为什么python不返回None？

Question

class tree :

    def __init__(self):

        self.val=0
        self.right=None
        self.left=None

def create_tree():

    x=input()

    if x==-1:
        return None

    root=tree()
    root.val=x

    print(root)
    print(root.val)
    print(root.right)
    print(root.left)



    while(True):
        print ("reach1")
        root.left=create_tree()
        root.right=create_tree()
        print("reach2")
        break


    return root

def main():

    root=tree()
    root=create_tree()

main()

Why None is not returned when x==-1 in create_tree() ?

sample output:

2 <__main__.tree object at 0x7f58cbf11128> 2 None None reach1
-1 <__main__.tree object at 0x7f58cbf11208>
-1 None None reach1

Answer 1

Why None is not returned when x==-1 in create_tree() ?

Because input() returns a string from stdin input. input reads a line from input, converts it to a string and returns that.

You can check this using type operator.

type_of = type(x)
>> string

The solution is to compare what you've entered with "-1"

if x == "-1":
    return None

or just use int method.

x = int(input())
if x == -1:
    return None