For a memoize function, try/except is perfect. You gain a slight speed benefit from function calls that are memoized as compared to the "if x not in y", and sin

For a memoize function, try/except is perfect. You gain a slight speed benefit from function calls that are memoized as compared to the "if x not in y", and since memoize is all about speeding up repetitive function calls, it's the most optimal choice. Using the "if x not in y" you are giving up speed of the "x is actually in the y" in exchange for a cheaper than "except", however if you are memoizing a function, chances are that the function is going to eat up processing power making your optimization of the "exceptional case" at the expense of the "normal case" not worth it. As far as people understanding code - this is a common pattern in python code, try / except should be easy for anyone to understand.