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

The OptionMenu widget in Tkinter allows users to select from a dropdown list of options. You can call a function when an option is selected by using the command parameter and a StringVar object to track the selected value.

Basic Setup

First, create a StringVar object to manage the selected value and define a callback function ?

from tkinter import *

# Create main window
root = Tk()
root.geometry("400x300")
root.title("OptionMenu Example")

# Create StringVar to store selected value
selected_city = StringVar()
selected_city.set("Select a City")

# Define callback function
def on_city_select(value):
    result_label.config(text=f"You selected: {value}")
    
# Create label to display selection
result_label = Label(root, text="No city selected", font=("Arial", 12))
result_label.pack(pady=20)

root.mainloop()

Complete Example with OptionMenu

Here's a complete example showing how to create an OptionMenu and call a function when an option is selected ?

from tkinter import *

# Create main window
root = Tk()
root.geometry("400x300")
root.title("City Selection")

# Callback function for OptionMenu selection
def city_selected(selected_value):
    result_label.config(text=f"You have selected: {selected_value}")
    print(f"User selected: {selected_value}")

# Create StringVar to hold the selected value
city_var = StringVar()
city_var.set("Choose a city")

# List of cities for the OptionMenu
cities = ["Mumbai", "Delhi", "Bangalore", "Chennai", "Kolkata", "Hyderabad"]

# Create OptionMenu widget
city_menu = OptionMenu(root, city_var, *cities, command=city_selected)
city_menu.config(width=20, font=("Arial", 10))
city_menu.pack(pady=30)

# Label to display the selection
result_label = Label(root, text="No city selected yet", 
                    font=("Arial", 12), fg="blue")
result_label.pack(pady=20)

# Start the GUI event loop
root.mainloop()

Key Components

• StringVar() − Manages the selected value from the OptionMenu

• command parameter − Specifies the function to call when selection changes

• *cities − Unpacks the list to provide options for the menu

• Callback function − Receives the selected value as a parameter

Alternative Approach Using trace()

You can also use the trace() method to monitor changes to the StringVar ?

from tkinter import *

root = Tk()
root.geometry("400x200")

def on_selection_change(*args):
    selected = city_var.get()
    status_label.config(text=f"Current selection: {selected}")

# Create StringVar and set up tracing
city_var = StringVar()
city_var.set("Select option")
city_var.trace('w', on_selection_change)  # 'w' for write operations

# Create OptionMenu without command parameter
cities = ["Tokyo", "Paris", "London", "New York"]
OptionMenu(root, city_var, *cities).pack(pady=20)

status_label = Label(root, text="Waiting for selection...")
status_label.pack(pady=10)

root.mainloop()

Comparison

Conclusion

Use the command parameter for simple callbacks when options are selected. The trace() method provides more control for monitoring StringVar changes in complex applications.