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<div class="section" id="module-chunk">
<h1>21.6. <tt class="xref docutils literal"><span class="pre">chunk</span></tt> — Read IFF chunked data<a class="headerlink" href="#module-chunk" title="Permalink to this headline">¶</a></h1>
<p id="index-236">This module provides an interface for reading files that use EA IFF 85 chunks.
<a class="footnote-reference" href="#id2" id="id1">[1]</a> This format is used in at least the Audio Interchange File Format
(AIFF/AIFF-C) and the Real Media File Format (RMFF). The WAVE audio file format
is closely related and can also be read using this module.</p>
<p>A chunk has the following structure:</p>
<table border="1" class="docutils">
<colgroup>
<col width="19%" />
<col width="17%" />
<col width="65%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Offset</th>
<th class="head">Length</th>
<th class="head">Contents</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>0</td>
<td>4</td>
<td>Chunk ID</td>
</tr>
<tr><td>4</td>
<td>4</td>
<td>Size of chunk in big-endian
byte order, not including the
header</td>
</tr>
<tr><td>8</td>
<td><em>n</em></td>
<td>Data bytes, where <em>n</em> is the
size given in the preceding
field</td>
</tr>
<tr><td>8 + <em>n</em></td>
<td>0 or 1</td>
<td>Pad byte needed if <em>n</em> is odd
and chunk alignment is used</td>
</tr>
</tbody>
</table>
<p>The ID is a 4-byte string which identifies the type of chunk.</p>
<p>The size field (a 32-bit value, encoded using big-endian byte order) gives the
size of the chunk data, not including the 8-byte header.</p>
<p>Usually an IFF-type file consists of one or more chunks. The proposed usage of
the <a title="chunk.Chunk" class="reference internal" href="#chunk.Chunk"><tt class="xref docutils literal"><span class="pre">Chunk</span></tt></a> class defined here is to instantiate an instance at the start
of each chunk and read from the instance until it reaches the end, after which a
new instance can be instantiated. At the end of the file, creating a new
instance will fail with a <a title="exceptions.EOFError" class="reference external" href="exceptions.html#exceptions.EOFError"><tt class="xref docutils literal"><span class="pre">EOFError</span></tt></a> exception.</p>
<dl class="class">
<dt id="chunk.Chunk">
<em class="property">class </em><tt class="descclassname">chunk.</tt><tt class="descname">Chunk</tt><big>(</big><em>file</em><span class="optional">[</span>, <em>align</em>, <em>bigendian</em>, <em>inclheader</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#chunk.Chunk" title="Permalink to this definition">¶</a></dt>
<dd><p>Class which represents a chunk. The <em>file</em> argument is expected to be a
file-like object. An instance of this class is specifically allowed. The
only method that is needed is <a title="chunk.Chunk.read" class="reference internal" href="#chunk.Chunk.read"><tt class="xref docutils literal"><span class="pre">read()</span></tt></a>. If the methods <a title="chunk.Chunk.seek" class="reference internal" href="#chunk.Chunk.seek"><tt class="xref docutils literal"><span class="pre">seek()</span></tt></a> and
<a title="chunk.Chunk.tell" class="reference internal" href="#chunk.Chunk.tell"><tt class="xref docutils literal"><span class="pre">tell()</span></tt></a> are present and don’t raise an exception, they are also used.
If these methods are present and raise an exception, they are expected to not
have altered the object. If the optional argument <em>align</em> is true, chunks
are assumed to be aligned on 2-byte boundaries. If <em>align</em> is false, no
alignment is assumed. The default value is true. If the optional argument
<em>bigendian</em> is false, the chunk size is assumed to be in little-endian order.
