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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 at(self, time_str):
 """ Schedule the job every day at a specific time. Calling this is only valid for jobs scheduled to run every N day(s). :param time_str: A string in `XX:YY` format. :return: The invoked job instance """

        assert self.unit in ('days', 'hours') or self.start_day
        hour, minute = time_str.split(':')
        minute = int(minute)
        if self.unit == 'days' or self.start_day:
            hour = int(hour)
            assert 0 <= hour <= 23
        elif self.unit == 'hours':
            hour = 0
        assert 0 <= minute <= 59
        self.at_time = datetime.time(hour, minute)
        return self