from datetime import date, timedelta

def iso_week_start(year, week):
    """Get the date of monday for any given iso week of any given year

    params:
    year, the year of intrest
    week, the week of which the date is desired
    """
  
    d = date(year, 1, 1)
    if d.isocalendar()[1] != 1:
        if d.weekday() > 3: #  Jan 1 is not in week one
            d += timedelta(7 - d.weekday())
    return d + timedelta(7 * (week - 1))


if __name__ == '__main__':
    assert(date(2012, 3, 26) == iso_week_start(2012, 13))
    assert(date(2012, 1, 2) == iso_week_start(2012, 1))
    assert(date(2004, 1, 1) == iso_week_start(2004, 1))
    assert(date(1998, 12, 31) == iso_week_start(1998, 53))