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.php for authoritative information.
The available exception and functions in this module are:
exceptionerror
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, 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.
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:
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.
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.
