您能否解释一下此解决方案中的变量选择来自哪里

Question

这是在这里发布的答案： Linking two OptionMenu widgets Tkinter

我只是想知道varmenu中的“选择”来自哪里，因为我们没有在command = self.varmenu中传递任何内容

预先感谢

from Tkinter import *

class app:
    def __init__(self, root):
    win1 = Frame(root)
    win1.grid(row=0,column=0)

    self.variable = StringVar(win1)                               
    self.variable.set(42)
    self.type = OptionMenu(win1, self.variable,
                      "None", "Clear", "Dark", "Heavy",
                      command = self.varMenu)
    self.type.grid(row=1, column=3, sticky="nsew", padx=1, pady=1)


    self.variableunit = StringVar(win1)
    self.variableunit.set('mm')
    self.unit = OptionMenu(win1,
                      self.variableunit, "mm", "colour", "shade")
    self.unit.grid(row=1, column=5, sticky="nsew", padx=1, pady=1)

def varMenu(self, selection):
    if selection == "Heavy":
        self.variableunit.set("colour")
        self.unit.config(state = DISABLED)
    else:
        self.variableunit.set("mm")
        self.unit.config(state = NORMAL)

root = Tk()
a = app(root)
root.mainloop()