Doc/library/zlib.rst

:mod:`zlib` --- Compression compatible with :program:`gzip`
===========================================================

.. module:: zlib
   :synopsis: Low-level interface to compression and decompression routines
              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 :mod:`gzip` module.

The available exception and functions in this module are:


.. exception:: error

   Exception raised on compression and decompression errors.


.. function:: 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.

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


.. function:: 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 :exc:`error` exception if any error occurs.


.. function:: 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 :program:`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 :class:`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.

   .. versionchanged:: 3.3
      Added the *zdict* parameter and keyword argument support.


.. function:: crc32(data[, value])

   .. index::
      single: Cyclic Redundancy Check
      single: checksum; Cyclic Redundancy Check

   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.

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


.. function:: 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 :exc:`error` exception if any error occurs.

   .. _decompress-wbits:

   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 :func:`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 :exc:`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 :c:func:`malloc`.  The default size is 16384.


.. function:: 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() <#decompress-wbits>`__.

   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 :class:`bytearray`), you must not
      modify its contents between the call to :func:`decompressobj` and the first
      call to the decompressor's ``decompress()`` method.

   .. versionchanged:: 3.3
      Added the *zdict* parameter.


Compression objects support the following methods:


.. method:: 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 :meth:`compress` method.  Some input may
   be kept in internal buffers for later processing.


.. method:: 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
   :const:`Z_SYNC_FLUSH`,  :const:`Z_FULL_FLUSH`,  or  :const:`Z_FINISH`,
   defaulting to :const:`Z_FINISH`.  :const:`Z_SYNC_FLUSH` and
   :const:`Z_FULL_FLUSH` allow compressing further bytestrings of data, while
   :const:`Z_FINISH` finishes the compressed stream and  prevents compressing any
   more data.  After calling :meth:`flush` with *mode* set to :const:`Z_FINISH`,
   the :meth:`compress` method cannot be called again; the only realistic action is
   to delete the object.


.. method:: 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:


.. attribute:: 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.


.. attribute:: Decompress.unconsumed_tail

   A bytes object that contains any data that was not consumed by the last
   :meth:`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
   :meth:`decompress` method call in order to get correct output.


.. attribute:: 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.

   .. versionadded:: 3.3


.. method:: 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
   :meth:`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
   :attr:`unconsumed_tail`. This bytestring must be passed to a subsequent call to
   :meth:`decompress` if decompression is to continue.  If *max_length* is not
   supplied then the whole input is decompressed, and :attr:`unconsumed_tail` is
   empty.


.. method:: Decompress.flush([length])

   All pending input is processed, and a bytes object containing the remaining
   uncompressed output is returned.  After calling :meth:`flush`, the
   :meth:`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.


.. method:: 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:


.. data:: 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 :const:`ZLIB_RUNTIME_VERSION`.


.. data:: ZLIB_RUNTIME_VERSION

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

   .. versionadded:: 3.3


.. seealso::

   Module :mod:`gzip`
      Reading and writing :program:`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.