Edit File: chunk.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>21.6. chunk — Read IFF chunked data — Python v2.6.6 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.6.6', COLLAPSE_MODINDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python v2.6.6 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python v2.6.6 documentation" href="../index.html" /> <link rel="up" title="21. 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Multimedia Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <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> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="wave.html" title="previous chapter">21.5. <tt class="docutils literal"><span class="pre">wave</span></tt> — Read and write WAV files</a></p> <h4>Next topic</h4> <p class="topless"><a href="colorsys.html" title="next chapter">21.7. <tt class="docutils literal"><span class="pre">colorsys</span></tt> — Conversions between color systems</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/chunk.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" >modules</a> |</li> <li class="right" > <a href="colorsys.html" title="21.7. colorsys — Conversions between color systems" >next</a> |</li> <li class="right" > <a href="wave.html" title="21.5. wave — Read and write WAV files" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="mm.html" >21. 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