SleepTimer in python, not to sleep to long

SleepTimer.py

class SleepTimer(object):
    """
    timer = SleepTime(10)
    do_something_for_3_sec()
    timer.sleep() # will sleep 7
    """
    def __init__(self, how_long):
        self.how_long = how_long
        self.start_time = datetime.datetime.now()
        self.i_have_slept = False

    def sleep(self):
        assert not self.i_have_slept, "I have already slept, create new object please."
        self.i_have_slept = True
        end_time = datetime.datetime.now()
        sec_already_passed = int((end_time - self.start_time).total_seconds())
        if sec_already_passed < self.how_long:
            time.sleep(self.how_long - sec_already_passed)