zlib
Larky module containing functions working with zlib.
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. 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 here for authoritative information.
For reading and writing .gz files see the Larky gzip module.
The available functions in this module are:
zlib.adler32(data[, value])zlib.compress(data, /, level=-1)zlib.compressobj(level=-1, method=DEFLATED, wbits=MAX_WBITS, memLevel=DEF\_MEM\_LEVEL, strategy=Z_DEFAULT_STRATEGY[, zdict])zlib.crc32(data[, value])zlib.decompress(data, /, wbits=MAX_WBITS, bufsize=DEF_BUF_SIZE)zlib.decompressobj(wbits=MAX_WBITS[, zdict])
Compression objects support the following methods:
Compress.compress(data)Compress.flush([mode])Compress.copy()
Decompression objects support the following methods and attributes:
Decompress.unused_dataDecompress.unconsumed_tailDecompress.eofDecompress.decompress(data, max_length=0)Decompress.flush([length])Decompress.copy()
Information about the version of the zlib library in use is available through the following constants:
zlib.ZLIB_VERSIONzlib.ZLIB_RUNTIME_VERSION
Similar to zlib
zlib.adler32(data, value=1)
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.). The result is an unsigned 32-bit integer. If value is present, it is used as the starting value of the checksum; otherwise, a default value of 1 is used. Passing in value 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.
Parameters:
data – starting value of the checksum.
Returns: the returned checksum is an integer.
zlib.compress(data, level=6)
Compresses the bytes in data, returning a bytes object containing compressed data. level is an integer from 0 to 9 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 default compromise between speed and compression (currently equivalent to level 6). Raises the error exception if any error occurs.
Parameters:
data – bytes to compress.
level – level of compression.
Returns: bytes object containing compressed data.
zlib.compressobj(level=6, method=8, wbits=15, memLevel=0, strategy=0, zdict=None)
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 0 to 9 or -1. A value of 1 (Z_BEST_SPEED) is fastest and produces the least compression, while a value of 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 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, defaulting to 15 (MAX_WBITS):
+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.
The memLevel argument controls the amount of memory used for the internal compression state. Valid values range from 1 to 9. Higher values use 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, Z_HUFFMAN_ONLY, Z_RLE (zlib 1.2.0.1) and Z_FIXED (zlib 1.2.2.2). 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.
Parameters:
level – compression level.
method – compression algorithm.
wbits – size of the history buffer.
memLevel – amount of memory used for the internal compression state.
strategy – is used to tune the compression algorithm.
zdict – predefined compression dictionary.
Returns: - compression object.
zlib.crc32(data, value=0)
Computes a CRC (Cyclic Redundancy Check) checksum of data. The result is an unsigned 32-bit integer. If value is present, it is used as the starting value of the checksum; otherwise, a default value of 0 is used. Passing in value 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.
Parameters:
data – starting value of the checksum.
Returns: the returned checksum is an integer.
zlib.decompress(data, wbits=0, bufsize=16384)
Decompresses the bytes in data, returning a bytes object containing the uncompressed data. The wbits parameter depends on the format of data, 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 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().
Parameters:
data – data in bytes to decompress.
wbits – controls the size of the history buffer.
bufsize – initial size of the output buffer.
Returns: decompressed bytes.
zlib.decompressobj(wbits=15, zdict=None)
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(). 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.
Parameters:
wbits – controls the size of the history buffer.
zdict – a predefined compression dictionary.
Returns: decompression object.
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