爲什麼遞歸沒有返回值

Question

無法理解，而不是爲什麼這個函數返回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) 

輸出：

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

Answer 1

您必須實際重新調用函數時返回的返回值遞歸。這樣，一旦你停止呼叫，你就可以正確地返回。它返回None，因爲這個調用什麼都沒有返回。看看這個週期：

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

一旦停止遞歸的filename返回到那裏你遞歸調用該函數的行，但它什麼都不做，所以你函數返回None。您必須添加回報。更改爲遞歸調用：

return existence_of_file() 

這將產生：

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

下面是完整的代碼：

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)