The title more or less says it all:

I have a function which takes symmetric input in two arguments, e.g. something like

def f(a1, a2):
    return heavy_stuff(abs(a1 - a2))

Now, I want to introduce some caching method. Would it be correct / pythonic / reasonably efficient to do something like this:

cache = {}
def g(a1, a2):
    return cache.setdefault(frozenset((tuple(a1), tuple(a2))), f(a1, a2))

Or would there be some better way?