"if in" is not the preferred way to check for the presence of a key in a dict. You should use setdefault. The thing is, you aren't searching for a key in a dict

"if in" is not the preferred way to check for the presence of a key in a dict. You should use setdefault. The thing is, you aren't searching for a key in a dict only. You're memoizing. And try/except is the most efficient way to do that. This article isn't teaching you how to program, it's teaching decorators.