How many working hours from friday 16:30 to monday 15:30?

working_hours.py

from datetime import time, datetime, timedelta


def working_hours(dt_start, dt_end, work_periods, work_days=range(5)):
    dur = (dt_end.date() - dt_start.date()).days + 1
    days = (dt_start.date() + timedelta(x) for x in range(dur))
    return sum(
        [(min(max(datetime.combine(d, h[1]), dt_start), dt_end) -
          min(max(datetime.combine(d, h[0]), dt_start), dt_end)
          ).total_seconds()/3600
         for d in days for h in work_periods
         if d.weekday() in work_days])


if __name__ == '__main__':
    work_periods = [(time(9), time(13)), (time(14, 30), time(18, 30))]
    dt0 = datetime(2013, 11, 22, 16, 30)  # Friday
    dt1 = datetime(2013, 11, 25, 15, 30)  # Monday
    from pprint import pprint
    print('%s -> %s:' % (dt0.isoformat(), dt1.isoformat()))
    pprint(working_hours(dt0, dt1, work_periods))

Remove "sum" at line 11 to better see what's going on...