attempt to prevent the timer from ever running its function

        :returns:
            ``True`` if it was successful, ``False`` if the timer had already
            run or been cancelled

def cancel(self):
        """attempt to prevent the timer from ever running its function

        :returns:
            ``True`` if it was successful, ``False`` if the timer had already
            run or been cancelled
        """
        done = self.finished.is_set()
        self.finished.set()
        return not done