The solution of adding the protection first is mentioned but there will be other situations later in which the use of a lot of self cannot be avoided, so it doe

The solution of adding the protection first is mentioned but there will be other situations later in which the use of a lot of self cannot be avoided, so it doesn't solve the general problem, if you call it that. What is being called "defensive" I would rather call "permissive". If at all possible, I would prefer raising a ValueError if x1 >= x2.