code: def margin_rate(self):
          
          try:
              return self.__dict__["margin_rate"]
          except (KeyError, ValueError):
              raise AttributeError(
                  "Instrument(order_book_id={}) has no attribute 'margin_rate' ".format(self.order_book_id)
              )
docs: [float] 合约最低保证金率（期货专用）