zlib — Compression compatible withgzip¶

For applications that require data compression, the functions in this moduleallow compression and decompression, using the zlib library. The zlib libraryhas its own home page athttp://www.zlib.net. There are knownincompatibilities between the Python module and versions of the zlib libraryearlier than 1.1.3; 1.1.3 has a security vulnerability, so we recommend using1.1.4 or later.

zlib’s functions have many options and often need to be used in a particularorder. This documentation doesn’t attempt to cover all of the permutations;consult the zlib manual athttp://www.zlib.net/manual.html for authoritativeinformation.

For reading and writing.gz files see thegzip module.

The available exception and functions in this module are:

exceptionzlib.error¶

Exception raised on compression and decompression errors.

zlib.adler32(data[,value])¶

Computes an Adler-32 checksum ofdata. (An Adler-32 checksum is almost asreliable as a CRC32 but can be computed much more quickly.) The resultis an unsigned 32-bit integer. Ifvalue is present, it is used asthe starting value of the checksum; otherwise, a default value of 1is used. Passing invalue allows computing a running checksum over theconcatenation of several inputs. The algorithm is not cryptographicallystrong, and should not be used for authentication or digital signatures. Sincethe algorithm is designed for use as a checksum algorithm, it is not suitablefor use as a general hash algorithm.

Changed in version 3.0:Always returns an unsigned value.To generate the same numeric value across all Python versions andplatforms, useadler32(data)&0xffffffff.

zlib.compress(data,level=-1)¶

Compresses the bytes indata, returning a bytes object containing compressed data.level is an integer from0 to9 or-1 controlling the level of compression;1 (Z_BEST_SPEED) is fastest and produces the least compression,9 (Z_BEST_COMPRESSION)is slowest and produces the most.0 (Z_NO_COMPRESSION) is no compression.The default value is-1 (Z_DEFAULT_COMPRESSION). Z_DEFAULT_COMPRESSION represents a defaultcompromise between speed and compression (currently equivalent to level 6).Raises theerror exception if any error occurs.

Changed in version 3.6:level can now be used as a keyword parameter.

zlib.compressobj(level=-1,method=DEFLATED,wbits=MAX_WBITS,memLevel=DEF_MEM_LEVEL,strategy=Z_DEFAULT_STRATEGY[,zdict])¶

Returns a compression object, to be used for compressing data streams that won’tfit into memory at once.

level is the compression level – an integer from0 to9 or-1.A value of1 (Z_BEST_SPEED) is fastest and produces the least compression,while a value of9 (Z_BEST_COMPRESSION) is slowest and produces the most.0 (Z_NO_COMPRESSION) 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 isDEFLATED.

Thewbits argument controls the size of the history buffer (or the“window size”) used when compressing data, and whether a header andtrailer is included in the output. It can take several ranges of values,defaulting to15 (MAX_WBITS):

• +9 to +15: The base-two logarithm of the window size, whichtherefore ranges between 512 and 32768. Larger values producebetter compression at the expense of greater memory usage. Theresulting output will include a zlib-specific header and trailer.

• −9 to −15: Uses the absolute value ofwbits as thewindow size logarithm, while producing a raw output stream with noheader or trailing checksum.

• +25 to +31 = 16 + (9 to 15): Uses the low 4 bits of the value as thewindow size logarithm, while including a basicgzip headerand trailing checksum in the output.

ThememLevel argument controls the amount of memory used for theinternal compression state. Valid values range from1 to9.Higher values use more memory, but are faster and produce smaller output.

strategy is used to tune the compression algorithm. Possible values areZ_DEFAULT_STRATEGY,Z_FILTERED,Z_HUFFMAN_ONLY,Z_RLE (zlib 1.2.0.1) andZ_FIXED (zlib 1.2.2.2).

zdict is a predefined compression dictionary. This is a sequence of bytes(such as abytes object) containing subsequences that are expectedto occur frequently in the data that is to be compressed. Those subsequencesthat are expected to be most common should come at the end of the dictionary.

Changed in version 3.3:Added thezdict parameter and keyword argument support.

zlib.crc32(data[,value])¶

Computes a CRC (Cyclic Redundancy Check) checksum ofdata. Theresult is an unsigned 32-bit integer. Ifvalue is present, it is usedas the starting value of the checksum; otherwise, a default value of 0is used. Passing invalue allows computing a running checksum over theconcatenation of several inputs. The algorithm is not cryptographicallystrong, and should not be used for authentication or digital signatures. Sincethe algorithm is designed for use as a checksum algorithm, it is not suitablefor use as a general hash algorithm.

