15.30. Timer Events

GUI programs run an “event loop” that continuously receive events from the operating system and “dispatches” those events to appropriate callback functions. Nothing happens in a GUI program without an event. The application logic for some problems requires that specific events happen at some specific times. For this reason tkinter includes a feature to generate events under software control. These are often refered to as timer events.

Every widget has an after method that will generate an event at a specific time interval from the time it is called. The method takes at least 2 arguments: the amount of time (in milliseconds) to wait before generating the event, and the callback function to call after the time has elapsed. In the example below, the function a_callback_function will be called one second (1000 milliseconds) after the timer-event was created.

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

my_button = tk.Button(application_window, text="Example")
my_button.after(1000, a_callback_function)

15.30.1. Animations and Repeated Tasks

If you want a specific task to be repeated on a regular interval, then the callback function that performs the task should create a new timer event each time it is called. The following example creates a callback function that is called 30 times per second. (Note that 1/30th of a second is 0.033 seconds, or 33 milliseconds.)

def animate():
    # Draw something
    my_button.after(33, animate)

my_button = tk.Button(application_window, text="Example")
my_button.after(33, animate)

A widget can have more than one timer event active at any time. In fact there is no limit to the number of timer events you can create.

Note that you should never use a loop to repeat a task in a GUI program. If you use a loop, the event-loop will be prevented from execution and no events will be processed while the loop is running. Always use a timer event for repeating a task, especially if a single execution of the task takes a considerable amount of CPU time.

15.30.2. Canceling Timer Events

In some cases you may need to cancel a timer event to prevent it from executing. This is straightforward using the after_cancel method. Remember that a widget can have multiple active timers, so the after_cancel method requires one parameter which specifies which timer event to cancel. If you need to cancel an event, you must capture the return value from the call to after when you created the event. Here is an example:

def do_something():
  # Some processing

my_button = tk.Button(application_window, text="Example")
timer_object = my_button.after(1000, do_something)


15.30.3. Multiple Parameters to Timer Callbacks

Timer callback function can have zero or more arguments passed to them when they are called. You specify the arguments when you create the event. This makes timer callback functions extremely flexible. Below is an example of three different callback functions, each of which receives a different number of arguments. You must specify the correct number of arguments for the callback function when you create the timer event.

def task1():
  # Do some processing

def task2(alpha):
  # Do some processing

def task3(beta, gamma):
  # Do some processing

my_button = tk.Button(application_window, text="Example")
my_button.after(1000, task1)
my_button.after(2000, task2, 3)     # 3 gets passed to the parameter alpha
my_button.after(5000, task3, a, b)  # a gets passed to the parameter beta
                                    # b gets passed to the parameter gamma
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