code: def _save(self):
          
  
          if self.__modified_flag:
              self.__filename_rep.update_id_counter()
              indexfilename = os.path.join(self.__dir, "index.dat")
              self._write_file(
                  indexfilename,
                  (self.__index,
                   self.__filename_rep))
  
              self.__modified_flag = False
docs: save the cache index, in case it was modified.
  
          Saves the index table and the file name repository in the file
          `index.dat`