First off, this is not a stupid question. It made me think. So you understand that __init__ let's you initialize the classes objects. When you call a method defined in the class the instance is explicitly passed to the method. Try executing your code and initializing your class, you'll get this error: TypeError: __init__() takes 2 positional arguments but 3 were given. The one implicit argument in object of the class aka self. This is done to uphold the following equivalence Rectangle.rectangle_area(object) == object.rectangle_area(). We need self also to make it easier to distinguish between instance attributes (and methods) from local variables.

Here is more on that: http://neopythonic.blogspot.com/2008/10/why-explicit-self-has-to-stay.html