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(python2.1-lib.info)chunk

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Read IFF chunked data
=====================

Module to read IFF chunks.  This module was written by Sjoerd Mullender
<sjoerd@acm.org>.
This manual section was written by Sjoerd Mullender <sjoerd@acm.org>.
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 a `EOFError' exception.

`Chunk(file[, align, bigendian, inclheader])'
     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 `IOError' if called after the
`close()' method has been called.

`isatty()'
     Returns `0'.

`seek(pos[, whence])'
     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])'
     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.  The bytes are returned as a string object.  An empty
     string 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 `'''.  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.