Python递归返回None但我明确设置了return

Question

I was trying to make some exercise from Stepic and almost all tests done but i
I far as i know when we do recursion cycle we need explicitly set return. But in this case something get wrong. This blow my mind for weeklong.

initialDict = {}

def add(child, parent):
    if child not in (initialDict):     
        initialDict.update({child: parent})
    else:
        initialDict[child].append(parent)

def heritage(parent, child):
    if parent == child:
        print('Yes')
    elif child in initialDict and parent in initialDict[child]:
        print('Yes')
    elif child not in initialDict or parent not in initialDict[child]:
        print('No')
    else:
        for i in range(len(initialDict[child])):
            return heritage(initialDict[child][i], initialDict[child])


n = int(input())
for _ in range(n):
    class_description = input().split()
    class_name = class_description[0]
    class_parents = class_description[2:]
    add(class_name, class_parents)

q = int(input())
for _ in range(q):
    question = input().split()
    parent = question[0]
    child = question[1]
    print(heritage(parent, child))

We set:
4
A
B : A
C : A
D : B C
4
A B
B D
C D
D A

Expected Result:

Yes
Yes
Yes
No

Actual Result:

Yes
None
Yes
None
Yes
None
No
None

Answer 1

Make the loop:

for _ in range(q):
    question = input().split()
    parent = question[0]
    child = question[1]
    heritage(parent, child)

Since heritage function contains print s, while print(print(..)) would give an extra None since print is actually None .

Answer 2

If you dont specify return, its by default None.

There are two solutions

1) you return string and print the result:

def heritage(parent, child):
    if parent == child:
        return 'Yes'
    elif child in initialDict and parent in initialDict[child]:
        return 'Yes'
    elif child not in initialDict or parent not in initialDict[child]:
        return 'No'
    else:
        for i in range(len(initialDict[child])):
            return heritage(initialDict[child][i], initialDict[child])

...
for _ in range(q):
    question = input().split()
    parent = question[0]
    child = question[1]
    print(heritage(parent, child))
...

2) or you print inside function, but you dont print result of the function, as the result is None

for _ in range(q):
    question = input().split()
    parent = question[0]
    child = question[1]
    heritage(parent, child) # You return None, but you dont print it

Essentially the problem is this:

def func_None():
    print('yes')

print(func_None())

>>> 
yes # this is printed inside function
None # this is printed result, because by default every function returns None if not specified

def func_not_None():
    return 'yes'

print(func_not_None())
>>>
yes