tkinter ttk OptionMenu losing checkmark highlight when updating option list

Question

When I update a ttk.OptionMenu widget using this example: https://stackoverflow.com/a/7403530 , I lose the check mark that showed up before when I selected an item if I was using the initial list of items. How do I get back the checkmark for the selected item?

Init Code: self.om = ttk.OptionMenu(self, self.om_variable,'a', *['a','b','c'])

Before:

After Update:

Code here:

import tkinter as tk

class SampleApp(tk.Tk):
    def __init__(self, *args, **kwargs):
        tk.Tk.__init__(self, *args, **kwargs)
        self.om_variable = tk.StringVar(self)

        b1 = tk.Button(self, text="Colors", width=8, command=self.use_colors)
        b2 = tk.Button(self, text="Sizes", width=8, command=self.use_sizes)

        self.om = tk.OptionMenu(self, self.om_variable, ())
        self.om.configure(width=20)
        self.use_colors()

        b1.pack(side="left")
        b2.pack(side="left")
        self.om.pack(side="left", fill="x", expand=True)


    def _reset_option_menu(self, options, index=None):
        '''reset the values in the option menu

        if index is given, set the value of the menu to
        the option at the given index
        '''
        menu = self.om["menu"]
        menu.delete(0, "end")
        for string in options:
            menu.add_command(label=string, 
                             command=lambda value=string:
                                  self.om_variable.set(value))
        if index is not None:
            self.om_variable.set(options[index])

    def use_colors(self):
        '''Switch the option menu to display colors'''
        self._reset_option_menu(["red","orange","green","blue"], 0)

    def use_sizes(self):
        '''Switch the option menu to display sizes'''
        self._reset_option_menu(["x-small", "small", "medium", "large"], 0)

if __name__ == "__main__":
    app = SampleApp()
    app.mainloop()

Answer 1

The ttk version, which you used previously, adds the checkmark functionality by default and a check will appear on the selected item. However, when you manually add items, you'll need to use the method add_radiobutton instead of add_command. This is what enables the check mark (on both tk and ttk versions).

import tkinter.tkk as tkk

def __init__(self, *args, **kwargs):
    ...
    self.om = ttk.OptionMenu(self, self.om_variable)
    ...

def _reset_option_menu(self, options, index=None):
    ...
    menu.add_radiobutton(
        label=string,
        command=tk._setit(self.om_variable, string)
    )
    ...