#!/usr/bin/env python3

import gi
gi.require_version("Gtk", "3.0")
from gi.repository import Gtk

class MyWindow(Gtk.Window):

    def __init__(self):
        Gtk.Window.__init__(self, title = "Hello World!")

        self.button = Gtk.Button(label = "Click Me")
        self.button.connect("clicked", self.on_button_clicked)
        self.add(self.button)

    def on_button_clicked(self, widget):
        print("Hello World!")

window = MyWindow()
window.connect("delete-event", Gtk.main_quit)
window.show_all()

Gtk.main()