为什么递归没有返回值

Question

无法理解，而不是为什么这个函数返回Nonefilename

import os 
def existence_of_file(): 
    filename = str(input("Give me the name: ")) 
    if os.path.exists(filename): 
     print("This file is already exists") 
     existence_of_file() 
    else: 
     print(filename) 
     return filename 
a = existence_of_file() 
print(a) 

输出：

Give me the name: app.py 
This file is already exists 
Give me the name: 3.txt 
3.txt 
None 

Answer 1

您必须实际重新调用函数时返回的返回值递归。这样，一旦你停止呼叫，你就可以正确地返回。它返回None，因为这个调用什么都没有返回。看看这个周期：

Asks for file -> Wrong one given -> Call again -> Right one given -> Stop recursion 

一旦停止递归的filename返回到那里你递归调用该函数的行，但它什么都不做，所以你函数返回None。您必须添加回报。更改为递归调用：

return existence_of_file() 

这将产生：

>>> 
Give me the name: bob.txt 
This file is already exists 
Give me the name: test.txt 
test.txt 
test.txt 

下面是完整的代码：

import os 
def existence_of_file(): 
    filename = str(input("Give me the name: ")) 
    if os.path.exists(filename): 
     print("This file is already exists") 
     return existence_of_file() 
    else: 
     print(filename) 
     return filename 
a = existence_of_file() 
print(a)