如何更新Tix.ComboBox的文本？

import tkinter as tk
import tix as tix

root = tk.Tk()

cb = tix.ComboBox(root, command=on_select)
cb.pack()

cb.add_item("Option 1")
cb.add_item("Option 2")
cb.add_item("Option 3")

def on_select():
    print("Selected:", cb.get())

root.mainloop()

cb.set("Option 2")