15.17. Command Events

When a user clicks on certain types of widgets, like a button, you typically want a specific action to be performed. This is accomplished by setting the command attribute of a widget to a specific event handler function. This can be any function that receives no arguments as parameters. You can set the event handler function using a “named parameter” when you create the widget, or set the widget’s command attribute using a dictionary lookup. For example:

def my_function():
  print("my_function was called.")

my_button = tk.Button(application_window, text="Example", command=my_function)

# or

my_button = tk.Button(application_window, text="Example")
my_button['command'] = my_function

Note that you are setting the command property of the widget to a function reference – you are not calling the function! Therefore, do not put parentheses after the function name when you make the assignment.

The following widgets define a command property which defines a function that is called from the application’s event loop whenever a specific, predefined event is performed by a user.

Widget	The user event that causes the command function to be executed:
Button	The user places their pointing device cursor over the button and presses and releases the left mouse button. The function is actually called on the button release.
Checkbutton	If the state of the check box changes, the function is called.
Radiobutton	If the state of the radio box changes, the function is called.
Scale	The function is called if the slider moves. The function is passed one argument, the new scale value.

The following widgets do not have a command property, but they use other properties to respond to user events:

Widget	Property	User events:
Menu	postcommand	Every time someone brings up this menu.
Combobox	postcommand	When the user clicks on the down-arrow.
Combobox	validatecommand	Dynamically validate the widget’s text content.
Entry	validatecommand	Dynamically validate the widget’s text content.

Note that the Label, Message, and Separator widgets do not respond to user events and therefore have no associated event handlers.

15.18. Hello World Again

As a simple example, here is an enhanced “Hello World” program that contains a quit button that has a single command event handler. If the quit button is pressed by a user, the window’s destroy method is called – which closes the window.

import tkinter as tk
from tkinter import ttk

# Create the application window
window = tk.Tk()

# Create the user interface
my_label = ttk.Label(window, text="Hello World")
my_label.grid(row=1, column=1)

quit_button = ttk.Button(window, text="Quit")
quit_button.grid(row=2, column=1)
quit_button['command'] = window.destroy

# Start the GUI event loop
window.mainloop()

15.19. Other Events

Using command events for simple actions is the easiest way to handle user events. For more complex situations, the next lesson explains how to associate “lower level” events with event handler functions.

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