code: def underlying_symbol(self):
          
          try:
              return self.__dict__["underlying_symbol"]
          except (KeyError, ValueError):
              raise AttributeError(
                  "Instrument(order_book_id={}) has no attribute 'underlying_symbol' ".format(self.order_book_id)
              )
docs: [str] 合约标的代码，目前除股指期货(IH, IF, IC)之外的期货合约，这一字段全部为’null’（期货专用）