2 :mod:`zlib` --- Compression compatible with :program:`gzip`
3 ===========================================================
6 :synopsis: Low-level interface to compression and decompression routines compatible with
10 For applications that require data compression, the functions in this module
11 allow compression and decompression, using the zlib library. The zlib library
12 has its own home page at http://www.zlib.net. There are known
13 incompatibilities between the Python module and versions of the zlib library
14 earlier than 1.1.3; 1.1.3 has a security vulnerability, so we recommend using
17 zlib's functions have many options and often need to be used in a particular
18 order. This documentation doesn't attempt to cover all of the permutations;
19 consult the zlib manual at http://www.zlib.net/manual.html for authoritative
22 For reading and writing ``.gz`` files see the :mod:`gzip` module. For
23 other archive formats, see the :mod:`bz2`, :mod:`zipfile`, and
24 :mod:`tarfile` modules.
26 The available exception and functions in this module are:
31 Exception raised on compression and decompression errors.
34 .. function:: adler32(string[, value])
36 Computes a Adler-32 checksum of *string*. (An Adler-32 checksum is almost as
37 reliable as a CRC32 but can be computed much more quickly.) If *value* is
38 present, it is used as the starting value of the checksum; otherwise, a fixed
39 default value is used. This allows computing a running checksum over the
40 concatenation of several input strings. The algorithm is not cryptographically
41 strong, and should not be used for authentication or digital signatures. Since
42 the algorithm is designed for use as a checksum algorithm, it is not suitable
43 for use as a general hash algorithm.
45 This function always returns an integer object.
47 .. versionchanged:: 2.6
48 For consistent cross-platform behavior we always return a signed integer.
49 ie: Results in the (2**31)...(2**32-1) range will be negative.
52 .. function:: compress(string[, level])
54 Compresses the data in *string*, returning a string contained compressed data.
55 *level* is an integer from ``1`` to ``9`` controlling the level of compression;
56 ``1`` is fastest and produces the least compression, ``9`` is slowest and
57 produces the most. The default value is ``6``. Raises the :exc:`error`
58 exception if any error occurs.
61 .. function:: compressobj([level])
63 Returns a compression object, to be used for compressing data streams that won't
64 fit into memory at once. *level* is an integer from ``1`` to ``9`` controlling
65 the level of compression; ``1`` is fastest and produces the least compression,
66 ``9`` is slowest and produces the most. The default value is ``6``.
69 .. function:: crc32(string[, value])
72 single: Cyclic Redundancy Check
73 single: checksum; Cyclic Redundancy Check
75 Computes a CRC (Cyclic Redundancy Check) checksum of *string*. If *value* is
76 present, it is used as the starting value of the checksum; otherwise, a fixed
77 default value is used. This allows computing a running checksum over the
78 concatenation of several input strings. The algorithm is not cryptographically
79 strong, and should not be used for authentication or digital signatures. Since
80 the algorithm is designed for use as a checksum algorithm, it is not suitable
81 for use as a general hash algorithm.
83 This function always returns an integer object.
85 .. versionchanged:: 2.6
86 For consistent cross-platform behavior we always return a signed integer.
87 ie: Results in the (2**31)...(2**32-1) range will be negative.
90 .. function:: decompress(string[, wbits[, bufsize]])
92 Decompresses the data in *string*, returning a string containing the
93 uncompressed data. The *wbits* parameter controls the size of the window
94 buffer. If *bufsize* is given, it is used as the initial size of the output
95 buffer. Raises the :exc:`error` exception if any error occurs.
97 The absolute value of *wbits* is the base two logarithm of the size of the
98 history buffer (the "window size") used when compressing data. Its absolute
99 value should be between 8 and 15 for the most recent versions of the zlib
100 library, larger values resulting in better compression at the expense of greater
101 memory usage. The default value is 15. When *wbits* is negative, the standard
102 :program:`gzip` header is suppressed; this is an undocumented feature of the
103 zlib library, used for compatibility with :program:`unzip`'s compression file
106 *bufsize* is the initial size of the buffer used to hold decompressed data. If
107 more space is required, the buffer size will be increased as needed, so you
108 don't have to get this value exactly right; tuning it will only save a few calls
109 to :cfunc:`malloc`. The default size is 16384.
