To unschedule a callback, we will first need the alarm identifier returned by after(). We will store this identifier in the scheduled_id attribute, since we will need it in a separate method:

    def start_action(self):
        self.button.config(state=tk.DISABLED)
        self.cancel.config(state=tk.NORMAL)
        self.scheduled_id = self.after(5000, self.init_buttons)

Then, this field is passed to after_cancel() in the callback of the Stop button:

    def cancel_action(self):
        print("Canceling scheduled", self.scheduled_id)
        self.after_cancel(self.scheduled_id)
        self.init_buttons()

In our case, it is important to disable the Start button once it is clicked, because if start_action() is called twice, scheduled_id would be overridden, and the Stop button could only cancel the last scheduled action.

As a side note, after_cancel() has no effect if we call it with an alarm identifier that has already been executed.