"""Fun with a label, a button and a callback."""

from Tkinter import *

class App:

    def __init__(self, parent):
        self.parent = parent
        self.create_widgets()

    def create_widgets(self):
        self.label1 = Label(
            self.parent,
            text="Hello world")
        self.label1.pack()
        self.button1 = Button(
            self.parent,
            text="Just do it",
            command=self.do_it)
        self.button1.pack()

    def do_it(self):
        self.label1.config(
          text="I'm doing it!")


# Boilerplate from here on...
if __name__ == '__main__':
    root = Tk()
    app = App(root)
    root.mainloop()