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# 13.1. `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 a 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.

  Always returns an unsigned 32-bit integer.

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.

`zlib.``compress`(data[, level])

  Compresses the bytes in data, returning a bytes object containing compressed data. level is an integer from `1` to `9` controlling the level of compression; `1` is fastest and produces the least compression, `9` is slowest and produces the most. 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 `1` to `9`. A value of `1` is fastest and produces the least compression, while a value of `9` is slowest and produces the most. The default value is `6`.

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

  wbits is the base two logarithm of the size of the window buffer. This should be an integer from `8` to `15`. Higher values give better compression, but use more memory.

  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.

  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 method, wbits, memlevel, strategy and zdict parameters.

`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.

  Always returns an unsigned 32-bit integer.

  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 32-bit binary representation regardless of sign.

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

  Decompresses the bytes in data, returning a bytes object containing the uncompressed data. The wbits parameter controls the size of the window buffer, 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 absolute value of wbits is the base two logarithm of the size of the history buffer (the “window size”) used when compressing data. Its absolute value should be between 8 and 15 for the most recent versions of the zlib library, larger values resulting in better compression at the expense of greater memory usage. When decompressing a stream, wbits must not be smaller than the size originally used to compress the stream; using a too-small value will result in an exception. The default value is therefore the highest value, 15. When wbits is negative, the standard gzip header is suppressed.

  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 window buffer.

  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 `""` 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 supplied 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.

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