The standard way to expel stated mutable state us to push it into function parameters and returned values. With Fibonacci numbers, you cab just compute two if

The standard way to expel stated mutable state us to push it into function parameters and returned values. With Fibonacci numbers, you cab just compute two if them outright: def fib_(n: int) -> tuple[int, int]: if n == 0: return (1, 1) prev, this = fib_(n - 1) return (this, this + prev) def fib(n): return fib_(n)[0] Now there is no mutable state that survives between function calls, the performance is linear. With true memoization though accessing a previously computed value.would be constant time.