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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def elapsed_time_s(self):
 """ Return the amount of time that has elapsed since the timer was started. Only works if the timer is active. """

        if self._start_time:
            return (datetime.datetime.now() - self._start_time).total_seconds()
        else:
            return 0