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| 54 | |
| 55 | <H1><A NAME="SECTION0091500000000000000000"> |
| 56 | 7.15 <tt class="module">zlib</tt> -- |
| 57 | Compression compatible with <b class="program">gzip</b></A> |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <A NAME="module-zlib"></A> |
| 62 | |
| 63 | <P> |
| 64 | For applications that require data compression, the functions in this |
| 65 | module allow compression and decompression, using the zlib library. |
| 66 | The zlib library has its own home page at <a class="url" href="http://www.gzip.org/zlib/">http://www.gzip.org/zlib/</a>. |
| 67 | There are known incompatibilities between the Python module and |
| 68 | versions of the zlib library earlier than 1.1.3; 1.1.3 has a security |
| 69 | vulnerability, so we recommend using 1.1.4 or later. |
| 70 | |
| 71 | <P> |
| 72 | The available exception and functions in this module are: |
| 73 | |
| 74 | <P> |
| 75 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-2815' xml:id='l2h-2815' class="exception">error</tt></b></dt> |
| 76 | <dd> |
| 77 | Exception raised on compression and decompression errors. |
| 78 | </dd></dl> |
| 79 | |
| 80 | <P> |
| 81 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 82 | <td><nobr><b><tt id='l2h-2816' xml:id='l2h-2816' class="function">adler32</tt></b>(</nobr></td> |
| 83 | <td><var>string</var><big>[</big><var>, value</var><big>]</big><var></var>)</td></tr></table></dt> |
| 84 | <dd> |
| 85 | Computes a Adler-32 checksum of <var>string</var>. (An Adler-32 |
| 86 | checksum is almost as reliable as a CRC32 but can be computed much |
| 87 | more quickly.) If <var>value</var> is present, it is used as the |
| 88 | starting value of the checksum; otherwise, a fixed default value is |
| 89 | used. This allows computing a running checksum over the |
| 90 | concatenation of several input strings. The algorithm is not |
| 91 | cryptographically strong, and should not be used for |
| 92 | authentication or digital signatures. Since the algorithm is |
| 93 | designed for use as a checksum algorithm, it is not suitable for |
| 94 | use as a general hash algorithm. |
| 95 | </dl> |
| 96 | |
| 97 | <P> |
| 98 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 99 | <td><nobr><b><tt id='l2h-2817' xml:id='l2h-2817' class="function">compress</tt></b>(</nobr></td> |
| 100 | <td><var>string</var><big>[</big><var>, level</var><big>]</big><var></var>)</td></tr></table></dt> |
| 101 | <dd> |
| 102 | Compresses the data in <var>string</var>, returning a string contained |
| 103 | compressed data. <var>level</var> is an integer from <code>1</code> to |
| 104 | <code>9</code> controlling the level of compression; <code>1</code> is fastest |
| 105 | and produces the least compression, <code>9</code> is slowest and produces |
| 106 | the most. The default value is <code>6</code>. Raises the |
| 107 | <tt class="exception">error</tt> exception if any error occurs. |
| 108 | </dl> |
| 109 | |
| 110 | <P> |
| 111 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 112 | <td><nobr><b><tt id='l2h-2818' xml:id='l2h-2818' class="function">compressobj</tt></b>(</nobr></td> |
| 113 | <td><var></var><big>[</big><var>level</var><big>]</big><var></var>)</td></tr></table></dt> |
| 114 | <dd> |
| 115 | Returns a compression object, to be used for compressing data streams |
| 116 | that won't fit into memory at once. <var>level</var> is an integer from |
| 117 | <code>1</code> to <code>9</code> controlling the level of compression; <code>1</code> is |
| 118 | fastest and produces the least compression, <code>9</code> is slowest and |
| 119 | produces the most. The default value is <code>6</code>. |
| 120 | </dl> |
| 121 | |
| 122 | <P> |
| 123 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 124 | <td><nobr><b><tt id='l2h-2819' xml:id='l2h-2819' class="function">crc32</tt></b>(</nobr></td> |
| 125 | <td><var>string</var><big>[</big><var>, value</var><big>]</big><var></var>)</td></tr></table></dt> |
| 126 | <dd> |
| 127 | Computes a CRC (Cyclic Redundancy Check)<a id='l2h-2828' xml:id='l2h-2828'></a> |
| 128 | checksum of <var>string</var>. If |
| 129 | <var>value</var> is present, it is used as the starting value of the |
| 130 | checksum; otherwise, a fixed default value is used. This allows |
| 131 | computing a running checksum over the concatenation of several |
| 132 | input strings. The algorithm is not cryptographically strong, and |
