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?

zondo · Accepted Answer · 2019-11-04 15:22:33Z

5

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())

edited Nov 4, 2019 at 15:22

answered Mar 13, 2016 at 18:44

zondo

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6 Comments

Pax Vobiscum Over a year ago

Thank you! Was going to have a hard time explaining to my teacher why I passed an unused variable...

Pax Vobiscum Over a year ago

Say if I wanted to use this functionality for a button instead, how could I pass the button itself as an argument (a calculator for example)?

zondo Over a year ago

Here is an answer that tells you how to do just that.

martineau Over a year ago

What you say about how it works is incorrect: It doesn't pass the OptionMenu instance to the function as an argument, it passes the value associated with the menu item that was selected (an int in this case).

zondo Over a year ago

@martineau: I can't believe I went so far as to post an answer without testing. I stand corrected.

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