Elaborate example of a custom widget implementing MVC

[Stefan de Konink]

Hello,

I have implemented a custom widget presenting a swimming lane graph that 
can interact with a model. It works pretty well, but there are some basic 
things I don't understand, for example the reason for adding 
QtCore.QModelIndex() to things like rowCount, and index.

Is there any good tutorial or are there any reference widgets written in 
Python that are considered best in class and good learning material?

-- 
Stefan