code: def query_symbol(self, asset: str) -> str:
          
          contract_address = self.get_asset_address(asset)
          method = 'symbol'
          invoke_code = build_native_invoke_code(contract_address, b'\x00', method, bytearray())
          tx = Transaction(0, 0xd1, int(time()), 0, 0, None, invoke_code, bytearray(), list())
          response = self.__sdk.rpc.send_raw_transaction_pre_exec(tx)
          symbol = ContractDataParser.to_utf8_str(response['Result'])
          return symbol
docs: This interface is used to query the asset's symbol of ONT or ONG.
  
          :param asset: a string which is used to indicate which asset's symbol we want to get.
          :return: asset's symbol in the form of string.