code: def prev_close(self):
          
          try:
              return self._data['prev_close']
          except (ValueError, KeyError):
              pass
  
          if self._prev_close is None:
              trading_dt = Environment.get_instance().trading_dt
              data_proxy = Environment.get_instance().data_proxy
              self._prev_close = data_proxy.get_prev_close(self._instrument.order_book_id, trading_dt)
          return self._prev_close
docs: [float] 昨日收盘价