zlib
— 压缩兼容
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
文件见
gzip
模块。
The available exception and functions in this module are:
zlib.
error
¶
Exception raised on compression and decompression errors.
zlib.
adler32
(
data
[
,
值
]
)
¶
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.) 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 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.
This function always returns an integer object.
注意
To generate the same numeric value across all Python versions and platforms use adler32(data) & 0xffffffff. If you are only using the checksum in packed binary format this is not necessary as the return value is the correct 32bit binary representation regardless of sign.
2.6 版改变： The return value is in the range [-2**31, 2**31-1] regardless of platform. In older versions the value is signed on some platforms and unsigned on others.
3.0 版改变： The return value is unsigned and in the range [0, 2**32-1] regardless of platform.
zlib.
compress
(
string
[
,
level
]
)
¶
Compresses the data in
string
, returning a string contained compressed data.
level
is an integer from
0
to
9
controlling the level of compression;
1
is fastest and produces the least compression,
9
is slowest and produces the most.
0
is no compression. The default value is
6
. Raises the
error
exception if any error occurs.
zlib.
compressobj
(
[
level
[
,
方法
[
,
wbits
[
,
memlevel
[
,
strategy
]
]
]
]
]
)
¶
Returns a compression object, to be used for compressing data streams that won’t fit into memory at once.
level
is an integer from
0
to
9
or
-1
, controlling the level of compression;
1
is fastest and produces the least compression,
9
is slowest and produces the most.
0
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).
方法
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. The default is 15.
+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.
memlevel
controls the amount of memory used for internal compression state. Valid values range from
1
to
9
. Higher values using more memory, but are faster and produce smaller output. The default is 8.
strategy
is used to tune the compression algorithm. Possible values are
Z_DEFAULT_STRATEGY
,
Z_FILTERED
，和
Z_HUFFMAN_ONLY
。默认为
Z_DEFAULT_STRATEGY
.
zlib.
crc32
(
data
[
,
值
]
)
¶
Computes a CRC (Cyclic Redundancy Check) checksum of data 。若 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 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.
This function always returns an integer object.
注意
To generate the same numeric value across all Python versions and platforms use crc32(data) & 0xffffffff. If you are only using the checksum in packed binary format this is not necessary as the return value is the correct 32bit binary representation regardless of sign.
2.6 版改变： The return value is in the range [-2**31, 2**31-1] regardless of platform. In older versions the value would be signed on some platforms and unsigned on others.
3.0 版改变： The return value is unsigned and in the range [0, 2**32-1] regardless of platform.
zlib.
decompress
(
string
[
,
wbits
[
,
bufsize
]
]
)
¶
Decompresses the data in
string
, returning a string containing the uncompressed data. The
wbits
parameter depends on the format of
string
, 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 is 15, which 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()
. 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 history buffer (or the “window size”), and what header and trailer format is expected. It has the same meaning as described for decompress() .
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
，或
Z_FINISH
，默认为
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
设为
Z_FINISH
，
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.
2.5 版新增。
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 non-zero 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()
，
decompress()
method cannot be called again; the only realistic action is to delete the object.
可选参数 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.
2.5 版新增。
另请参阅
gzip
读写 gzip -format files.
The zlib library home page.
The zlib manual explains the semantics and usage of the library’s many functions.