root.title('Say Hello')
label = tk.Label(root, text='Hello World')
label.pack(side='top', fill='both', expand=True, padx=20, pady=20)
button = tk.Button(root, text='OK', command=lambda : root.destroy())
button.pack(side='bottom', fill='none', expand=True)
root.mainloop()
Tkinter messagebox without window?