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zlib – zlib compression & decompression

This module implements a subset of the corresponding CPython module, as described below. For more information, refer to the original CPython documentation: zlib.

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


Prefer to use 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.


  • From MicroPython v1.21 onwards, this module may not be present by default on all MicroPython firmware as it duplicates functionality available in the deflate module.

  • A copy of this module can be installed (or frozen) from micropython-lib (source). See Package management for more information. This documentation describes that module.

  • Requires the built-in deflate module (available since MicroPython v1.21)

  • Compression support will only be available if compression support is enabled in the built-in deflate module.


zlib.decompress(data, wbits=15, /)

Decompresses data into a bytes object.

The wbits parameter works the same way as for 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 zlib.compress(), see the CPython documentation for zlib for more information about the wbits parameter. As for zlib.compress(), MicroPython also supports smaller window sizes than CPython. See more MicroPython-specific details in the deflate module documentation.

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

zlib.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 CPython documentation for 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 MicroPython-specific details in the deflate module documentation.