code: def board_type(self):
          
          try:
              return self.__dict__["board_type"]
          except (KeyError, ValueError):
              raise AttributeError(
                  "Instrument(order_book_id={}) has no attribute 'board_type' ".format(self.order_book_id)
              )
docs: [str] 板块类别，’MainBoard’ - 主板,’GEM’ - 创业板（股票专用）