How to update the command of an OptionMenu

Question

I am trying to set or update the command of an OptionMenu after its instantiation.

The widget.configure(command=foo) statement works for Button

Answer 1

It is possible to change the commands associated with OptionMenu widets if you're careful (as @Bryan Oakley commented). Below is an example of doing it.

The tricky part is you have to reconfigure all the menu items, not just one of them. This requires some extra bookkeeping (and some processing overhead, but it's unnoticeable).

Initially the menu has three items, each with a different function to be called when selected, one of which changes the menu. If the latter is selected the menu is changed to only have two menu items both of which call the same function.

from tkinter import *

root = Tk()
var = StringVar()
var.set('Select')

def foo(value):
    var.set(value)
    print("foo1" + value)

def foo2(value):
    var.set(value)
    print("foo2 " + value)

def foo3(value):
    var.set(value)
    print("foo3 " + value)

def change_menu(value):
    var.set('Select')
    print('changing optionmenu commands')
    populate_menu(optionmenu, one=foo3, two=foo3)

def populate_menu(optionmenu, **cmds):
    menu = optionmenu['menu']
    menu.delete(0, "end")
    for name, func in cmds.items():
        menu.add_command(label=name, command=
                         lambda name=name, func=func: func(name))

optionmenu = OptionMenu(root, var, ())  # no choices supplied here
optionmenu.pack()
Label(root, textvariable=var).pack()

populate_menu(optionmenu, one=foo, two=foo2, change=change_menu)

root.mainloop()

Answer 2

Good question! Its a good thing I never had to do this in any one of my projects before because (unless someone proves me wrong here) you can't set/update the command of a OptionMenu widget once its already defined.

If Tkinter wanted you to be able to do that, it definitely would've included it to be edited by .configure()

There is a handy function called .keys() which you can call with a widget object to see all available traits that can be used with .configure().

Button example:

from tkinter import *

master = Tk()

def callback():
    print ("click!")

b = Button(master, text="OK", command=callback)
print (b.keys()) #Printing .keys()
b.pack()

mainloop()

Which results in :

Notice how in this huge list of keys, 'command' is on the second line? That is because a button's command CAN be used in .configure()

OptionMenu example:

from tkinter import *

root = Tk()
var = StringVar()

def foo(val):
    print ("HI")

widget = OptionMenu(root, var, "one", 'two')
print(widget.keys())
widget.pack()
root.mainloop()

Which results in:

Notice how there is no 'command' on line 2 this time. This is because you cant configure command with an OptionMenu widget.

Hopefully this problem doesn't hinder your program too much and I hope my answer helped you understand better!

Answer 3

I think what you're really asking is how to associate a command to an Optionmenu, rather than update a command (there is no command, so there's nothing to update).

If you want a function to be called every time a value is selected from an Optionmenu, you can add a trace on the related variable. The trace will call a function whenever that variable changes, whether through the Optionmenu or any other means.

For example:

...
var = tk.StringVar()
def foo(*args):
    print "the value changed...", var.get()
var.trace("w", foo)
...

When the function is called it will pass three arguments, which you can safely ignore in this case.

For more information on variable traces see http://effbot.org/tkinterbook/variable.htm

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You might also want to consider switching to the ttk combobox. It supports binding to <<ComboboxSelected>>, which is every so slightly less clunky than doing a variable trace.