
``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.

The available exception and functions in this module are:

exception exception zlib.error

   Exception raised on compression and decompression errors.

zlib.adler32(data[, value])

   Computes a Adler-32 checksum of *data*.  (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 inputs.  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.

   Always returns an unsigned 32-bit integer.

Note: To generate the same numeric value across all Python versions and
  platforms use adler32(data) & 0xffffffff.  If you are only using the
  checksum in packed binary format this is not necessary as the return
  value is the correct 32bit binary representation regardless of sign.

zlib.compress(data[, level])

   Compresses the bytes in *data*, returning a bytes object containing
   compressed data.  *level* is an integer from ``0`` to ``9``
   controlling the level of compression; ``1`` is fastest and produces
   the least compression, ``9`` is slowest and produces the most.
   ``0`` is no compression. 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 ``0`` to ``9`` controlling the level of compression; ``1`` is
   fastest and produces the least compression, ``9`` is slowest and
   produces the most.  ``0`` is no compression.  The default value is
   ``6``.

zlib.crc32(data[, value])

   Computes a CRC (Cyclic Redundancy Check)  checksum of *data*. 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
   inputs.  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.

   Always returns an unsigned 32-bit integer.

Note: To generate the same numeric value across all Python versions and
  platforms use crc32(data) & 0xffffffff.  If you are only using the
  checksum in packed binary format this is not necessary as the return
  value is the correct 32bit binary representation regardless of sign.

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

   Decompresses the bytes in *data*, returning a bytes object
   containing the uncompressed data.  The *wbits* parameter controls
   the size of the window buffer, and is discussed further below. 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.  When
   decompressing a stream, *wbits* must not be smaller than the size
   originally used to compress the stream; using a too-small value
   will result in an exception. The default value is therefore the
   highest value, 15.  When *wbits* is negative, the standard **gzip**
   header is suppressed.

   *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(data)

   Compress *data*, returning a bytes object containing compressed
   data for at least part of the data in *data*.  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 bytes object 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 bytestrings 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.

Decompression objects support the following methods, and two
attributes:

Decompress.unused_data

   A bytes object 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
   bytestring turned out to contain compressed data, this is ``b""``,
   an empty bytes object.

   The only way to determine where a bytestring 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 bytestring into a decompression object's ``decompress()``
   method until the ``unused_data`` attribute is no longer empty.

Decompress.unconsumed_tail

   A bytes object 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(data[, max_length])

   Decompress *data*, returning a bytes object 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
   bytestring 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
   empty.

Decompress.flush([length])

   All pending input is processed, and a bytes object 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.

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.
