22.5. chunk — Read IFF chunked data

Source code: Lib/chunk.py

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This module provides an interface for reading files that use EA IFF 85 chunks. 1 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.

A chunk has the following structure:

Offset	Length	Contents
0	4	Chunk ID
4	4	Size of chunk in big-endian byte order, not including the header
8	n	Data bytes, where n is the size given in the preceding field
8 + n	0 or 1	Pad byte needed if n is odd and chunk alignment is used

The ID is a 4-byte string which identifies the type of chunk.

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.

Usually an IFF-type file consists of one or more chunks. The proposed usage of the Chunk 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 an EOFError exception.

class chunk.Chunk(file, align=True, bigendian=True, inclheader=False)

Class which represents a chunk. The file argument is expected to be a file-like object. An instance of this class is specifically allowed. The only method that is needed is read(). If the methods seek() and tell() 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 align is true, chunks are assumed to be aligned on 2-byte boundaries. If align is false, no alignment is assumed. The default value is true. If the optional argument bigendian 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 inclheader is true, the size given in the chunk header includes the size of the header. The default value is false.

A Chunk object supports the following methods:

getname()

Returns the name (ID) of the chunk. This is the first 4 bytes of the chunk.

getsize()

Returns the size of the chunk.

close()

Close and skip to the end of the chunk. This does not close the underlying file.

The remaining methods will raise OSError if called after the close() method has been called. Before Python 3.3, they used to raise IOError, now an alias of OSError.

isatty()

Returns False.

seek(pos, whence=0)

Set the chunk’s current position. The whence argument is optional and defaults to 0 (absolute file positioning); other values are 1 (seek relative to the current position) and 2 (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.

tell()

Return the current position into the chunk.

read(size=-1)

Read at most size bytes from the chunk (less if the read hits the end of the chunk before obtaining size bytes). If the size argument is negative or omitted, read all data until the end of the chunk. An empty bytes object is returned when the end of the chunk is encountered immediately.

skip()

Skip to the end of the chunk. All further calls to read() for the chunk will return b''. 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.

Footnotes

1

《EA IFF 85》 Standard for Interchange Format Files, Jerry Morrison, Electronic Arts, January 1985.