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<H1><A NAME="SECTION0091500000000000000000">
7.15 <tt class="module">zlib</tt> --
         Compression compatible with <b class="program">gzip</b></A>
</H1>

<P>
<A NAME="module-zlib"></A>

<P>
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 <a class="url" href="http://www.gzip.org/zlib/">http://www.gzip.org/zlib/</a>.  
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.

<P>
The available exception and functions in this module are:

<P>
<dl><dt><b><span class="typelabel">exception</span>&nbsp;<tt id='l2h-2815' xml:id='l2h-2815' class="exception">error</tt></b></dt>
<dd>
  Exception raised on compression and decompression errors.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2816' xml:id='l2h-2816' class="function">adler32</tt></b>(</nobr></td>
  <td><var>string</var><big>[</big><var>, value</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
   Computes a Adler-32 checksum of <var>string</var>.  (An Adler-32
   checksum is almost as reliable as a CRC32 but can be computed much
   more quickly.)  If <var>value</var> 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.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2817' xml:id='l2h-2817' class="function">compress</tt></b>(</nobr></td>
  <td><var>string</var><big>[</big><var>, level</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Compresses the data in <var>string</var>, returning a string contained
  compressed data.  <var>level</var> is an integer from <code>1</code> to
  <code>9</code> controlling the level of compression; <code>1</code> is fastest
  and produces the least compression, <code>9</code> is slowest and produces
  the most.  The default value is <code>6</code>.  Raises the
  <tt class="exception">error</tt> exception if any error occurs.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2818' xml:id='l2h-2818' class="function">compressobj</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>level</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Returns a compression object, to be used for compressing data streams
  that won't fit into memory at once.  <var>level</var> is an integer from
  <code>1</code> to <code>9</code> controlling the level of compression; <code>1</code> is
  fastest and produces the least compression, <code>9</code> is slowest and
  produces the most.  The default value is <code>6</code>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2819' xml:id='l2h-2819' class="function">crc32</tt></b>(</nobr></td>
  <td><var>string</var><big>[</big><var>, value</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Computes a CRC (Cyclic Redundancy Check)<a id='l2h-2828' xml:id='l2h-2828'></a>
  checksum of <var>string</var>. If
  <var>value</var> 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.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2820' xml:id='l2h-2820' class="function">decompress</tt></b>(</nobr></td>
  <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>
<dd>
  Decompresses the data in <var>string</var>, returning a string containing
  the uncompressed data.  The <var>wbits</var> parameter controls the size of
  the window buffer.  If <var>bufsize</var> is given, it is used as the
  initial size of the output buffer.  Raises the <tt class="exception">error</tt>
  exception if any error occurs.

<P>
The absolute value of <var>wbits</var> 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 <var>wbits</var> is negative, the standard
<b class="program">gzip</b> header is suppressed; this is an undocumented feature
of the zlib library, used for compatibility with <b class="program">unzip</b>'s
compression file format.

<P>
<var>bufsize</var> 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 <tt class="cfunction">malloc()</tt>.  The
default size is 16384.

<P>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2821' xml:id='l2h-2821' class="function">decompressobj</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>wbits</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Returns a decompression object, to be used for decompressing data
  streams that won't fit into memory at once.  The <var>wbits</var>
  parameter controls the size of the window buffer.
</dl>

<P>
Compression objects support the following methods:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2822' xml:id='l2h-2822' class="method">compress</tt></b>(</nobr></td>
  <td><var>string</var>)</td></tr></table></dt>
<dd>
Compress <var>string</var>, returning a string containing compressed data
for at least part of the data in <var>string</var>.  This data should be
concatenated to the output produced by any preceding calls to the
<tt class="method">compress()</tt> method.  Some input may be kept in internal buffers
for later processing.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2823' xml:id='l2h-2823' class="method">flush</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>mode</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
All pending input is processed, and a string containing the remaining
compressed output is returned.  <var>mode</var> can be selected from the
constants <tt class="constant">Z_SYNC_FLUSH</tt>,  <tt class="constant">Z_FULL_FLUSH</tt>,  or 
<tt class="constant">Z_FINISH</tt>, defaulting to <tt class="constant">Z_FINISH</tt>.  <tt class="constant">Z_SYNC_FLUSH</tt> and 
<tt class="constant">Z_FULL_FLUSH</tt> allow compressing further strings of data and
are used to allow partial error recovery on decompression, while
<tt class="constant">Z_FINISH</tt> finishes the compressed stream and 
prevents compressing any more data.  After calling
<tt class="method">flush()</tt> with <var>mode</var> set to <tt class="constant">Z_FINISH</tt>, the
<tt class="method">compress()</tt> method cannot be called again; the only realistic
action is to delete the object.  
</dl>

<P>
Decompression objects support the following methods, and two attributes:

<P>
<dl><dt><b><tt id='l2h-2824' xml:id='l2h-2824' class="member">unused_data</tt></b></dt>
<dd>
A string which contains any bytes past the end of the compressed data.
That is, this remains <code>""</code> until the last byte that contains
compression data is available.  If the whole string turned out to
contain compressed data, this is <code>""</code>, the empty string.

<P>
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 <tt class="method">decompress</tt> method until the
<tt class="member">unused_data</tt> attribute is no longer the empty string.
</dl>

<P>
<dl><dt><b><tt id='l2h-2825' xml:id='l2h-2825' class="member">unconsumed_tail</tt></b></dt>
<dd>
A string that contains any data that was not consumed by the last
<tt class="method">decompress</tt> 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 <tt class="method">decompress</tt> method
call in order to get correct output.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2826' xml:id='l2h-2826' class="method">decompress</tt></b>(</nobr></td>
  <td><var>string</var><big>[</big><var>, max_length</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Decompress <var>string</var>, returning a string containing the
uncompressed data corresponding to at least part of the data in
<var>string</var>.  This data should be concatenated to the output produced
by any preceding calls to the
<tt class="method">decompress()</tt> method.  Some of the input data may be preserved
in internal buffers for later processing.

<P>
If the optional parameter <var>max_length</var> is supplied then the return value
will be no longer than <var>max_length</var>. This may mean that not all of the
compressed input can be processed; and unconsumed data will be stored
in the attribute <tt class="member">unconsumed_tail</tt>. This string must be passed
to a subsequent call to <tt class="method">decompress()</tt> if decompression is to
continue.  If <var>max_length</var> is not supplied then the whole input is
decompressed, and <tt class="member">unconsumed_tail</tt> is an empty string.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2827' xml:id='l2h-2827' class="method">flush</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
All pending input is processed, and a string containing the remaining
uncompressed output is returned.  After calling <tt class="method">flush()</tt>, the
<tt class="method">decompress()</tt> method cannot be called again; the only realistic
action is to delete the object.
</dl>

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-gzip.html">gzip</a></tt>:</b>
    <dd>Reading and writing <b class="program">gzip</b>-format files.
  </dl>
  <dl compact="compact" class="seeurl">
    <dt><a href="http://www.gzip.org/zlib/"
        class="url">http://www.gzip.org/zlib/</a></dt>
    <dd>The zlib library home page.</dd>
  </dl>
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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>
290	</div>
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