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. For
other archive formats, see the bz2, zipfile, and
tarfile modules.
The available exception and functions in this module are:
-
exception zlib.error
- Exception raised on compression and decompression errors.
-
zlib.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.
This function always returns an integer object.
Changed in version 2.6: For consistent cross-platform behavior we always return a signed integer.
ie: Results in the (2**31)...(2**32-1) range will be negative.
-
zlib.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.
-
zlib.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.
-
zlib.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.
This function always returns an integer object.
Changed in version 2.6: For consistent cross-platform behavior we always return a signed integer.
ie: Results in the (2**31)...(2**32-1) range will be negative.
-
zlib.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.
-
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
window buffer.
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 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.
-
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.
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