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