如何使tkinter按钮只打开一个Toplevel？

import tkinter as tk

def open_toplevel():
    global toplevel_opened
    if not toplevel_opened:
        toplevel_opened = True
        toplevel = tk.Toplevel(root)
        toplevel.protocol("WM_DELETE_WINDOW", on_toplevel_close)

def on_toplevel_close():
    global toplevel_opened
    toplevel_opened = False
    toplevel.destroy()

root = tk.Tk()
toplevel_opened = False

button = tk.Button(root, text="Open Toplevel", command=open_toplevel)
button.pack()

root.mainloop()