Mostly you shouldn't subclass #Python built-in types. But if you do, dict subclasses can define __missing__: it's called when a key is missing. Instead of hiding a dict in a function as a cache, how about hiding a function in a dict!? A Fibonacci dictionary:
@nedbat that’s mind blowing 🤯. How does this compare to @lru_cache ?
@juliensalort Easy: @lru_cache is a good idea, and this is not! :D