code: def chunklentype(self):
          
  
          x = self.file.read(8)
          if not x:
              return None
          if len(x) != 8:
              raise FormatError(
                'End of file whilst reading chunk length and type.')
          length,type = struct.unpack('!I4s', x)
          type = bytestostr(type)
          if length > 2**31-1:
              raise FormatError('Chunk %s is too large: %d.' % (type,length))
          return length,type
docs: Reads just enough of the input to determine the next
          chunk's length and type, returned as a (*length*, *type*) pair
          where *type* is a string.  If there are no more chunks, ``None``
          is returned.