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Code Editor : zlib.html

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<title>12.1. zlib — Compression compatible with gzip &#8212; Python 2.7.18 documentation</title>
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 <section id="module-zlib">
<h1>12.1. <a class="reference internal" href="#module-zlib" title="zlib: Low-level interface to compression and decompression routines compatible with gzip."><code class="xref py py-mod docutils literal notranslate">zlib</code></a> — Compression compatible with gzip<a class="headerlink" href="#module-zlib" title="Permalink to this headline">¶</a></h1>
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="reference external" href="http://www.zlib.net">http://www.zlib.net</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.
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 <a class="reference external" href="http://www.zlib.net/manual.html">http://www.zlib.net/manual.html</a> for authoritative
information.
For reading and writing <code class="docutils literal notranslate">.gz</code> files see the <a class="reference internal" href="gzip.html#module-gzip" title="gzip: Interfaces for gzip compression and decompression using file objects."><code class="xref py py-mod docutils literal notranslate">gzip</code></a> module.
The available exception and functions in this module are:
<dl class="py exception">
<dt class="sig sig-object py" id="zlib.error">
exception zlib.error<a class="headerlink" href="#zlib.error" title="Permalink to this definition">¶</a></dt>
<dd>Exception raised on compression and decompression errors.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="zlib.adler32">
zlib.adler32(data[, value])<a class="headerlink" href="#zlib.adler32" title="Permalink to this definition">¶</a></dt>
<dd>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.
</dd></dl>

<div class="admonition note">
Note
To generate the same numeric value across all Python versions and
platforms use adler32(data) &amp; 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.
</div>
<div class="versionchanged">
Changed in version 2.6: 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.
</div>
<div class="versionchanged">
Changed in version 3.0: The return value is unsigned and in the range [0, 2**32-1]
regardless of platform.
</div>
<dl class="py function">
<dt class="sig sig-object py" id="zlib.compress">
zlib.compress(string[, level])<a class="headerlink" href="#zlib.compress" title="Permalink to this definition">¶</a></dt>
<dd>Compresses the data in string, returning a string contained compressed data.
level is an integer from <code class="docutils literal notranslate">0</code> to <code class="docutils literal notranslate">9</code> controlling the level of compression;
<code class="docutils literal notranslate">1</code> is fastest and produces the least compression, <code class="docutils literal notranslate">9</code> is slowest and
produces the most. <code class="docutils literal notranslate">0</code> is no compression. The default value is <code class="docutils literal notranslate">6</code>.
Raises the <a class="reference internal" href="#zlib.error" title="zlib.error"><code class="xref py py-exc docutils literal notranslate">error</code></a> exception if any error occurs.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="zlib.compressobj">
zlib.compressobj([level[, method[, wbits[, memlevel[, strategy]]]]])<a class="headerlink" href="#zlib.compressobj" title="Permalink to this definition">¶</a></dt>
<dd>Returns a compression object, to be used for compressing data streams that won’t
fit into memory at once. level is an integer from
<code class="docutils literal notranslate">0</code> to <code class="docutils literal notranslate">9</code> or <code class="docutils literal notranslate">-1</code>, controlling
the level of compression; <code class="docutils literal notranslate">1</code> is fastest and produces the least compression,
<code class="docutils literal notranslate">9</code> is slowest and produces the most. <code class="docutils literal notranslate">0</code> is no compression. The default
value is <code class="docutils literal notranslate">-1</code> (Z_DEFAULT_COMPRESSION). Z_DEFAULT_COMPRESSION represents a default
compromise between speed and compression (currently equivalent to level 6).
method is the compression algorithm. Currently, the only supported value is
<code class="docutils literal notranslate">DEFLATED</code>.
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.
<ul class="simple">
<li>+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.</li>
<li>−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.</li>
<li>+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.</li>
</ul>
memlevel controls the amount of memory used for internal compression state.
Valid values range from <code class="docutils literal notranslate">1</code> to <code class="docutils literal notranslate">9</code>. 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
<code class="docutils literal notranslate">Z_DEFAULT_STRATEGY</code>, <code class="docutils literal notranslate">Z_FILTERED</code>, and <code class="docutils literal notranslate">Z_HUFFMAN_ONLY</code>. The default
is <code class="docutils literal notranslate">Z_DEFAULT_STRATEGY</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="zlib.crc32">
zlib.crc32(data[, value])<a class="headerlink" href="#zlib.crc32" title="Permalink to this definition">¶</a></dt>
<dd>Computes a CRC (Cyclic Redundancy Check) checksum of data. 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.
</dd></dl>

