TypeError: _str_returned non-string (Type Nonetype)

Today, listening to the net class encountered a problem: TypeError: __str__returned non-string (Type Nonetype) and the solution, share to everyone. (I'm studying Python3.)

The error message translated into Chinese means: "Type error: returned non-string type".

I think this is a problem in class __str__, this part of the code is as follows:

The problem is that in this else part, I change this part of the code to return, and the program will run smoothly without any errors, as follows:

It appears that in Python3, getting a string in __str__ requires return.

TypeError: _str_returned non-string (Type Nonetype)