python-gtk-3-tutorial(1)

Getting Started

File:simple_example.py

#!/usr/bin/python
from gi.repository import Gtk

win = Gtk.Window()
win.connect("delete-event", Gtk.main_quit)
win.show_all()
Gtk.main()

Let me explain:  

(1) from gi.repository import Gtk

To import Gtk class from gi.repository

(2) win = Gtk.Window()

To create a empty windows object.

(3) win.connect("delete-event", Gtk.main_quit)

To connect the window's delete event.

(4) win.show_all()

we display the window

Gtk.main()

we start the GTK+ processing loop which we quit when the window is closed (see line 5).