Once you start doing it, pylint will enforce it for you in that it will inform you if you access an instance variable from another class beginning with underscore. It drives me mad because in the PyDev debugger in eclipse, all such underscore-variables in the Variables view are under a twistie called "protected variables" and if you have to go down several layers in a complicated object containing references to other objects, you have to click through all these twisties, twice as many clicks.