12.1. 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 an 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.

This function always returns an integer object.

zlib.compress(string[, level])

Compresses the data in string, returning a string contained compressed data. level is an integer from 0 to 9 controlling the level of compression; 1 is fastest and produces the least compression, 9 is slowest and produces the most. 0 is no compression. The default value is 6. Raises the error exception if any error occurs.

zlib.compressobj([level[, method[, wbits[, memlevel[, strategy]]]]])

Returns a compression object, to be used for compressing data streams that won’t fit into memory at once. level is an integer from 0 to 9 or -1, controlling the level of compression; 1 is fastest and produces the least compression, 9 is slowest and produces the most. 0 is no compression. The default value is -1 (Z_DEFAULT_COMPRESSION). Z_DEFAULT_COMPRESSION represents a default compromise between speed and compression (currently equivalent to level 6).

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

The wbits argument controls the size of the history buffer (or the “window size”) used when compressing data, and whether a header and trailer is included in the output. It can take several ranges of values. The default is 15.

• +9 to +15: The base-two logarithm of the window size, which therefore ranges between 512 and 32768. Larger values produce better compression at the expense of greater memory usage. The resulting output will include a zlib-specific header and trailer.
• −9 to −15: Uses the absolute value of wbits as the window size logarithm, while producing a raw output stream with no header or trailing checksum.
• +25 to +31 = 16 + (9 to 15): Uses the low 4 bits of the value as the window size logarithm, while including a basic gzip header and trailing checksum in the output.

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. The default is 8.

strategy is used to tune the compression algorithm. Possible values are Z_DEFAULT_STRATEGY, Z_FILTERED, and Z_HUFFMAN_ONLY. The default is Z_DEFAULT_STRATEGY.

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.

This function always returns an integer object.

zlib.decompress(string[, wbits[, bufsize]])

Decompresses the data in string, returning a string containing the uncompressed data. The wbits parameter depends on the format of string, 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 wbits parameter controls the size of the history buffer (or “window size”), and what header and trailer format is expected. It is similar to the parameter for compressobj(), but accepts more ranges of values:

• +8 to +15: The base-two logarithm of the window size. The input must include a zlib header and trailer.
• 0: Automatically determine the window size from the zlib header. Only supported since zlib 1.2.3.5.
• −8 to −15: Uses the absolute value of wbits as the window size logarithm. The input must be a raw stream with no header or trailer.
• +24 to +31 = 16 + (8 to 15): Uses the low 4 bits of the value as the window size logarithm. The input must include a gzip header and trailer.
• +40 to +47 = 32 + (8 to 15): Uses the low 4 bits of the value as the window size logarithm, and automatically accepts either the zlib or gzip format.

When decompressing a stream, the window size must not be smaller than the size originally used to compress the stream; using a too-small value may result in an error exception. The default wbits value is 15, which corresponds to the largest window size and requires a zlib header and trailer to be included.

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])

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 history buffer (or the “window size”), and what header and trailer format is expected. It has the same meaning as described for decompress().

Compression objects support the following methods:

Compress.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.

Compress.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, 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.

New in version 2.5.

Decompression objects support the following methods, and two attributes:

Decompress.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.

Decompress.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.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 non-zero 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.

Decompress.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.

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.

New in version 2.5.