13.1. "zlib" — Compression compatible with **gzip**
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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 zlib.error

   Exception raised on compression and decompression errors.

zlib.adler32(data[, value])

   Computes an Adler-32 checksum of *data*.  (An Adler-32 checksum is
   almost as reliable as a CRC32 but can be computed much more
   quickly.)  The result is an unsigned 32-bit integer.  If *value* is
   present, it is used as the starting value of the checksum;
   otherwise, a default value of 1 is used.  Passing in *value* 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.

   Changed in version 3.0: Always returns an unsigned value. To
   generate the same numeric value across all Python versions and
   platforms, use "adler32(data) & 0xffffffff".

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=-1, method=DEFLATED, wbits=15, memLevel=8, strategy=Z_DEFAULT_STRATEGY[, zdict])

   Returns a compression object, to be used for compressing data
   streams that won’t fit into memory at once.

   *level* is the compression level – an integer from "0" to "9" or
   "-1". A value of "1" is fastest and produces the least compression,
   while a value of "9" is slowest and produces the most. "0" is no
   compression. The default value is "-1" (Z_DEFAULT_COMPRESSION).
   Z_DEFAULT_COMPRESSION represents a default compromise between speed
   and compression (currently equivalent to level 6).

   *method* is the compression algorithm. Currently, the only
   supported value is "DEFLATED".

   The *wbits* argument controls the size of the history buffer (or
   the “window size”) used when compressing data, and whether a header
   and trailer is included in the output.  It can take several ranges
   of values:

   * +9 to +15: The base-two logarithm of the window size, which
     therefore ranges between 512 and 32768.  Larger values produce
     better compression at the expense of greater memory usage.  The
     resulting output will include a zlib-specific header and trailer.

   * −9 to −15: Uses the absolute value of *wbits* as the window
     size logarithm, while producing a raw output stream with no
     header or trailing checksum.

   * +25 to +31 = 16 + (9 to 15): Uses the low 4 bits of the value
     as the window size logarithm, while including a basic **gzip**
     header and trailing checksum in the output.

   The *memLevel* argument controls the amount of memory used for the
   internal compression state. Valid values range from "1" to "9".
   Higher values use more memory, but are faster and produce smaller
   output.

   *strategy* is used to tune the compression algorithm. Possible
   values are "Z_DEFAULT_STRATEGY", "Z_FILTERED", and
   "Z_HUFFMAN_ONLY".

   *zdict* is a predefined compression dictionary. This is a sequence
   of bytes (such as a "bytes" object) containing subsequences that
   are expected to occur frequently in the data that is to be
   compressed. Those subsequences that are expected to be most common
   should come at the end of the dictionary.

   Changed in version 3.3: Added the *zdict* parameter and keyword
   argument support.

zlib.crc32(data[, value])

   Computes a CRC (Cyclic Redundancy Check) checksum of *data*. The
   result is an unsigned 32-bit integer. If *value* is present, it is
   used as the starting value of the checksum; otherwise, a default
   value of 0 is used.  Passing in *value* 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.

   Changed in version 3.0: Always returns an unsigned value. To
   generate the same numeric value across all Python versions and
   platforms, use "crc32(data) & 0xffffffff".

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

   Decompresses the bytes in *data*, returning a bytes object
   containing the uncompressed data.  The *wbits* parameter depends on
   the format of *data*, 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 *wbits* parameter controls the size of the history buffer (or
   “window size”), and what header and trailer format is expected. It
   is similar to the parameter for "compressobj()", but accepts more
   ranges of values:

   * +8 to +15: The base-two logarithm of the window size.  The
     input must include a zlib header and trailer.

   * 0: Automatically determine the window size from the zlib
     header. Only supported since zlib 1.2.3.5.

   * −8 to −15: Uses the absolute value of *wbits* as the window
     size logarithm.  The input must be a raw stream with no header or
     trailer.

   * +24 to +31 = 16 + (8 to 15): Uses the low 4 bits of the value
     as the window size logarithm.  The input must include a gzip
     header and trailer.

   * +40 to +47 = 32 + (8 to 15): Uses the low 4 bits of the value
     as the window size logarithm, and automatically accepts either
     the zlib or gzip format.

   When decompressing a stream, the window size must not be smaller
   than the size originally used to compress the stream; using a too-
   small value may result in an "error" exception. The default *wbits*
   value is 15, which corresponds to the largest window size and
   requires a zlib header and trailer to be included.

   *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=15[, zdict])

   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 history buffer (or
   the “window size”), and what header and trailer format is expected.
   It has the same meaning as described for decompress().

   The *zdict* parameter specifies a predefined compression
   dictionary. If provided, this must be the same dictionary as was
   used by the compressor that produced the data that is to be
   decompressed.

   Note: If *zdict* is a mutable object (such as a "bytearray"), you
     must not modify its contents between the call to
     "decompressobj()" and the first call to the decompressor’s
     "decompress()" method.

   Changed in version 3.3: Added the *zdict* parameter.

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 attributes:

Decompress.unused_data

   A bytes object which contains any bytes past the end of the
   compressed data. That is, this remains "b""" 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.

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.eof

   A boolean indicating whether the end of the compressed data stream
   has been reached.

   This makes it possible to distinguish between a properly-formed
   compressed stream, and an incomplete or truncated one.

   New in version 3.3.

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 non-zero 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.

Information about the version of the zlib library in use is available
through the following constants:

zlib.ZLIB_VERSION

   The version string of the zlib library that was used for building
   the module. This may be different from the zlib library actually
   used at runtime, which is available as "ZLIB_RUNTIME_VERSION".

zlib.ZLIB_RUNTIME_VERSION

   The version string of the zlib library actually loaded by the
   interpreter.

   New in version 3.3.

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.
