JezK
Edit File: 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 a 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. """ class Chunk: def __init__(self, file, align=True, bigendian=True, inclheader=False): import struct self.closed = False self.align = align # whether to align to word (2-byte) boundaries if bigendian: strflag = '>' else: strflag = '<' self.file = file self.chunkname = file.read(4) if len(self.chunkname) < 4: raise EOFError try: self.chunksize = struct.unpack(strflag+'L', file.read(4))[0] except struct.error: raise EOFError if inclheader: self.chunksize = self.chunksize - 8 # subtract header self.size_read = 0 try: self.offset = self.file.tell() except (AttributeError, IOError): self.seekable = False else: self.seekable = True def getname(self): """Return the name (ID) of the current chunk.""" return self.chunkname def getsize(self): """Return the size of the current chunk.""" return self.chunksize def close(self): if not self.closed: self.skip() self.closed = True def isatty(self): if self.closed: raise ValueError, "I/O operation on closed file" return False def seek(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. """ if self.closed: raise ValueError, "I/O operation on closed file" if not self.seekable: raise IOError, "cannot seek" if whence == 1: pos = pos + self.size_read elif whence == 2: pos = pos + self.chunksize if pos < 0 or pos > self.chunksize: raise RuntimeError self.file.seek(self.offset + pos, 0) self.size_read = pos def tell(self): if self.closed: raise ValueError, "I/O operation on closed file" return self.size_read def read(self, size=-1): """Read at most size bytes from the chunk. If size is omitted or negative, read until the end of the chunk. """ if self.closed: raise ValueError, "I/O operation on closed file" if self.size_read >= self.chunksize: return '' if size < 0: size = self.chunksize - self.size_read if size > self.chunksize - self.size_read: size = self.chunksize - self.size_read data = self.file.read(size) self.size_read = self.size_read + len(data) if self.size_read == self.chunksize and \ self.align and \ (self.chunksize & 1): dummy = self.file.read(1) self.size_read = self.size_read + len(dummy) return data def skip(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. """ if self.closed: raise ValueError, "I/O operation on closed file" if self.seekable: try: n = self.chunksize - self.size_read # maybe fix alignment if self.align and (self.chunksize & 1): n = n + 1 self.file.seek(n, 1) self.size_read = self.size_read + n return except IOError: pass while self.size_read < self.chunksize: n = min(8192, self.chunksize - self.size_read) dummy = self.read(n) if not dummy: raise EOFError