def METHOD_NAME(self):
    test_window = toga.Window()
    self.assertEqual(len(self.app.windows), 0)
    self.app.windows += test_window
    self.assertEqual(len(self.app.windows), 1)
    self.app.windows += test_window
    self.assertEqual(len(self.app.windows), 1)
    self.assertIs(test_window.app, self.app)
    not_a_window = "not_a_window"
    with self.assertRaises(TypeError):
        self.app.windows += not_a_window