def __init__(self, period, init_value=0, init_endpoint=0):
        """
        first: if True, triggers at the first time
        """
        U.assert_type(period, int)
        assert period > 0
        U.assert_type(init_value, int)
        self.period = period
        self.value = init_value
        self._endpoint = init_endpoint

        first: if True, triggers at the first time
        
surreal/session/tracker.py