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 |
|---|---|
|
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. |
|
If the state of the check box changes, the function is called. |
|
If the state of the radio box changes, the function is called. |
|
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: |
|---|---|---|
|
|
Every time someone brings up this menu. |
|
|
When the user clicks on the down-arrow. |
|
|
Dynamically validate the widget’s text content. |
|
|
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.