问无法使tkinter的StringVar的trace方法工作

EN

from tkinter import OptionMenu, StringVar, Tk

class Foo(OptionMenu):

    def __init__(self, master, options):
        self.bar = StringVar()
        self.bar.set(options[0])
        self.bar.trace("w", lambda: self.mouseCallback("foobar"))
        super().__init__(master, self.bar, *options)

    def mouseCallback(self, baz):
        print(baz)

def mainCycle():
    while True:
        root.update()

if __name__ == "__main__":
    opts = ["A", "LONG", "LIST", "OF", "OPTIONS"]
    root = Tk()
    foobarbaz = Foo(root, opts)
    foobarbaz.pack()
    mainCycle()

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Program Files\Python35\lib\tkinter\__init__.py", line 1549, in __call__
    return self.func(*args)
  File "C:\Program Files\Python35\lib\tkinter\__init__.py", line 3285, in __call__
    self.__var.set(self.__value)
  File "C:\Program Files\Python35\lib\tkinter\__init__.py", line 260, in set
    return self._tk.globalsetvar(self._name, value)
_tkinter.TclError: can't set "PY_VAR0":