Python学习——尾递归及装饰器优化

def recsum(x):
    if x == 1:
        return x
    else:
        return x+recsum(x-1)  # 这里的return语句返回的是表达式

recsum(5)
5 + recsum(4)
5 + (4 + recsum(3))
5 + (4 + (3 + recsum(2)))
5 + (4 + (3 + (2 + recsum(1))))
5 + (4 + (3 + (2 +1)))
5 + (4 + (3 + 3))
5 + (4 + 6)
5 + 10
15

def tailrecsum(x,running_total=0):
    if x == 0:
        return running_total
    else:
        return tailrecsum(x - 1,running_total + x)  # 尾递归的返回是调用自身

tailrecum(5,0)
tailrecum(4,5)
tailrecum(3,9)
tailrecum(2,12)
tailrecum(1,14)
tailrecum(0,15)
15

# Python3的装饰器
import sys
class TailRecurseException(BaseException):
    def __init__(self,args,kwargs):
        self.args = args
        self.kwargs = kwargs
def tail_call_optimized(g):
    def func(*args,**kwargs):
        f = sys._getframe()
        #如果产生新的递归调用栈帧时
        if f.f_back and f.f_back.f_back \          
            and f.f_back.f_back.f_code == f.f_code:
            # 捕获当前尾调用函数的参数，并抛出异常
            raise TailRecurseException(args,kwargs)
        else:
            while 1:
                try:
                    return g(*args,**kwargs)
                except TailRecurseException as e:
                    args = e.args
                    kwargs = e.kwargs
    func.__doc__ = g.__doc__
    return func

@tail_call_optimized
def fact_iter(num,product):
    print(product)
    if num == 1:
        return product
    return fact_iter(num-1,product+1)