"""Simple class to read IFF chunks.

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An IFF chunk (used in formats such as AIFF, TIFF, RMFF (RealMedia File

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Format)) has the following structure:

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+----------------+

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| ID (4 bytes) |

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+----------------+

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| size (4 bytes) |

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+----------------+

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| data |

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| ... |

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+----------------+

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

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The size field (a 32-bit value, encoded using big-endian byte order)

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gives the size of the whole chunk, including the 8-byte header.

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Usually an IFF-type file consists of one or more chunks. The proposed

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usage of the Chunk class defined here is to instantiate an instance at

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the start of each chunk and read from the instance until it reaches

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the end, after which a new instance can be instantiated. At the end

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of the file, creating a new instance will fail with an EOFError

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exception.

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Usage:

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while True:

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try:

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chunk = Chunk(file)

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except EOFError:

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break

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chunktype = chunk.getname()

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while True:

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data = chunk.read(nbytes)

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if not data:

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pass

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# do something with data

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The interface is file-like. The implemented methods are:

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read, close, seek, tell, isatty.

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Extra methods are: skip() (called by close, skips to the end of the chunk),

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getname() (returns the name (ID) of the chunk)

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The __init__ method has one required argument, a file-like object

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(including a chunk instance), and one optional argument, a flag which

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specifies whether or not chunks are aligned on 2-byte boundaries. The

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default is 1, i.e. aligned.

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"""

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class Chunk:

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def __init__(self, file, align=True, bigendian=True, inclheader=False):

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import struct

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self.closed = False

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self.align = align # whether to align to word (2-byte) boundaries

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if bigendian:

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strflag = '>'

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else:

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strflag = '<'

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self.file = file

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self.chunkname = file.read(4)

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if len(self.chunkname) < 4:

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raise EOFError

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try:

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self.chunksize = struct.unpack_from(strflag+'L', file.read(4))[0]

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except struct.error:

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raise EOFError

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if inclheader:

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self.chunksize = self.chunksize - 8 # subtract header

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self.size_read = 0

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try:

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self.offset = self.file.tell()

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except (AttributeError, OSError):

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self.seekable = False

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else:

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self.seekable = True

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def getname(self):

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"""Return the name (ID) of the current chunk."""

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return self.chunkname

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def getsize(self):

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"""Return the size of the current chunk."""

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return self.chunksize

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def close(self):

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if not self.closed:

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try:

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self.skip()

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finally:

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self.closed = True

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def isatty(self):

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if self.closed:

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raise ValueError("I/O operation on closed file")

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return False

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def seek(self, pos, whence=0):

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"""Seek to specified position into the chunk.

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Default position is 0 (start of chunk).

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If the file is not seekable, this will result in an error.

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"""

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if self.closed:

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raise ValueError("I/O operation on closed file")

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if not self.seekable:

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raise OSError("cannot seek")

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if whence == 1:

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pos = pos + self.size_read

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elif whence == 2:

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pos = pos + self.chunksize

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if pos < 0 or pos > self.chunksize:

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raise RuntimeError

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self.file.seek(self.offset + pos, 0)

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self.size_read = pos

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def tell(self):

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if self.closed:

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raise ValueError("I/O operation on closed file")

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return self.size_read

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def read(self, size=-1):

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"""Read at most size bytes from the chunk.

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If size is omitted or negative, read until the end

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of the chunk.

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"""

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if self.closed:

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raise ValueError("I/O operation on closed file")

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if self.size_read >= self.chunksize:

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return b''

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if size < 0:

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size = self.chunksize - self.size_read

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if size > self.chunksize - self.size_read:

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size = self.chunksize - self.size_read

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data = self.file.read(size)

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self.size_read = self.size_read + len(data)

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if self.size_read == self.chunksize and \

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self.align and \

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(self.chunksize & 1):

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dummy = self.file.read(1)

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self.size_read = self.size_read + len(dummy)

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return data

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def skip(self):

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"""Skip the rest of the chunk.

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If you are not interested in the contents of the chunk,

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this method should be called so that the file points to

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the start of the next chunk.

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"""

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if self.closed:

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raise ValueError("I/O operation on closed file")

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if self.seekable:

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try:

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n = self.chunksize - self.size_read

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# maybe fix alignment

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if self.align and (self.chunksize & 1):

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n = n + 1

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self.file.seek(n, 1)

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self.size_read = self.size_read + n

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return

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except OSError:

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pass

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while self.size_read < self.chunksize:

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n = min(8192, self.chunksize - self.size_read)

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dummy = self.read(n)

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if not dummy:

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raise EOFError

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