| 133 | should not be used for authentication or digital signatures. Since |
| 134 | the algorithm is designed for use as a checksum algorithm, it is not |
| 135 | suitable for use as a general hash algorithm. |
| 136 | </dl> |
| 137 | |
| 138 | <P> |
| 139 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 140 | <td><nobr><b><tt id='l2h-2820' xml:id='l2h-2820' class="function">decompress</tt></b>(</nobr></td> |
| 141 | <td><var>string</var><big>[</big><var>, wbits</var><big>[</big><var>, bufsize</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> |
| 142 | <dd> |
| 143 | Decompresses the data in <var>string</var>, returning a string containing |
| 144 | the uncompressed data. The <var>wbits</var> parameter controls the size of |
| 145 | the window buffer. If <var>bufsize</var> is given, it is used as the |
| 146 | initial size of the output buffer. Raises the <tt class="exception">error</tt> |
| 147 | exception if any error occurs. |
| 148 | |
| 149 | <P> |
| 150 | The absolute value of <var>wbits</var> is the base two logarithm of the |
| 151 | size of the history buffer (the ``window size'') used when compressing |
| 152 | data. Its absolute value should be between 8 and 15 for the most |
| 153 | recent versions of the zlib library, larger values resulting in better |
| 154 | compression at the expense of greater memory usage. The default value |
| 155 | is 15. When <var>wbits</var> is negative, the standard |
| 156 | <b class="program">gzip</b> header is suppressed; this is an undocumented feature |
| 157 | of the zlib library, used for compatibility with <b class="program">unzip</b>'s |
| 158 | compression file format. |
| 159 | |
| 160 | <P> |
| 161 | <var>bufsize</var> is the initial size of the buffer used to hold |
| 162 | decompressed data. If more space is required, the buffer size will be |
| 163 | increased as needed, so you don't have to get this value exactly |
| 164 | right; tuning it will only save a few calls to <tt class="cfunction">malloc()</tt>. The |
| 165 | default size is 16384. |
| 166 | |
| 167 | <P> |
| 168 | </dl> |
| 169 | |
| 170 | <P> |
| 171 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 172 | <td><nobr><b><tt id='l2h-2821' xml:id='l2h-2821' class="function">decompressobj</tt></b>(</nobr></td> |
| 173 | <td><var></var><big>[</big><var>wbits</var><big>]</big><var></var>)</td></tr></table></dt> |
| 174 | <dd> |
| 175 | Returns a decompression object, to be used for decompressing data |
| 176 | streams that won't fit into memory at once. The <var>wbits</var> |
| 177 | parameter controls the size of the window buffer. |
| 178 | </dl> |
| 179 | |
| 180 | <P> |
| 181 | Compression objects support the following methods: |
| 182 | |
| 183 | <P> |
| 184 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 185 | <td><nobr><b><tt id='l2h-2822' xml:id='l2h-2822' class="method">compress</tt></b>(</nobr></td> |
| 186 | <td><var>string</var>)</td></tr></table></dt> |
| 187 | <dd> |
| 188 | Compress <var>string</var>, returning a string containing compressed data |
| 189 | for at least part of the data in <var>string</var>. This data should be |
| 190 | concatenated to the output produced by any preceding calls to the |
| 191 | <tt class="method">compress()</tt> method. Some input may be kept in internal buffers |
| 192 | for later processing. |
| 193 | </dl> |
| 194 | |
| 195 | <P> |
| 196 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 197 | <td><nobr><b><tt id='l2h-2823' xml:id='l2h-2823' class="method">flush</tt></b>(</nobr></td> |
| 198 | <td><var></var><big>[</big><var>mode</var><big>]</big><var></var>)</td></tr></table></dt> |
| 199 | <dd> |
| 200 | All pending input is processed, and a string containing the remaining |
| 201 | compressed output is returned. <var>mode</var> can be selected from the |
| 202 | constants <tt class="constant">Z_SYNC_FLUSH</tt>, <tt class="constant">Z_FULL_FLUSH</tt>, or |
| 203 | <tt class="constant">Z_FINISH</tt>, defaulting to <tt class="constant">Z_FINISH</tt>. <tt class="constant">Z_SYNC_FLUSH</tt> and |
| 204 | <tt class="constant">Z_FULL_FLUSH</tt> allow compressing further strings of data and |
| 205 | are used to allow partial error recovery on decompression, while |
| 206 | <tt class="constant">Z_FINISH</tt> finishes the compressed stream and |
| 207 | prevents compressing any more data. After calling |
| 208 | <tt class="method">flush()</tt> with <var>mode</var> set to <tt class="constant">Z_FINISH</tt>, the |
