class Memoize(object):

    def __init__(self, f):
        self.f = f
        self.computed = {}

    def __call__(self, *args):
        xs = self.computed.get(args)
        if xs :
            return xs
        else :
            value = self.f(*args)
            self.computed[args] = value
            return value

    def unmemoize(self):
        return self.f


def fact(n):
    print("computing", n)
    if n == 0:
        return 1
    else:
        return fact(n-1) * n

fact = Memoize(fact)

# fact(5)
# fact(6)
# ...

fact = fact.unmemoize()