code: def set_pre_handler(self, handler):
          '''pre handler push
          return: ret_error or ret_ok
          '''
          set_flag = False
          for protoc in self._pre_handler_table:
              if isinstance(handler, self._pre_handler_table[protoc]["type"]):
                  self._pre_handler_table[protoc]["obj"] = handler
                  return RET_OK
  
          if set_flag is False:
              return RET_ERROf set_pre_handler(self, handler):
          '''pre handler push
          return: ret_error or ret_ok
          '''
          set_flag = False
          for protoc in self._pre_handler_table:
              if isinstance(handler, self._pre_handler_table[protoc]["type"]):
                  self._pre_handler_table[protoc]["obj"] = handler
                  return RET_OK
  
          if set_flag is False:
              return RET_ERROR
docs: pre handler push
          return: ret_error or ret_ok