<div class="admonition note">
Note
To generate the same numeric value across all Python versions and
platforms use crc32(data) &amp; 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.
</div>
<div class="versionchanged">
Changed in version 2.6: 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.
</div>
<div class="versionchanged">
Changed in version 3.0: The return value is unsigned and in the range [0, 2**32-1]
regardless of platform.
</div>
<dl class="py function">
<dt class="sig sig-object py" id="zlib.decompress">
zlib.decompress(string[, wbits[, bufsize]])<a class="headerlink" href="#zlib.decompress" title="Permalink to this definition">¶</a></dt>
<dd>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 <a class="reference internal" href="#zlib.error" title="zlib.error"><code class="xref py py-exc docutils literal notranslate">error</code></a> 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 <a class="reference internal" href="#zlib.compressobj" title="zlib.compressobj"><code class="xref py py-func docutils literal notranslate">compressobj()</code></a>, but accepts
more ranges of values:
<ul class="simple">
<li>+8 to +15: The base-two logarithm of the window size. The input
must include a zlib header and trailer.</li>
<li>0: Automatically determine the window size from the zlib header.
Only supported since zlib 1.2.3.5.</li>
<li>−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.</li>
<li>+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.</li>
<li>+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.</li>
</ul>
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 <a class="reference internal" href="#zlib.error" title="zlib.error"><code class="xref py py-exc docutils literal notranslate">error</code></a> 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 <code class="xref c c-func docutils literal notranslate">malloc()</code>. The default size is 16384.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="zlib.decompressobj">
zlib.decompressobj([wbits])<a class="headerlink" href="#zlib.decompressobj" title="Permalink to this definition">¶</a></dt>
<dd>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 <a class="reference external" href="#decompress-wbits">described for decompress()</a>.
</dd></dl>

Compression objects support the following methods:
<dl class="py method">
<dt class="sig sig-object py" id="zlib.Compress.compress">
Compress.compress(string)<a class="headerlink" href="#zlib.Compress.compress" title="Permalink to this definition">¶</a></dt>
<dd>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 <a class="reference internal" href="#zlib.compress" title="zlib.compress"><code class="xref py py-meth docutils literal notranslate">compress()</code></a> method. Some input may
be kept in internal buffers for later processing.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="zlib.Compress.flush">
Compress.flush([mode])<a class="headerlink" href="#zlib.Compress.flush" title="Permalink to this definition">¶</a></dt>
<dd>All pending input is processed, and a string containing the remaining compressed
output is returned. mode can be selected from the constants
<code class="xref py py-const docutils literal notranslate">Z_SYNC_FLUSH</code>, <code class="xref py py-const docutils literal notranslate">Z_FULL_FLUSH</code>, or <code class="xref py py-const docutils literal notranslate">Z_FINISH</code>,
defaulting to <code class="xref py py-const docutils literal notranslate">Z_FINISH</code>. <code class="xref py py-const docutils literal notranslate">Z_SYNC_FLUSH</code> and
<code class="xref py py-const docutils literal notranslate">Z_FULL_FLUSH</code> allow compressing further strings of data, while
<code class="xref py py-const docutils literal notranslate">Z_FINISH</code> finishes the compressed stream and prevents compressing any
more data. After calling <a class="reference internal" href="#zlib.Compress.flush" title="zlib.Compress.flush"><code class="xref py py-meth docutils literal notranslate">flush()</code></a> with mode set to <code class="xref py py-const docutils literal notranslate">Z_FINISH</code>,
the <a class="reference internal" href="#zlib.compress" title="zlib.compress"><code class="xref py py-meth docutils literal notranslate">compress()</code></a> method cannot be called again; the only realistic action is
to delete the object.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="zlib.Compress.copy">
Compress.copy()<a class="headerlink" href="#zlib.Compress.copy" title="Permalink to this definition">¶</a></dt>
<dd>Returns a copy of the compression object. This can be used to efficiently
compress a set of data that share a common initial prefix.
<div class="versionadded">
New in version 2.5.
</div>
</dd></dl>

