How to call a function using the OptionMenu widget in Tkinter?

When creating user interfaces in Python using Tkinter, you might find yourself in a scenario where you need to call a certain function every time the option chosen from an OptionMenu widget changes.

The OptionMenu widget provides a dropdown menu from which users can choose an option. In this article, we will learn how to use the OptionMenu widget in Tkinter and how to call a function every time an option is selected.

Creating the OptionMenu

Before we can call a function using the OptionMenu widget, we first need to create the widget itself. We can do this by using the OptionMenu constructor, specifying the parent frame and the options we want the user to choose from. Here’s an example:

from tkinter import *

root = Tk()

options = ["Option 1", "Option 2", "Option 3"]

var = StringVar()
var.set(options[0])

option_menu = OptionMenu(root, var, *options)
option_menu.pack()

root.mainloop()

In this example, we create a window and define a list of three options. We then create a StringVar object and set its value to the first option. The OptionMenu constructor then creates the widget, specifying the parent frame, the StringVar object to which the selected option will be assigned, and using the *options syntax to specify all the options.

Calling a Function on Selection

Now that we have created the OptionMenu widget, we can proceed to call a function every time the user selects an option. We can do this by using the command argument of the OptionMenu constructor, which specifies the function to call. Here’s an example:

from tkinter import *

root = Tk()

options = ["Option 1", "Option 2", "Option 3"]

var = StringVar()
var.set(options[0])

def on_select(option):
    print("Selected option:", option)

option_menu = OptionMenu(root, var, *options, command=on_select)
option_menu.pack()

root.mainloop()

In this example, we define a function on_select that takes a single argument option, which will be the option selected by the user. We then pass this function as the command argument of the OptionMenu constructor. Every time the user selects an option, the on_select function will be called with the selected option as the argument.

Modifying the OptionMenu Options

We might find ourselves in a scenario where we want to modify the options available in the OptionMenu widget dynamically. We can do this by modifying the list of options and using the set_menu method of the OptionMenu widget to update the widget with the new options. Here’s an example:

from tkinter import *

root = Tk()

options = ["Option 1", "Option 2", "Option 3"]

var = StringVar()
var.set(options[0])

def on_select(option):
    print("Selected option:", option)

option_menu = OptionMenu(root, var, *options, command=on_select)
option_menu.pack()

new_options = ["Option 4", "Option 5", "Option 6"]

options += new_options
option_menu['menu'].delete(0, END)

for option in options:
    option_menu['menu'].add_command(label=option, command=lambda value=option: var.set(value))

root.mainloop()

In this example, we define a new list of options new_options, which we want to add to the existing options. We append the new options to the existing list and use the delete method of the menu attribute of the OptionMenu widget to remove all the options from the widget. We then loop through the updated list of options and use the add_command method of the menu attribute to add each option back to the widget.

Conclusion

In this article, we have learned how to create and use the OptionMenu widget in Tkinter. We have seen how to call a function every time an option is selected, and also how to modify the options available in the widget dynamically. With this knowledge, you should now be able to create powerful user interfaces that respond dynamically to user input.