It's a feature! Use this memoization pattern and you'll never forget it

  def foo(x, cache={}):
    if x in cache:
      return cache[x]
    val = cache[x] = x*x+1
    return val

But, be warned, there's no mechanism for cache eviction; use @lru_cache if you don't know ahead of time that x will take a reasonably small number of values