Decompression objects support the following methods, and two attributes:
<dl class="py attribute">
<dt class="sig sig-object py" id="zlib.Decompress.unused_data">
Decompress.unused_data<a class="headerlink" href="#zlib.Decompress.unused_data" title="Permalink to this definition">¶</a></dt>
<dd>A string which contains any bytes past the end of the compressed data. That is,
this remains <code class="docutils literal notranslate">&quot;&quot;</code> until the last byte that contains compression data is
available. If the whole string turned out to contain compressed data, this is
<code class="docutils literal notranslate">&quot;&quot;</code>, 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
<a class="reference internal" href="#zlib.decompress" title="zlib.decompress"><code class="xref py py-meth docutils literal notranslate">decompress()</code></a> method until the <a class="reference internal" href="#zlib.Decompress.unused_data" title="zlib.Decompress.unused_data"><code class="xref py py-attr docutils literal notranslate">unused_data</code></a> attribute is no longer
the empty string.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="zlib.Decompress.unconsumed_tail">
Decompress.unconsumed_tail<a class="headerlink" href="#zlib.Decompress.unconsumed_tail" title="Permalink to this definition">¶</a></dt>
<dd>A string that contains any data that was not consumed by the last
<a class="reference internal" href="#zlib.decompress" title="zlib.decompress"><code class="xref py py-meth docutils literal notranslate">decompress()</code></a> 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
<a class="reference internal" href="#zlib.decompress" title="zlib.decompress"><code class="xref py py-meth docutils literal notranslate">decompress()</code></a> method call in order to get correct output.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="zlib.Decompress.decompress">
Decompress.decompress(string[, max_length])<a class="headerlink" href="#zlib.Decompress.decompress" title="Permalink to this definition">¶</a></dt>
<dd>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
<a class="reference internal" href="#zlib.decompress" title="zlib.decompress"><code class="xref py py-meth docutils literal notranslate">decompress()</code></a> 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
<a class="reference internal" href="#zlib.Decompress.unconsumed_tail" title="zlib.Decompress.unconsumed_tail"><code class="xref py py-attr docutils literal notranslate">unconsumed_tail</code></a>. This string must be passed to a subsequent call to
<a class="reference internal" href="#zlib.decompress" title="zlib.decompress"><code class="xref py py-meth docutils literal notranslate">decompress()</code></a> if decompression is to continue. If max_length is not
supplied then the whole input is decompressed, and <a class="reference internal" href="#zlib.Decompress.unconsumed_tail" title="zlib.Decompress.unconsumed_tail"><code class="xref py py-attr docutils literal notranslate">unconsumed_tail</code></a> is an
empty string.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="zlib.Decompress.flush">
Decompress.flush([length])<a class="headerlink" href="#zlib.Decompress.flush" title="Permalink to this definition">¶</a></dt>
<dd>All pending input is processed, and a string containing the remaining
uncompressed output is returned. After calling <a class="reference internal" href="#zlib.Decompress.flush" title="zlib.Decompress.flush"><code class="xref py py-meth docutils literal notranslate">flush()</code></a>, the
<a class="reference internal" href="#zlib.decompress" title="zlib.decompress"><code class="xref py py-meth docutils literal notranslate">decompress()</code></a> method cannot be called again; the only realistic action is
to delete the object.
The optional parameter length sets the initial size of the output buffer.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="zlib.Decompress.copy">
Decompress.copy()<a class="headerlink" href="#zlib.Decompress.copy" title="Permalink to this definition">¶</a></dt>
<dd>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.
<div class="versionadded">
New in version 2.5.
</div>
</dd></dl>

<div class="admonition seealso">
See also
<dl class="simple">
<dt>Module <a class="reference internal" href="gzip.html#module-gzip" title="gzip: Interfaces for gzip compression and decompression using file objects."><code class="xref py py-mod docutils literal notranslate">gzip</code></a></dt><dd>Reading and writing gzip-format files.
</dd>
<dt><a class="reference external" href="http://www.zlib.net">http://www.zlib.net</a></dt><dd>The zlib library home page.
</dd>
<dt><a class="reference external" href="http://www.zlib.net/manual.html">http://www.zlib.net/manual.html</a></dt><dd>The zlib manual explains the semantics and usage of the library’s many
functions.
</dd>
</dl>
</div>
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