python - OptionMenu command function requires argument

Question

This is a pretty general question about the optionmenu widget in tkinter.

When defining an OptionMenu widget, and assigning a function as its command, why does it require an argument?

My code:

from tkinter import *

def update():
    x = optionvar.get()
    x = str(x)
    mylabel.config(text=x)

root = Tk()

l = []
for n in range(10):
    l.append(n)

t = tuple(l)

optionvar = IntVar()

optionvar.set('hello stackoverflow')

mymenu  = OptionMenu(root, optionvar, *t, command=update)
mylabel = Label(root)

mymenu.pack()
mylabel.pack()

My errors:

TypeError: update() takes 0 positional arguments but 1 was given

Simply defining update with

def update(foo):

seems to work. But why?

Answer 1

The callback usually wants to know which item was selected, so the value of the IntVar is passed as an argument. If you want to ignore that argument, you can simply use a lambda (_ is a valid name that is commonly used to indicate that it is a throwaway variable):

mymenu = OptionMenu(root, optionvar, *t, command=lambda _: update())