import tkinter as tk
from tkinter import messagebox


def about_app():
    messagebox.showinfo("App", "The application\nthat does nothing")


window = tk.Tk()

# main menu creation
main_menu = tk.Menu(window)
window.config(menu=main_menu)

# 1st main menu item: an empty (as far) submenu
sub_menu_file = tk.Menu(main_menu)
main_menu.add_cascade(label="File", menu=sub_menu_file)

# 2nd main menu item: a simple callback
sub_menu_help = tk.Menu(main_menu)
main_menu.add_command(label="About...", command=about_app)

window.mainloop()