code: def prev_settlement(self):
          
          try:
              return self._data['prev_settlement']
          except (ValueError, KeyError):
              pass
  
          if self._prev_settlement is None:
              trading_dt = Environment.get_instance().trading_dt
              data_proxy = Environment.get_instance().data_proxy
              self._prev_settlement = data_proxy.get_prev_settlement(self._instrument.order_book_id, trading_dt)
          return self._prev_settlement
docs: [float] 昨日结算价（期货专用）