I don't believe it is.  lru_cache only guarantees that you will get the same result back for identical arguments, not that the function will only be called once.  Seems to me if you call it, then in the middle of caching the value, there's a thread change, you could get to the function wrapped by lru_cache twice (or more times).  In order to implement once, it needs to contain a thread lock to ensure its "once" moniker and support the singleton pattern for which it is currently being used (apparently incorrectly) in django and other places.  Am I understanding threading correctly here?