In my MVC example, the Viewer expected a 'shape' object to support a specific API: verts and edges needed to be available as attributes.
Let me try to practice better what I think I am preaching and follow your lead in making the discussion more concrete and less theoretical. I have a line segment - defined as the segment connecting 2 points. The interface allows the points to be picked and moved. The line segment has a length. I choose line.getLength() as my API which I feel idiomatically expresses in Python that we are calculating the length as it happens to be when the call is made. I could quite easily wrap it in a property function and have a line.length attribute - but find it less expressive. Am I - in fact - violating the Uniform Access Principal. If I am, am I violating good programming practice? If I am in violation of the UAP principal, but not in violation of good programming practice - what are the bounds of the UAP principal. And how do we teach that? If I am not violating the Uniform Access Principal how do we express on what basis I am not? Art