chunk.py

"""Simple class to read IFF chunks.

An IFF chunk (used in formats such as AIFF, TIFF, RMFF (RealMedia File
Format)) has the following structure:

+----------------+
| ID (4 bytes) |
+----------------+
| size (4 bytes) |
+----------------+
| data |
| ... |
+----------------+

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 whole chunk, 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.

Usage:
while True:
 try:
 chunk = Chunk(file)
 except EOFError:
 break
 chunktype = chunk.getname()
 while True:
 data = chunk.read(nbytes)
 if not data:
 pass
 # do something with data

The interface is file-like. The implemented methods are:
read, close, seek, tell, isatty.
Extra methods are: skip() (called by close, skips to the end of the chunk),
getname() (returns the name (ID) of the chunk)

The __init__ method has one required argument, a file-like object
(including a chunk instance), and one optional argument, a flag which
specifies whether or not chunks are aligned on 2-byte boundaries. The
default is 1, i.e. aligned.
"""

classChunk:
def__init__(self, file, align=True, bigendian=True, inclheader=False):
importstruct
self.closed=False
self.align=align# whether to align to word (2-byte) boundaries
ifbigendian:
strflag='>'
else:
strflag='<'
self.file=file
self.chunkname=file.read(4)
iflen(self.chunkname) <4:
raiseEOFError
try:
self.chunksize=struct.unpack_from(strflag+'L', file.read(4))[0]
exceptstruct.error:
raiseEOFErrorfromNone
ifinclheader:
self.chunksize=self.chunksize-8# subtract header
self.size_read=0
try:
self.offset=self.file.tell()
except (AttributeError, OSError):
self.seekable=False
else:
self.seekable=True

defgetname(self):
"""Return the name (ID) of the current chunk."""
returnself.chunkname

defgetsize(self):
"""Return the size of the current chunk."""
returnself.chunksize

defclose(self):
ifnotself.closed:
try:
self.skip()
finally:
self.closed=True

defisatty(self):
ifself.closed:
raiseValueError("I/O operation on closed file")
returnFalse

defseek(self, pos, whence=0):
"""Seek to specified position into the chunk.
 Default position is 0 (start of chunk).
 If the file is not seekable, this will result in an error.
 """

ifself.closed:
raiseValueError("I/O operation on closed file")
ifnotself.seekable:
raiseOSError("cannot seek")
ifwhence==1:
pos=pos+self.size_read
elifwhence==2:
pos=pos+self.chunksize
ifpos<0orpos>self.chunksize:
raiseRuntimeError
self.file.seek(self.offset+pos, 0)
self.size_read=pos

deftell(self):
ifself.closed:
raiseValueError("I/O operation on closed file")
returnself.size_read

defread(self, size=-1):
"""Read at most size bytes from the chunk.
 If size is omitted or negative, read until the end
 of the chunk.
 """

ifself.closed:
raiseValueError("I/O operation on closed file")
ifself.size_read>=self.chunksize:
returnb''
ifsize<0:
size=self.chunksize-self.size_read
ifsize>self.chunksize-self.size_read:
size=self.chunksize-self.size_read
data=self.file.read(size)
self.size_read=self.size_read+len(data)
ifself.size_read==self.chunksizeand \
self.alignand \
 (self.chunksize&1):
dummy=self.file.read(1)
self.size_read=self.size_read+len(dummy)
returndata

defskip(self):
"""Skip the rest of the chunk.
 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.
 """

ifself.closed:
raiseValueError("I/O operation on closed file")
ifself.seekable:
try:
n=self.chunksize-self.size_read
# maybe fix alignment
ifself.alignand (self.chunksize&1):
n=n+1
self.file.seek(n, 1)
self.size_read=self.size_read+n
return
exceptOSError:
pass
whileself.size_read<self.chunksize:
n=min(8192, self.chunksize-self.size_read)
dummy=self.read(n)
ifnotdummy:
raiseEOFError