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

        @param interval: The number of seconds between calls.  May be
        less than one.  Precision will depend on the underlying
        platform, the available hardware, and the load on the system.

        @param now: If True, run this call right now.  Otherwise, wait
        until the interval has elapsed before beginning.

        @return: A Deferred whose callback will be invoked with
        C{self} when C{self.stop} is called, or whose errback will be
        invoked when the function raises an exception or returned a
        deferred that has its errback invoked.
        """
        assert not self.running, ("Tried to start an already running "
                                  "LoopingCall.")
        if interval < 0:
            raise ValueError("interval must be >= 0")
        self.running = True
        # Loop might fail to start and then self._deferred will be cleared.
        # This why the local C{deferred} variable is used.
        deferred = self._deferred = defer.Deferred()
        self.starttime = self.clock.seconds()
        self.interval = interval
        self._runAtStart = now
        if now:
            self()
        else:
            self._scheduleFrom(self.starttime)
        return deferred

        Start running function every interval seconds.

        @param interval: The number of seconds between calls.  May be
        less than one.  Precision will depend on the underlying
        platform, the available hardware, and the load on the system.

        @param now: If True, run this call right now.  Otherwise, wait
        until the interval has elapsed before beginning.

        @return: A Deferred whose callback will be invoked with
        C{self} when C{self.stop} is called, or whose errback will be
        invoked when the function raises an exception or returned a
        deferred that has its errback invoked.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/task.py