python - 递归:看起来像相同的函数，但打印出不同的执行流程

Question

我尝试打印阶乘递归函数的执行流程。

def factorial(n):
    space=' '*(4*n)
    print(space,'factorial ',n)
    if n==0:
        print(space,'returning 1')
        return 1
    else:
        recurse=factorial(n-1)
        result=n*recurse
        print(space,'returning ', result)
        return n*recurse

使用阶乘(2) 结果将是正确的:

        factorial 2
    factorial 1
factorial 0
returning 1
    returning 1
        returning 2
2

但是当我不使用变量 recurse=factorial(n-1) 时:

def factorial(n):
    space=' '*(4*n)
    print(space,'factorial ',n)
    if n==0:
        print(space,'returning 1')
        return 1
    else:
        result=n*factorial(n-1)
        print(space,'returning ', result)
        return n*factorial(n-1)

然后使用阶乘(2)打印出不同的流程:

             factorial  2
     factorial  1
 factorial  0
 returning 1
     returning  1
 factorial  0
 returning 1
         returning  2
     factorial  1
 factorial  0
 returning 1
     returning  1
 factorial  0
 returning 1
2

我的问题是:为什么执行流程不同[有和没有变量 recurse=factorial(n-1)]？

Answer 1

这是由于代码中的以下额外函数调用所致

def factorial(n):
    space=' '*(4*n)
    print(space,'factorial ',n)
    if n==0:
        print(space,'returning 1')
        return 1
    else:
        result=n*factorial(n-1)
        print(space,'returning ', result)
        return n*factorial(n-1)   # factorial(n-1) is called second time so that this function is executed once again

您可以按照以下方式执行类似的操作

def factorial(n):
    space=' '*(4*n)
    print(space,'factorial ',n)
    if n==0:
        print(space,'returning 1')
        return 1
    else:
        result=n*factorial(n-1)
        print(space,'returning ', result)
        return result