when to use dunder and when not to?

Hi there, E. T.! Your own custom classes/objects don't naturally have access to some things you're used to. For instance, imagine that you had a Student object that had attributes of name, major, grade point average etc. And let's say you made an instance of that object with new_student = Student(). What do you suppose would happen if you did len(new_student). What would you expect it to return? Well, that's entirely up to you, the developer. But you could have it return the length of their full name, or just their last name or anything you like. And you would do that by creating a __len__. The __len__ is what runs every time you do a len() of something

As for the, __contains__ it is what is run every time you do a for something in other_thing. Specifically, it's that in word there that makes it look for a __contains__ on the object. I suggest taking a peek at this Python documentation about dunder contains

Hope this helps!