code: def _parse_this_box(self, fptr, box_id, start, num_bytes):
          
          try:
              parser = _BOX_WITH_ID[box_id].parse
  
          except KeyError:
              # We don't recognize the box ID, so create an UnknownBox and be
              # done with it.
              msg = ('Unrecognized box ({box_id}) encountered at byte offset '
                     '{offset}.')
              msg = msg.format(box_id=box_id, offset=fptr.tell() - 8)
              warnings.warn(msg, UserWarning)
              box = UnknownBox(box_id, offset=start, length=num_bytes,
                               longname='Unknown')
  
              return box
  
          try:
              box = parser(fptr, start, num_bytes)
          except ValueError as err:
              msg = ("Encountered an unrecoverable ValueError while parsing a "
                     "{box_id} box at byte offset {offset}.  The original error "
                     "message was \"{original_error_message}\".")
              msg = msg.format(box_id=_BOX_WITH_ID[box_id].longname,
                               offset=start,
                               original_error_message=str(err))
              warnings.warn(msg, UserWarning)
              box = UnknownBox(box_id.decode('utf-8'),
                               length=num_bytes,
                               offset=start, longname='Unknown')
  
          return box
docs: Parse the current box.
  
          Parameters
          ----------
          fptr : file
              Open file object, currently points to start of box payload, not the
              start of the box.
          box_id : str
              4-letter identifier for the current box.
          start, num_bytes : int
              Byte offset and length of the current box.
  
          Returns
          -------
          Jp2kBox
              Object corresponding to the current box.