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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.gzip.org/zlib/. Version
1.1.3 is the most recent version as of September 2000; use a later version if one is
available. There are known incompatibilities between the Python module and earlier versions of
the zlib library.
The available exception and functions in this module are:
- exception error
- 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.
-
- 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.
-
- 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.
-
- 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, 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.
-
- 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 and are used
to allow partial error recovery on decompression, 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.
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.
-
- [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.
-
- 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.
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