rec.py

class cont:

    def __init__(self, fn):
        self.fn = fn

    def __call__(self, *args, **kwargs):
        return (lambda: self.fn(*args, self, **kwargs))

class tailrec:

    def __init__(self, fn):
        self.fn = fn

    def __call__(self, *args, **kwargs):
        a = self.fn(*args, cont(self.fn), **kwargs)
#        count = 0
        while callable(a):
#            print('loop %d' % (count,))
#            count = count + 1
            a = a()
        return a

@tailrec
def factorial_rec(n, acc, k):
    if n == 1:
        return acc
    else:
        acc = n * acc
        n = n - 1
        return k(n, acc)

def factorial(n):
    return factorial_rec(n, 1)

if __name__=='__main__':
    print(factorial(5))
    print(factorial(50))