def fact(n):
    return fact_iter(n, 1)
 
def fact_iter(num, product):
    if num == 1:
        return product
    return fact_iter(num - 1, num * product)
    
# fact(5)的调用过程
===> fact_iter(5, 1)
===> fact_iter(4, 5)
===> fact_iter(3, 20)
===> fact_iter(2, 60)
===> fact_iter(1, 120)
===> 120

>>> fact(1000)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in fact
  ...
  File "<stdin>", line 4, in fact
RuntimeError: maximum recursion depth exceeded