简单的python函数返回None，并且我正在使用return语句

Question

I have no idea why this is returning None

def list_union(self,A,B):
    A = self.separate(A["fill"])
    B = self.separate(B["fill"])
    print A
    print B
    if len(A)==1 and A[0]=="no":
        return B.sort()
    if len(B)==1 and B[0]=="no":
        return ["no"]

Just assume and A and B are being returned as a list type( which it is and it is working fine)

A and B may contain the strings yes or no. I just don't get why they are returning None.

Let me add, I don't care if these two are statements are false because there is more to this program, but I figure whatever issue I am having can be solved by figuring out why this isn't working.

Answer 1

By default a function in Python returns None . In your case, it has two exit points, but they will be reached only if one of the conditions is True . Ask yourself, what happens if both conditions are False ? add another return at the end, to handle this case and return an appropriate value (or raise an exception, if it makes sense):

return "Unexpected result" # just an example 

Also, notice that sort() does not return a value (it sorts the list in-place), so it'll evaluate to None , too! In other words, you should change this line:

return B.sort()

Do this instead:

B.sort()
return B

Or do this, which does return a new sorted list:

return sorted(B)

Answer 2

B.sort() doesn't return B, it returns None ! (But B is sorted as you wanted)

You can use sorted(B) that will return what you want.

Indeed, the built-in sort() method modifies the list in-place.

sorted() built-in function builds a new sorted list.