import Tkinter


class MainWindow(Tkinter.Frame):

    def __init__(self, parent):
        Tkinter.Frame.__init__(self, parent, background="white")
        self.parent = parent
        self.initUI()

    def initUI(self):
        self.parent.title("Main Window")
        self.pack(fill=Tkinter.BOTH, expand=1)
        window_button = Tkinter.Button(self, text="New Window", command=self.show_new_window)
        window_button.pack()

    def show_new_window(self):
        new_window = ChildWindow()
        new_window.title("New window")


class ChildWindow(Tkinter.Toplevel):
    pass


if __name__ == "__main__":
    root = Tkinter.Tk()
    root.geometry("250x150+300+300")
    main_window = MainWindow(root)
    root.mainloop()