12.1. "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 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.)  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.

   This function always returns an integer object.

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.

Changed in version 2.6: The return value is in the range [-2**31,
2**31-1] regardless of platform.  In older versions the value is
signed on some platforms and unsigned on others.

Changed in version 3.0: The return value is unsigned and in the range
[0, 2**32-1] regardless of platform.

zlib.compress(string[, level])

   Compresses the data in *string*, returning a string contained
   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[, method[, wbits[, memlevel[, strategy]]]]])

   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" or "-1", 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
   "-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. The default is 15.

   * +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.

   *memlevel* controls the amount of memory used for internal
   compression state. Valid values range from "1" to "9". Higher
   values using more memory, but are faster and produce smaller
   output. The default is 8.

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

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.

   This function always returns an integer object.

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.

Changed in version 2.6: The return value is in the range [-2**31,
2**31-1] regardless of platform.  In older versions the value would be
signed on some platforms and unsigned on others.

Changed in version 3.0: The return value is unsigned and in the range
[0, 2**32-1] regardless of platform.

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

   Decompresses the data in *string*, returning a string containing
   the uncompressed data.  The *wbits* parameter depends on the format
   of *string*, 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])

   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().

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 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 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.
