def week_start_date(year, week):
    d = date(year, 1, 1)
delta_days = d.isoweekday() - 1
delta_weeks = week
if year == d.isocalendar()[0]:
    delta_weeks -= 1
delta = timedelta(days=-delta_days, weeks=delta_weeks)
return d + delta
Как в Python найти дату первого понедельника данной недели?