Operational Overloading

Question

I understand these works .Can someone make more clear why s1 N s3 will result a error if
__str__ operation is not defined/
https://code.sololearn.com/ctjh7I5PjKhf/#py

Answer 1

It won't give an error if __str__ is not defined because Python provides a __str__ method by default

Answer 2

python then does not know, how to print the var student.
to print itself , an object needs __str__ or __repr__