112 .. function:: decompressobj([wbits])
114 Returns a decompression object, to be used for decompressing data streams that
115 won't fit into memory at once. The *wbits* parameter controls the size of the
118 Compression objects support the following methods:
121 .. method:: Compress.compress(string)
123 Compress *string*, returning a string containing compressed data for at least
124 part of the data in *string*. This data should be concatenated to the output
125 produced by any preceding calls to the :meth:`compress` method. Some input may
126 be kept in internal buffers for later processing.
129 .. method:: Compress.flush([mode])
131 All pending input is processed, and a string containing the remaining compressed
132 output is returned. *mode* can be selected from the constants
133 :const:`Z_SYNC_FLUSH`, :const:`Z_FULL_FLUSH`, or :const:`Z_FINISH`,
134 defaulting to :const:`Z_FINISH`. :const:`Z_SYNC_FLUSH` and
135 :const:`Z_FULL_FLUSH` allow compressing further strings of data, while
136 :const:`Z_FINISH` finishes the compressed stream and prevents compressing any
137 more data. After calling :meth:`flush` with *mode* set to :const:`Z_FINISH`,
138 the :meth:`compress` method cannot be called again; the only realistic action is
139 to delete the object.
142 .. method:: Compress.copy()
144 Returns a copy of the compression object. This can be used to efficiently
145 compress a set of data that share a common initial prefix.
147 .. versionadded:: 2.5
149 Decompression objects support the following methods, and two attributes:
152 .. attribute:: Decompress.unused_data
154 A string which contains any bytes past the end of the compressed data. That is,
155 this remains ``""`` until the last byte that contains compression data is
156 available. If the whole string turned out to contain compressed data, this is
157 ``""``, the empty string.
159 The only way to determine where a string of compressed data ends is by actually
160 decompressing it. This means that when compressed data is contained part of a
161 larger file, you can only find the end of it by reading data and feeding it
162 followed by some non-empty string into a decompression object's
163 :meth:`decompress` method until the :attr:`unused_data` attribute is no longer
167 .. attribute:: Decompress.unconsumed_tail
169 A string that contains any data that was not consumed by the last
170 :meth:`decompress` call because it exceeded the limit for the uncompressed data
171 buffer. This data has not yet been seen by the zlib machinery, so you must feed
172 it (possibly with further data concatenated to it) back to a subsequent
173 :meth:`decompress` method call in order to get correct output.
176 .. method:: Decompress.decompress(string[, max_length])
178 Decompress *string*, returning a string containing the uncompressed data
179 corresponding to at least part of the data in *string*. This data should be
180 concatenated to the output produced by any preceding calls to the
181 :meth:`decompress` method. Some of the input data may be preserved in internal
182 buffers for later processing.
184 If the optional parameter *max_length* is supplied then the return value will be
185 no longer than *max_length*. This may mean that not all of the compressed input
186 can be processed; and unconsumed data will be stored in the attribute
187 :attr:`unconsumed_tail`. This string must be passed to a subsequent call to
188 :meth:`decompress` if decompression is to continue. If *max_length* is not
189 supplied then the whole input is decompressed, and :attr:`unconsumed_tail` is an
193 .. method:: Decompress.flush([length])
195 All pending input is processed, and a string containing the remaining
196 uncompressed output is returned. After calling :meth:`flush`, the
197 :meth:`decompress` method cannot be called again; the only realistic action is
198 to delete the object.
200 The optional parameter *length* sets the initial size of the output buffer.
203 .. method:: Decompress.copy()
205 Returns a copy of the decompression object. This can be used to save the state
206 of the decompressor midway through the data stream in order to speed up random
207 seeks into the stream at a future point.
209 .. versionadded:: 2.5
215 Reading and writing :program:`gzip`\ -format files.
218 The zlib library home page.
220 http://www.zlib.net/manual.html
221 The zlib manual explains the semantics and usage of the library's many