added button

app.py

@@ -3,7 +3,20 @@ import gi
 gi.require_version("Gtk", "3.0")
 from gi.repository import Gtk
 
-win = Gtk.Window()
+
+class MyWindow(Gtk.Window):
+    def __init__(self):
+        super().__init__(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")
+
+
+win = MyWindow()
 win.connect("destroy", Gtk.main_quit)
 win.show_all()
 Gtk.main()