如何从OptionMenu (tkinter)赋值以供以后使用？

selected_value = tkinter.StringVar()

option_menu = tkinter.OptionMenu(root, selected_value, "选项1", "选项2", "选项3")
option_menu.pack()

value = selected_value.get()

import tkinter

def get_selected_value():
    value = selected_value.get()
    print("选中的值是：", value)

root = tkinter.Tk()

selected_value = tkinter.StringVar()
option_menu = tkinter.OptionMenu(root, selected_value, "选项1", "选项2", "选项3")
option_menu.pack()

button = tkinter.Button(root, text="获取选中的值", command=get_selected_value)
button.pack()

root.mainloop()