cpython/Doc/library/chunk.rst

:mod:`chunk` --- Read IFF chunked data
======================================

.. module:: chunk
   :synopsis: Module to read IFF chunks.

.. moduleauthor:: Sjoerd Mullender <sjoerd@acm.org>
.. sectionauthor:: Sjoerd Mullender <sjoerd@acm.org>

**Source code:** :source:`Lib/chunk.py`

.. index::
   single: Audio Interchange File Format
   single: AIFF
   single: AIFF-C
   single: Real Media File Format
   single: RMFF

--------------

This module provides an interface for reading files that use EA IFF 85 chunks.
[#]_  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 :class:`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 :exc:`EOFError` exception.


.. class:: 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 :meth:`~io.IOBase.read`.  If the methods
   :meth:`~io.IOBase.seek` and :meth:`~io.IOBase.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 :class:`Chunk` object supports the following methods:


   .. method:: getname()

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


   .. method:: getsize()

      Returns the size of the chunk.


   .. method:: close()

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

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


   .. method:: isatty()

      Returns ``False``.


   .. method:: 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.


   .. method:: tell()

      Return the current position into the chunk.


   .. method:: 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.


   .. method:: skip()

      Skip to the end of the chunk.  All further calls to :meth:`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.


.. rubric:: Footnotes

.. [#] "EA IFF 85" Standard for Interchange Format Files, Jerry Morrison, Electronic
   Arts, January 1985.