SB Command Events

Section 15.6 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.

Table 15.6.1.

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:

Table 15.6.2.

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.

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