def stop(self, n=1):
    if (self.start_time is not None):
        delta = (time.time() - self.start_time)
        self.sum += delta
        self.n += n
        self.start_time = None
def stop(self, n=1):
    if (self.start_time is not None):
        delta = (time.time() - self.start_time)
        self.sum += delta
        self.n += n
        self.start_time = None