# 7.15 `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.gzip.org/zlib/. 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.

The available exception and functions in this module are:

exception `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. Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm.

`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. Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm.

`decompress`( | string[, wbits[, bufsize]])

  Decompresses the data in string, returning a string containing the uncompressed data. The wbits parameter controls the size of the window buffer. 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. The default value is 15. When wbits is negative, the standard gzip header is suppressed; this is an undocumented feature of the zlib library, used for compatibility with unzip's compression file format.

  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.

`decompressobj`( | [wbits])

  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.

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, and two attributes:

`unused_data`
  A string 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 string turned out to contain compressed data, this is `""`, the empty string.

  The only way to determine where a string of compressed data ends is by actually decompressing it. This means that when compressed data is contained part of a larger file, you can only find the end of it by reading data and feeding it followed by some non-empty string into a decompression object's `decompress` method until the `unused_data` attribute is no longer the empty string.

`unconsumed_tail`
  A string 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`( | string[, max_length])

  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.

  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 string 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 an empty string.

`flush`( | [length])

  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.

  The optional parameter length sets the initial size of the output buffer.

See Also:

Module `gzip`:
  Reading and writing gzip-format files.

http://www.gzip.org/zlib/
  The zlib library home page.