/Doc/library/zlib.rst
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1 2:mod:`zlib` --- Compression compatible with :program:`gzip` 3=========================================================== 4 5.. module:: zlib 6 :synopsis: Low-level interface to compression and decompression routines compatible with 7 gzip. 8 9 10For applications that require data compression, the functions in this module 11allow compression and decompression, using the zlib library. The zlib library 12has its own home page at http://www.zlib.net. There are known 13incompatibilities between the Python module and versions of the zlib library 14earlier than 1.1.3; 1.1.3 has a security vulnerability, so we recommend using 151.1.4 or later. 16 17zlib's functions have many options and often need to be used in a particular 18order. This documentation doesn't attempt to cover all of the permutations; 19consult the zlib manual at http://www.zlib.net/manual.html for authoritative 20information. 21 22For reading and writing ``.gz`` files see the :mod:`gzip` module. For 23other archive formats, see the :mod:`bz2`, :mod:`zipfile`, and 24:mod:`tarfile` modules. 25 26The available exception and functions in this module are: 27 28 29.. exception:: error 30 31 Exception raised on compression and decompression errors. 32 33 34.. function:: adler32(data[, value]) 35 36 Computes a Adler-32 checksum of *data*. (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 inputs. 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. 44 45 This function always returns an integer object. 46 47.. note:: 48 To generate the same numeric value across all Python versions and 49 platforms use adler32(data) & 0xffffffff. If you are only using 50 the checksum in packed binary format this is not necessary as the 51 return value is the correct 32bit binary representation 52 regardless of sign. 53 54.. versionchanged:: 2.6 55 The return value is in the range [-2**31, 2**31-1] 56 regardless of platform. In older versions the value is 57 signed on some platforms and unsigned on others. 58 59.. versionchanged:: 3.0 60 The return value is unsigned and in the range [0, 2**32-1] 61 regardless of platform. 62 63 64.. function:: compress(string[, level]) 65 66 Compresses the data in *string*, returning a string contained compressed data. 67 *level* is an integer from ``1`` to ``9`` controlling the level of compression; 68 ``1`` is fastest and produces the least compression, ``9`` is slowest and 69 produces the most. The default value is ``6``. Raises the :exc:`error` 70 exception if any error occurs. 71 72 73.. function:: compressobj([level]) 74 75 Returns a compression object, to be used for compressing data streams that won't 76 fit into memory at once. *level* is an integer from ``1`` to ``9`` controlling 77 the level of compression; ``1`` is fastest and produces the least compression, 78 ``9`` is slowest and produces the most. The default value is ``6``. 79 80 81.. function:: crc32(data[, value]) 82 83 .. index:: 84 single: Cyclic Redundancy Check 85 single: checksum; Cyclic Redundancy Check 86 87 Computes a CRC (Cyclic Redundancy Check) checksum of *data*. If *value* is 88 present, it is used as the starting value of the checksum; otherwise, a fixed 89 default value is used. This allows computing a running checksum over the 90 concatenation of several inputs. The algorithm is not cryptographically 91 strong, and should not be used for authentication or digital signatures. Since 92 the algorithm is designed for use as a checksum algorithm, it is not suitable 93 for use as a general hash algorithm. 94 95 This function always returns an integer object. 96 97.. note:: 98 To generate the same numeric value across all Python versions and 99 platforms use crc32(data) & 0xffffffff. If you are only using 100 the checksum in packed binary format this is not necessary as the 101 return value is the correct 32bit binary representation 102 regardless of sign. 103 104.. versionchanged:: 2.6 105 The return value is in the range [-2**31, 2**31-1] 106 regardless of platform. In older versions the value would be 107 signed on some platforms and unsigned on others. 108 109.. versionchanged:: 3.0 110 The return value is unsigned and in the range [0, 2**32-1] 111 regardless of platform. 112 113 114.. function:: decompress(string[, wbits[, bufsize]]) 115 116 Decompresses the data in *string*, returning a string containing the 117 uncompressed data. The *wbits* parameter controls the size of the window 118 buffer. If *bufsize* is given, it is used as the initial size of the output 119 buffer. Raises the :exc:`error` exception if any error occurs. 120 121 The absolute value of *wbits* is the base two logarithm of the size of the 122 history buffer (the "window size") used when compressing data. Its absolute 123 value should be between 8 and 15 for the most recent versions of the zlib 124 library, larger values resulting in better compression at the expense of greater 125 memory usage. The default value is 15. When *wbits* is negative, the standard 126 :program:`gzip` header is suppressed; this is an undocumented feature of the 127 zlib library, used for compatibility with :program:`unzip`'s compression file 128 format. 