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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 start(self, interval, now=True):
 """Start running pre-set function every interval seconds. """

        if interval < 0:
            raise ValueError('interval must be >= 0')

        if self._running:
            self.stop()

        self._running = True
        self._interval = interval
        if now:
            self._self_thread = hub.spawn_after(0, self)
        else:
            self._self_thread = hub.spawn_after(self._interval, self)