code: def reserve_position(fp, fmt='I'):
      
      position = fp.tell()
      fp.seek(struct.calcsize(str('>' + fmt)), 1)
      return position
docs: Reserves the current position for write.
  
      Use with `write_position`.
  
      :param fp: file-like object
      :param fmt: format of the reserved position
      :return: the position