That's true though kind of an implementation detail. Conceptually x += y is always x = x.__iadd__(y), and x can have its own semantics for what iadd does. Conve

That's true though kind of an implementation detail. Conceptually x += y is always x = x.__iadd__(y), and x can have its own semantics for what iadd does. Conventionally that is that mutable objects modify themselves in place and return themselves, while immutable objects create a new object and return that. This behavior for list and tuple is familiar from regular local variables. Together with the fact that obj.x = ... always means "create or rebind an instance variable" never "rebind the A.x class variable", I think the example is less surprising.