def lru(self):
        """ Returns the Least Recently Used key """
        if self._maxsize:
            self._lock.acquire()
            try:
                return self._head._next._key
            finally:
                self._lock.release()
        else:
            return None
 Returns the Least Recently Used key 
recipes/Python/302997_Threadsafe_caching_object_file_HTTP/recipe-302997.py