# 7.12 `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.cdrom.com/pub/infozip/zlib/. Version 1.1.3 is the most recent version as of April 1999; use a later version if one is available. There are known incompatibilities between the Python module and earlier versions of the zlib library.

The documentation for this module is woefully out of date. In some cases, the doc strings have been updated more recently. In other cases, they are both stale.

The available exception and functions in this module are:

`error`
  Exception raised on compression and decompression errors.

`adler32` (string[, value])
  Computes a Adler-32 checksum of string. (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 input strings. The algorithm is not cryptographically strong, and should not be used for authentication or digital signatures.

`compress` (string[, level])
  Compresses the data in string, returning a string contained 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.

`compressobj` ([level])
  Returns a compression object, to be used for compressing data streams that won't fit into memory at once. 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`.

`crc32` (string[, value])
  Computes a CRC (Cyclic Redundancy Check) checksum of string. 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 input strings. The algorithm is not cryptographically strong, and should not be used for authentication or digital signatures.

`decompress` (string[, wbits[, buffsize]])
  Decompresses the data in string, returning a string containing the uncompressed data. The wbits parameter controls the size of the window buffer. If buffsize is given, it is used as the initial size of the output buffer. Raises the `error` exception if any error occurs.

`decompressobj` ([wbits])
  Returns a compression 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.

Compression objects support the following methods:

`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.

`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 and are used to allow partial error recovery on decompression, 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.

Decompression objects support the following methods:

`decompress` (string)
  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.

`flush` ()
  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.

See Also:

Module `gzip` (reading and writing gzip-format files)

The zlib library home page is located at http://www.cdrom.com/pub/infozip/zlib/.