up random seeks into the stream at a future
   point.

   New in version 2.5.

See also:

   Module ``gzip``
      Reading and writing **gzip**-format files.

   http://www.zlib.net
      The zlib library home page.

   http://www.zlib.net/manual.html
      The zlib manual explains  the semantics and usage of the
      library's many functions.