| 209 | <tt class="method">compress()</tt> method cannot be called again; the only realistic |
| 210 | action is to delete the object. |
| 211 | </dl> |
| 212 | |
| 213 | <P> |
| 214 | Decompression objects support the following methods, and two attributes: |
| 215 | |
| 216 | <P> |
| 217 | <dl><dt><b><tt id='l2h-2824' xml:id='l2h-2824' class="member">unused_data</tt></b></dt> |
| 218 | <dd> |
| 219 | A string which contains any bytes past the end of the compressed data. |
| 220 | That is, this remains <code>""</code> until the last byte that contains |
| 221 | compression data is available. If the whole string turned out to |
| 222 | contain compressed data, this is <code>""</code>, the empty string. |
| 223 | |
| 224 | <P> |
| 225 | The only way to determine where a string of compressed data ends is by |
| 226 | actually decompressing it. This means that when compressed data is |
| 227 | contained part of a larger file, you can only find the end of it by |
| 228 | reading data and feeding it followed by some non-empty string into a |
| 229 | decompression object's <tt class="method">decompress</tt> method until the |
| 230 | <tt class="member">unused_data</tt> attribute is no longer the empty string. |
| 231 | </dl> |
| 232 | |
| 233 | <P> |
| 234 | <dl><dt><b><tt id='l2h-2825' xml:id='l2h-2825' class="member">unconsumed_tail</tt></b></dt> |
| 235 | <dd> |
| 236 | A string that contains any data that was not consumed by the last |
| 237 | <tt class="method">decompress</tt> call because it exceeded the limit for the |
| 238 | uncompressed data buffer. This data has not yet been seen by the zlib |
| 239 | machinery, so you must feed it (possibly with further data |
| 240 | concatenated to it) back to a subsequent <tt class="method">decompress</tt> method |
| 241 | call in order to get correct output. |
| 242 | </dl> |
| 243 | |
| 244 | <P> |
| 245 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 246 | <td><nobr><b><tt id='l2h-2826' xml:id='l2h-2826' class="method">decompress</tt></b>(</nobr></td> |
| 247 | <td><var>string</var><big>[</big><var>, max_length</var><big>]</big><var></var>)</td></tr></table></dt> |
| 248 | <dd> |
| 249 | Decompress <var>string</var>, returning a string containing the |
| 250 | uncompressed data corresponding to at least part of the data in |
| 251 | <var>string</var>. This data should be concatenated to the output produced |
| 252 | by any preceding calls to the |
| 253 | <tt class="method">decompress()</tt> method. Some of the input data may be preserved |
| 254 | in internal buffers for later processing. |
| 255 | |
| 256 | <P> |
| 257 | If the optional parameter <var>max_length</var> is supplied then the return value |
| 258 | will be no longer than <var>max_length</var>. This may mean that not all of the |
| 259 | compressed input can be processed; and unconsumed data will be stored |
| 260 | in the attribute <tt class="member">unconsumed_tail</tt>. This string must be passed |
| 261 | to a subsequent call to <tt class="method">decompress()</tt> if decompression is to |
| 262 | continue. If <var>max_length</var> is not supplied then the whole input is |
| 263 | decompressed, and <tt class="member">unconsumed_tail</tt> is an empty string. |
| 264 | </dl> |
| 265 | |
| 266 | <P> |
| 267 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 268 | <td><nobr><b><tt id='l2h-2827' xml:id='l2h-2827' class="method">flush</tt></b>(</nobr></td> |
| 269 | <td><var></var>)</td></tr></table></dt> |
| 270 | <dd> |
| 271 | All pending input is processed, and a string containing the remaining |
| 272 | uncompressed output is returned. After calling <tt class="method">flush()</tt>, the |
| 273 | <tt class="method">decompress()</tt> method cannot be called again; the only realistic |
| 274 | action is to delete the object. |
| 275 | </dl> |
| 276 | |
| 277 | <P> |
| 278 | <div class="seealso"> |
| 279 | <p class="heading">See Also:</p> |
| 280 | |
| 281 | <dl compact="compact" class="seemodule"> |
| 282 | <dt>Module <b><tt class="module"><a href="module-gzip.html">gzip</a></tt>:</b> |
| 283 | <dd>Reading and writing <b class="program">gzip</b>-format files. |
| 284 | </dl> |
| 285 | <dl compact="compact" class="seeurl"> |
| 286 | <dt><a href="http://www.gzip.org/zlib/" |
| 287 | class="url">http://www.gzip.org/zlib/</a></dt> |
| 288 | <dd>The zlib library home page.</dd> |
| 289 | </dl> |
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