Changed in version 3.0:Always returns an unsigned value.To generate the same numeric value across all Python versions andplatforms, usecrc32(data)&0xffffffff.

zlib.decompress(data,wbits=MAX_WBITS,bufsize=DEF_BUF_SIZE)¶

Decompresses the bytes indata, returning a bytes object containing theuncompressed data. Thewbits parameter depends onthe format ofdata, and is discussed further below.Ifbufsize is given, it is used as the initial size of the outputbuffer. Raises theerror exception if any error occurs.

Thewbits 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 forcompressobj(), but acceptsmore ranges of values:

• +8 to +15: The base-two logarithm of the window size. The inputmust 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 ofwbits as the window sizelogarithm. 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 asthe window size logarithm. The input must include a gzip header andtrailer.

• +40 to +47 = 32 + (8 to 15): Uses the low 4 bits of the value asthe window size logarithm, and automatically accepts eitherthe zlib or gzip format.

When decompressing a stream, the window size must not be smallerthan the size originally used to compress the stream; using a too-smallvalue may result in anerror exception. The defaultwbits valuecorresponds to the largest window size and requires a zlib header andtrailer to be included.

bufsize is the initial size of the buffer used to hold decompressed data. Ifmore space is required, the buffer size will be increased as needed, so youdon’t have to get this value exactly right; tuning it will only save a few callstomalloc().

Changed in version 3.6:wbits andbufsize can be used as keyword arguments.

zlib.decompressobj(wbits=MAX_WBITS[,zdict])¶

Returns a decompression object, to be used for decompressing data streams thatwon’t fit into memory at once.

Thewbits parameter controls the size of the history buffer (or the“window size”), and what header and trailer format is expected. It hasthe same meaning asdescribed for decompress().

Thezdict parameter specifies a predefined compression dictionary. Ifprovided, this must be the same dictionary as was used by the compressor thatproduced the data that is to be decompressed.

Note

Ifzdict is a mutable object (such as abytearray), you must notmodify its contents between the call todecompressobj() and the firstcall to the decompressor’sdecompress() method.

Changed in version 3.3:Added thezdict parameter.

Compression objects support the following methods:

Compress.compress(data)¶

Compressdata, returning a bytes object containing compressed data for at leastpart of the data indata. This data should be concatenated to the outputproduced by any preceding calls to thecompress() method. Some input maybe kept in internal buffers for later processing.

Compress.flush([mode])¶

All pending input is processed, and a bytes object containing the remaining compressedoutput is returned.mode can be selected from the constantsZ_NO_FLUSH,Z_PARTIAL_FLUSH,Z_SYNC_FLUSH,Z_FULL_FLUSH,Z_BLOCK (zlib 1.2.3.4), orZ_FINISH,defaulting toZ_FINISH. ExceptZ_FINISH, all constantsallow compressing further bytestrings of data, whileZ_FINISH finishes thecompressed stream and prevents compressing any more data. After callingflush()withmode set toZ_FINISH, thecompress() 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 efficientlycompress 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 remainsb"" until the last byte that contains compression data isavailable. If the whole bytestring turned out to contain compressed data, this isb"", an empty bytes object.

Decompress.unconsumed_tail¶

A bytes object that contains any data that was not consumed by the lastdecompress() call because it exceeded the limit for the uncompressed databuffer. This data has not yet been seen by the zlib machinery, so you must feedit (possibly with further data concatenated to it) back to a subsequentdecompress() method call in order to get correct output.

Decompress.eof¶

A boolean indicating whether the end of the compressed data stream has beenreached.

This makes it possible to distinguish between a properly-formed compressedstream, and an incomplete or truncated one.

New in version 3.3.

Decompress.decompress(data,max_length=0)¶

Decompressdata, returning a bytes object containing the uncompressed datacorresponding to at least part of the data instring. This data should beconcatenated to the output produced by any preceding calls to thedecompress() method. Some of the input data may be preserved in internalbuffers for later processing.

If the optional parametermax_length is non-zero then the return value will beno longer thanmax_length. This may mean that not all of the compressed inputcan be processed; and unconsumed data will be stored in the attributeunconsumed_tail. This bytestring must be passed to a subsequent call todecompress() if decompression is to continue. Ifmax_length is zerothen the whole input is decompressed, andunconsumed_tail is empty.

Changed in version 3.6:max_length can be used as a keyword argument.

Decompress.flush([length])¶

All pending input is processed, and a bytes object containing the remaininguncompressed output is returned. After callingflush(), thedecompress() method cannot be called again; the only realistic action isto delete the object.

The optional parameterlength sets the initial size of the output buffer.

Decompress.copy()¶

Returns a copy of the decompression object. This can be used to save the stateof the decompressor midway through the data stream in order to speed up randomseeks into the stream at a future point.

Information about the version of the zlib library in use is available throughthe 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, whichis available asZLIB_RUNTIME_VERSION.

zlib.ZLIB_RUNTIME_VERSION¶

The version string of the zlib library actually loaded by the interpreter.

New in version 3.3.