code: def maybe_clean(self):
          
  
          now = time.time()
          if self.next_cleaning <= now:
              keys_to_delete = []
              for (k, v) in self.data.iteritems():
                  if v.expiration <= now:
                      keys_to_delete.append(k)
              for k in keys_to_delete:
                  del self.data[k]
              now = time.time()
              self.next_cleaning = now + self.cleaning_interval
docs: Clean the cache if it's time to do so.