def set_frequency(self, host, sem=None, interval=None):
        """Set frequency for host with sem and interval."""
        # single sem or global sem
        sem = sem or self.sem
        interval = self.interval if interval is None else interval
        frequency = Frequency(sem, interval, host)
        frequencies = {host: frequency}
        self.update_frequency(frequencies)
        return frequency
Set frequency for host with sem and interval.