2 :mod:`chunk` --- Read IFF chunked data
3 ======================================
6 :synopsis: Module to read IFF chunks.
7 .. moduleauthor:: Sjoerd Mullender <sjoerd@acm.org>
8 .. sectionauthor:: Sjoerd Mullender <sjoerd@acm.org>
12 single: Audio Interchange File Format
15 single: Real Media File Format
18 This module provides an interface for reading files that use EA IFF 85 chunks.
19 [#]_ This format is used in at least the Audio Interchange File Format
20 (AIFF/AIFF-C) and the Real Media File Format (RMFF). The WAVE audio file format
21 is closely related and can also be read using this module.
23 A chunk has the following structure:
25 +---------+--------+-------------------------------+
26 | Offset | Length | Contents |
27 +=========+========+===============================+
29 +---------+--------+-------------------------------+
30 | 4 | 4 | Size of chunk in big-endian |
31 | | | byte order, not including the |
33 +---------+--------+-------------------------------+
34 | 8 | *n* | Data bytes, where *n* is the |
35 | | | size given in the preceding |
37 +---------+--------+-------------------------------+
38 | 8 + *n* | 0 or 1 | Pad byte needed if *n* is odd |
39 | | | and chunk alignment is used |
40 +---------+--------+-------------------------------+
42 The ID is a 4-byte string which identifies the type of chunk.
44 The size field (a 32-bit value, encoded using big-endian byte order) gives the
45 size of the chunk data, not including the 8-byte header.
47 Usually an IFF-type file consists of one or more chunks. The proposed usage of
48 the :class:`Chunk` class defined here is to instantiate an instance at the start
49 of each chunk and read from the instance until it reaches the end, after which a
50 new instance can be instantiated. At the end of the file, creating a new
51 instance will fail with a :exc:`EOFError` exception.
54 .. class:: Chunk(file[, align, bigendian, inclheader])
56 Class which represents a chunk. The *file* argument is expected to be a
57 file-like object. An instance of this class is specifically allowed. The
58 only method that is needed is :meth:`read`. If the methods :meth:`seek` and
59 :meth:`tell` are present and don't raise an exception, they are also used.
60 If these methods are present and raise an exception, they are expected to not
61 have altered the object. If the optional argument *align* is true, chunks
62 are assumed to be aligned on 2-byte boundaries. If *align* is false, no
63 alignment is assumed. The default value is true. If the optional argument
64 *bigendian* is false, the chunk size is assumed to be in little-endian order.
65 This is needed for WAVE audio files. The default value is true. If the
66 optional argument *inclheader* is true, the size given in the chunk header
67 includes the size of the header. The default value is false.
69 A :class:`Chunk` object supports the following methods:
72 .. method:: Chunk.getname()
74 Returns the name (ID) of the chunk. This is the first 4 bytes of the chunk.
77 .. method:: Chunk.getsize()
79 Returns the size of the chunk.
82 .. method:: Chunk.close()
84 Close and skip to the end of the chunk. This does not close the underlying
87 The remaining methods will raise :exc:`IOError` if called after the
88 :meth:`close` method has been called.
91 .. method:: Chunk.isatty()
96 .. method:: Chunk.seek(pos[, whence])
98 Set the chunk's current position. The *whence* argument is optional and
99 defaults to ``0`` (absolute file positioning); other values are ``1`` (seek
100 relative to the current position) and ``2`` (seek relative to the file's end).
101 There is no return value. If the underlying file does not allow seek, only
102 forward seeks are allowed.
105 .. method:: Chunk.tell()
107 Return the current position into the chunk.
110 .. method:: Chunk.read([size])
112 Read at most *size* bytes from the chunk (less if the read hits the end of the
113 chunk before obtaining *size* bytes). If the *size* argument is negative or
114 omitted, read all data until the end of the chunk. The bytes are returned as a
115 string object. An empty string is returned when the end of the chunk is
116 encountered immediately.
119 .. method:: Chunk.skip()
121 Skip to the end of the chunk. All further calls to :meth:`read` for the chunk
122 will return ``''``. If you are not interested in the contents of the chunk,
123 this method should be called so that the file points to the start of the next
126 .. rubric:: Footnotes
128 .. [#] "EA IFF 85" Standard for Interchange Format Files, Jerry Morrison, Electronic