129 130 *bufsize* is the initial size of the buffer used to hold decompressed data. If 131 more space is required, the buffer size will be increased as needed, so you 132 don't have to get this value exactly right; tuning it will only save a few calls 133 to :cfunc:`malloc`. The default size is 16384. 134 135 136.. function:: decompressobj([wbits]) 137 138 Returns a decompression object, to be used for decompressing data streams that 139 won't fit into memory at once. The *wbits* parameter controls the size of the 140 window buffer. 141 142Compression objects support the following methods: 143 144 145.. method:: Compress.compress(string) 146 147 Compress *string*, returning a string containing compressed data for at least 148 part of the data in *string*. This data should be concatenated to the output 149 produced by any preceding calls to the :meth:`compress` method. Some input may 150 be kept in internal buffers for later processing. 151 152 153.. method:: Compress.flush([mode]) 154 155 All pending input is processed, and a string containing the remaining compressed 156 output is returned. *mode* can be selected from the constants 157 :const:`Z_SYNC_FLUSH`, :const:`Z_FULL_FLUSH`, or :const:`Z_FINISH`, 158 defaulting to :const:`Z_FINISH`. :const:`Z_SYNC_FLUSH` and 159 :const:`Z_FULL_FLUSH` allow compressing further strings of data, while 160 :const:`Z_FINISH` finishes the compressed stream and prevents compressing any 161 more data. After calling :meth:`flush` with *mode* set to :const:`Z_FINISH`, 162 the :meth:`compress` method cannot be called again; the only realistic action is 163 to delete the object. 164 165 166.. method:: Compress.copy() 167 168 Returns a copy of the compression object. This can be used to efficiently 169 compress a set of data that share a common initial prefix. 170 171 .. versionadded:: 2.5 172 173Decompression objects support the following methods, and two attributes: 174 175 176.. attribute:: Decompress.unused_data 177 178 A string which contains any bytes past the end of the compressed data. That is, 179 this remains ``""`` until the last byte that contains compression data is 180 available. If the whole string turned out to contain compressed data, this is 181 ``""``, the empty string. 182 183 The only way to determine where a string of compressed data ends is by actually 184 decompressing it. This means that when compressed data is contained part of a 185 larger file, you can only find the end of it by reading data and feeding it 186 followed by some non-empty string into a decompression object's 187 :meth:`decompress` method until the :attr:`unused_data` attribute is no longer 188 the empty string. 189 190 191.. attribute:: Decompress.unconsumed_tail 192 193 A string that contains any data that was not consumed by the last 194 :meth:`decompress` call because it exceeded the limit for the uncompressed data 195 buffer. This data has not yet been seen by the zlib machinery, so you must feed 196 it (possibly with further data concatenated to it) back to a subsequent 197 :meth:`decompress` method call in order to get correct output. 198 199 200.. method:: Decompress.decompress(string[, max_length]) 201 202 Decompress *string*, returning a string containing the uncompressed data 203 corresponding to at least part of the data in *string*. This data should be 204 concatenated to the output produced by any preceding calls to the 205 :meth:`decompress` method. Some of the input data may be preserved in internal 206 buffers for later processing. 207 208 If the optional parameter *max_length* is supplied then the return value will be 209 no longer than *max_length*. This may mean that not all of the compressed input 210 can be processed; and unconsumed data will be stored in the attribute 211 :attr:`unconsumed_tail`. This string must be passed to a subsequent call to 212 :meth:`decompress` if decompression is to continue. If *max_length* is not 213 supplied then the whole input is decompressed, and :attr:`unconsumed_tail` is an 214 empty string. 215 216 217.. method:: Decompress.flush([length]) 218 219 All pending input is processed, and a string containing the remaining 220 uncompressed output is returned. After calling :meth:`flush`, the 221 :meth:`decompress` method cannot be called again; the only realistic action is 222 to delete the object. 223 224 The optional parameter *length* sets the initial size of the output buffer. 225 226 227.. method:: Decompress.copy() 228 229 Returns a copy of the decompression object. This can be used to save the state 230 of the decompressor midway through the data stream in order to speed up random 231 seeks into the stream at a future point. 232 233 .. versionadded:: 2.5 234 235 236.. seealso:: 237 238 Module :mod:`gzip` 239 Reading and writing :program:`gzip`\ -format files. 240 241 http://www.zlib.net 242 The zlib library home page. 243 244 http://www.zlib.net/manual.html 245 The zlib manual explains the semantics and usage of the library's many 246 functions. 247