zlib.rst

:mod:`zlib` -- zlib compression & decompression

.. module:: zlib
   :synopsis: zlib compression & decompression

|see_cpython_module| :mod:`python:zlib`.

This module allows compression and decompression of binary data with the DEFLATE algorithm (commonly used in the zlib library and gzip archiver).

Note

Prefer to use :class:`deflate.DeflateIO` instead of the functions in this module as it provides a streaming interface to compression and decompression which is convenient and more memory efficient when working with reading or writing compressed data to a file, socket, or stream.

Availability:

• From MicroPython v1.21 onwards, this module may not be present by default on all MicroPython firmware as it duplicates functionality available in the :mod:`deflate <deflate>` module.
• A copy of this module can be installed (or frozen) from :term:`micropython-lib` (source). See :ref:`packages` for more information. This documentation describes that module.
• Requires the built-in :mod:`deflate <deflate>` module (available since MicroPython v1.21)
• Compression support will only be available if compression support is enabled in the built-in :mod:`deflate <deflate>` module.

Functions

.. function:: decompress(data, wbits=15, /)

   Decompresses *data* into a bytes object.

   The *wbits* parameter works the same way as for :meth:`zlib.compress`
   with the following additional valid values:

   * ``0``: Automatically determine the window size from the zlib header
     (*data* must be in zlib format).
   * ``35`` to ``47``: Auto-detect either the zlib or gzip format.

   As for :meth:`zlib.compress`, see the :mod:`CPython documentation for zlib <python:zlib>`
   for more information about the *wbits* parameter. As for :meth:`zlib.compress`,
   MicroPython also supports smaller window sizes than CPython. See more
   :ref:`MicroPython-specific details <deflate_wbits>` in the
   :mod:`deflate <deflate>` module documentation.

   If the data to be decompressed requires a larger window size, it will
   fail during decompression.

.. function:: compress(data, wbits=15, /)

   Compresses *data* into a bytes object.

   *wbits* allows you to configure the DEFLATE dictionary window size and the
   output format. The window size allows you to trade-off memory usage for
   compression level. A larger window size will allow the compressor to
   reference fragments further back in the input. The output formats are "raw"
   DEFLATE (no header/footer), zlib, and gzip, where the latter two
   include a header and checksum.

   The low four bits of the absolute value of *wbits* set the base-2 logarithm of
   the DEFLATE dictionary window size. So for example, ``wbits=10``,
   ``wbits=-10``, and ``wbits=26`` all set the window size to 1024 bytes. Valid
   window sizes are ``5`` to ``15`` inclusive (corresponding to 32 to 32k bytes).

   Negative values of *wbits* between ``-5`` and ``-15`` correspond to "raw"
   output mode, positive values between ``5`` and ``15`` correspond to zlib
   output mode, and positive values between ``21`` and ``31`` correspond to
   gzip output mode.

   See the :mod:`CPython documentation for zlib <python:zlib>` for more
   information about the *wbits* parameter. Note that MicroPython allows
   for smaller window sizes, which is useful when memory is constrained while
   still achieving a reasonable level of compression. It also speeds up
   the compressor. See more :ref:`MicroPython-specific details <deflate_wbits>`
   in the :mod:`deflate <deflate>` module documentation.