PyGTK 3

gistfile1.py

import sys
from gi.repository import Gtk

UI_FILE = 'src/your_application.ui'

class AppUI:
    def __init__(self):
        self.builder = Gtk.Builder()
        self.builder.add_from_file(UI_FILE)
        self.builder.connect_signals(self)

        window = self.builder.get_object('window')
        window.show_all()

    def destroy(window, self):
        Gtk.main_quit()

def main():
    app = AppUI()
    Gtk.main()

if __name__ == '__main__':
    sys.exit(main())