This is intended as a straightforward example of how to build a pure-python widget that uses a Model-View-Controller design. It works with ipywidgets 5+ and traitlets 4.1+. This is not currently packaged for installation via pypi. Instead, download it as a zip from github, or, even better, clone it and improve it!
The widget is a simple password generator.
The file model.py contains the data model: a class that has as traitlet properties the parameters that go into generating the password and a method for doing the generation based on the parameters. The observe decorator indicates which method gets called when properties of the widgets change because the user has adjusted them; this is one piece of the controller.
The file gui.py does two things: set up the actual widgets (the view) and link properties of the widget elements to traits in the model (one piece of the controller).