We begin the chapter by setting up the structural skeleton for our application GUI. This should be logically separate from the rest of our code so we give it a subpackage of its own. Inside the top level pythongis folder, create a package folder called app with an __init__.py file inside it. Have it import the rest of the modules we will be creating:

from . import builder
from . import dialogues
from . import toolkit
from . import icons 

To make our app package accessible from our top level pythongis package, we similarly need to import it in pythongis/__init__.py as follows:

from . import app

The purpose of the app package is that we should be able to define how our GUI looks and behaves, and with a single line of code, pythongis.app.run(), we should be able to invoke it. The actual definition and layout of our GUI should be contained in a module we call app/builder.py (which we return to at the end of the chapter). The builder in turn relies on a set of predefined...