22.5. "chunk" --- Read IFF chunked data
***************************************

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

======================================================================

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.

一个chunk具有以下结构:

+-----------+----------+---------------------------------+
| 偏移      | 长度     | 内容                            |
+===========+==========+=================================+
| 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.

-[ 脚注 ]-

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