code: def out_deque(self):
          
          self.expire()
          expired = {k: v for (k, v) in self._tracking.items() if self.get(k) is None}
          for k, v in expired.items():
              del self._tracking[k]
              self._out_deque.append(v)
          return self._out_deque
docs: The :class:`collections.deque` to which expired items are added.