This is needed for WAVE audio files. The default value is true. If the
optional argument <em>inclheader</em> is true, the size given in the chunk header
includes the size of the header. The default value is false.</p>
<p>A <a title="chunk.Chunk" class="reference internal" href="#chunk.Chunk"><tt class="xref docutils literal"><span class="pre">Chunk</span></tt></a> object supports the following methods:</p>
<dl class="method">
<dt id="chunk.Chunk.getname">
<tt class="descname">getname</tt><big>(</big><big>)</big><a class="headerlink" href="#chunk.Chunk.getname" title="Permalink to this definition">¶</a></dt>
<dd>Returns the name (ID) of the chunk. This is the first 4 bytes of the
chunk.</dd></dl>
<dl class="method">
<dt id="chunk.Chunk.getsize">
<tt class="descname">getsize</tt><big>(</big><big>)</big><a class="headerlink" href="#chunk.Chunk.getsize" title="Permalink to this definition">¶</a></dt>
<dd>Returns the size of the chunk.</dd></dl>
<dl class="method">
<dt id="chunk.Chunk.close">
<tt class="descname">close</tt><big>(</big><big>)</big><a class="headerlink" href="#chunk.Chunk.close" title="Permalink to this definition">¶</a></dt>
<dd>Close and skip to the end of the chunk. This does not close the
underlying file.</dd></dl>
<p>The remaining methods will raise <a title="exceptions.IOError" class="reference external" href="exceptions.html#exceptions.IOError"><tt class="xref docutils literal"><span class="pre">IOError</span></tt></a> if called after the
<a title="chunk.Chunk.close" class="reference internal" href="#chunk.Chunk.close"><tt class="xref docutils literal"><span class="pre">close()</span></tt></a> method has been called.</p>
<dl class="method">
<dt id="chunk.Chunk.isatty">
<tt class="descname">isatty</tt><big>(</big><big>)</big><a class="headerlink" href="#chunk.Chunk.isatty" title="Permalink to this definition">¶</a></dt>
<dd>Returns <tt class="xref docutils literal"><span class="pre">False</span></tt>.</dd></dl>
<dl class="method">
<dt id="chunk.Chunk.seek">
<tt class="descname">seek</tt><big>(</big><em>pos</em><span class="optional">[</span>, <em>whence</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#chunk.Chunk.seek" title="Permalink to this definition">¶</a></dt>
<dd>Set the chunk’s current position. The <em>whence</em> argument is optional and
defaults to <tt class="docutils literal"><span class="pre">0</span></tt> (absolute file positioning); other values are <tt class="docutils literal"><span class="pre">1</span></tt>
(seek relative to the current position) and <tt class="docutils literal"><span class="pre">2</span></tt> (seek relative to the
file’s end). There is no return value. If the underlying file does not
allow seek, only forward seeks are allowed.</dd></dl>
<dl class="method">
<dt id="chunk.Chunk.tell">
<tt class="descname">tell</tt><big>(</big><big>)</big><a class="headerlink" href="#chunk.Chunk.tell" title="Permalink to this definition">¶</a></dt>
<dd>Return the current position into the chunk.</dd></dl>
<dl class="method">
<dt id="chunk.Chunk.read">
<tt class="descname">read</tt><big>(</big><span class="optional">[</span><em>size</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#chunk.Chunk.read" title="Permalink to this definition">¶</a></dt>
<dd>Read at most <em>size</em> bytes from the chunk (less if the read hits the end of
the chunk before obtaining <em>size</em> bytes). If the <em>size</em> argument is
negative or omitted, read all data until the end of the chunk. The bytes
are returned as a string object. An empty string is returned when the end
of the chunk is encountered immediately.</dd></dl>
<dl class="method">
<dt id="chunk.Chunk.skip">
<tt class="descname">skip</tt><big>(</big><big>)</big><a class="headerlink" href="#chunk.Chunk.skip" title="Permalink to this definition">¶</a></dt>
<dd>Skip to the end of the chunk. All further calls to <a title="chunk.Chunk.read" class="reference internal" href="#chunk.Chunk.read"><tt class="xref docutils literal"><span class="pre">read()</span></tt></a> for the
chunk will return <tt class="docutils literal"><span class="pre">''</span></tt>. If you are not interested in the contents of
the chunk, this method should be called so that the file points to the
start of the next chunk.</dd></dl>
</dd></dl>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>“EA IFF 85” Standard for Interchange Format Files, Jerry Morrison, Electronic
Arts, January 1985.</td></tr>
</tbody>
</table>
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