Fixed number of arguments

Fixed number of arguments#

The problem lies in the decorator cached_factory:

def cached_factory(f):
    cache = {}
    def cached_f(n):
        if n not in cache:
            cache[n] = f(n)
        return cache[n]

    return cached_f

Note how the inner function cached_f only accepts one argument n, which is then passed into the function f:

Diagram showing how the number of arguments that the decorated function accepts is tied to the number of arguments the inner function accepts, and how we assume that's a single argument.

You are assuming that the functions decorated by cached_factory only accept a single argument, and that assumption is baked right into your code! However, the function combinations_len accepts two arguments, so that’s why the decorator cached_factory doesn’t work…

But hey, you can just fix it by changing the number of arguments that cached_f accepts, right?

def cached_factory_two(f):
    cache = {}
    def cached_f(a, b):
        if (a, b) not in cache:
            cache[a, b] = f(a, b)
        return cache[a, b]

    return cached_f

This works, but now you have two decorators that are essentially the same…

Diagram showing how the two cache decorators are almost the same, up to a small difference in that one works with the value and the other works with the values and together.

Surely, there must be a way to write a single decorator that handles these two cases? Better yet, there must be a way to write a single decorator that handles functions with arbitrary numbers of arguments, right?