1:mod:`chunk` --- Read IFF chunked data
2======================================
3
4.. module:: chunk
5   :synopsis: Module to read IFF chunks.
6
7.. moduleauthor:: Sjoerd Mullender <sjoerd@acm.org>
8.. sectionauthor:: Sjoerd Mullender <sjoerd@acm.org>
9
10**Source code:** :source:`Lib/chunk.py`
11
12.. index::
13   single: Audio Interchange File Format
14   single: AIFF
15   single: AIFF-C
16   single: Real Media File Format
17   single: RMFF
18
19--------------
20
21This module provides an interface for reading files that use EA IFF 85 chunks.
22[#]_  This format is used in at least the Audio Interchange File Format
23(AIFF/AIFF-C) and the Real Media File Format (RMFF).  The WAVE audio file format
24is closely related and can also be read using this module.
25
26A chunk has the following structure:
27
28+---------+--------+-------------------------------+
29| Offset  | Length | Contents                      |
30+=========+========+===============================+
31| 0       | 4      | Chunk ID                      |
32+---------+--------+-------------------------------+
33| 4       | 4      | Size of chunk in big-endian   |
34|         |        | byte order, not including the |
35|         |        | header                        |
36+---------+--------+-------------------------------+
37| 8       | *n*    | Data bytes, where *n* is the  |
38|         |        | size given in the preceding   |
39|         |        | field                         |
40+---------+--------+-------------------------------+
41| 8 + *n* | 0 or 1 | Pad byte needed if *n* is odd |
42|         |        | and chunk alignment is used   |
43+---------+--------+-------------------------------+
44
45The ID is a 4-byte string which identifies the type of chunk.
46
47The size field (a 32-bit value, encoded using big-endian byte order) gives the
48size of the chunk data, not including the 8-byte header.
49
50Usually an IFF-type file consists of one or more chunks.  The proposed usage of
51the :class:`Chunk` class defined here is to instantiate an instance at the start
52of each chunk and read from the instance until it reaches the end, after which a
53new instance can be instantiated. At the end of the file, creating a new
54instance will fail with an :exc:`EOFError` exception.
55
56
57.. class:: Chunk(file, align=True, bigendian=True, inclheader=False)
58
59   Class which represents a chunk.  The *file* argument is expected to be a
60   file-like object.  An instance of this class is specifically allowed.  The
61   only method that is needed is :meth:`~io.IOBase.read`.  If the methods
62   :meth:`~io.IOBase.seek` and :meth:`~io.IOBase.tell` are present and don't
63   raise an exception, they are also used.
64   If these methods are present and raise an exception, they are expected to not
65   have altered the object.  If the optional argument *align* is true, chunks
66   are assumed to be aligned on 2-byte boundaries.  If *align* is false, no
67   alignment is assumed.  The default value is true.  If the optional argument
68   *bigendian* is false, the chunk size is assumed to be in little-endian order.
69   This is needed for WAVE audio files. The default value is true.  If the
70   optional argument *inclheader* is true, the size given in the chunk header
71   includes the size of the header.  The default value is false.
72
73   A :class:`Chunk` object supports the following methods:
74
75
76   .. method:: getname()
77
78      Returns the name (ID) of the chunk.  This is the first 4 bytes of the
79      chunk.
80
81
82   .. method:: getsize()
83
84      Returns the size of the chunk.
85
86
87   .. method:: close()
88
89      Close and skip to the end of the chunk.  This does not close the
90      underlying file.
91
92   The remaining methods will raise :exc:`OSError` if called after the
93   :meth:`close` method has been called.  Before Python 3.3, they used to
94   raise :exc:`IOError`, now an alias of :exc:`OSError`.
95
96
97   .. method:: isatty()
98
99      Returns ``False``.
100
101
102   .. method:: seek(pos, whence=0)
103
104      Set the chunk's current position.  The *whence* argument is optional and
105      defaults to ``0`` (absolute file positioning); other values are ``1``
106      (seek relative to the current position) and ``2`` (seek relative to the
107      file's end).  There is no return value. If the underlying file does not
108      allow seek, only forward seeks are allowed.
109
110
111   .. method:: tell()
112
113      Return the current position into the chunk.
114
115
116   .. method:: read(size=-1)
117
118      Read at most *size* bytes from the chunk (less if the read hits the end of
119      the chunk before obtaining *size* bytes).  If the *size* argument is
120      negative or omitted, read all data until the end of the chunk.  An empty
121      bytes object is returned when the end of the chunk is encountered
122      immediately.
123
124
125   .. method:: skip()
126
127      Skip to the end of the chunk.  All further calls to :meth:`read` for the
128      chunk will return ``b''``.  If you are not interested in the contents of
129      the chunk, this method should be called so that the file points to the
130      start of the next chunk.
131
132
133.. rubric:: Footnotes
134
135.. [#] "EA IFF 85" Standard for Interchange Format Files, Jerry Morrison, Electronic
136   Arts, January 1985.
137
138