code: def universe(self):
          
          # avoid windowing every time
          # if calling and on same date return
          # cached value
          if self.now == self._last_chk:
              return self._funiverse
          else:
              self._last_chk = self.now
              self._funiverse = self._universe.loc[:self.now]
              return self._funiverse
docs: Data universe available at the current time.
          Universe contains the data passed in when creating a Backtest.
          Use this data to determine strategy logic.