§ Fixpoint as decorator

#!/usr/bin/env python3
class Thunk:
    def __init__(self, func, *args):
        self.func = func
        self.args = args
    def force(self):
        return self.func(*self.args)

def fix(f):
    return f(Thunk(fix, f))

@fix
def fact(f):
    def fact_n(n):
        if n == 0: return 1
        else: return n * (f.force())(n-1)
    return fact_n

print(fact(5))