Agree with that "try except" is better than "if else" for file handling ("with" is even better).

I am not sure if "try except" is really faster than "if else" in some edge cases in a memoization context, as you claim.

What I am sure is that in a didactic context, where you want people to understand code, something like:

    if args not in stored_results:
        stored_results[args] = fn(*args)

    return stored_results[args]

is much better than that (from the OP):

    try:
        # try to get the cached result
        return stored_results[args]
    except KeyError:
        # nothing was cached for those args. let's fix that.
        result = stored_results[args] = fn(*args)
        return result