week_start_day.py

from datetime import datetime, timedelta, date
from unittest import TestCase

def get_week_start_day(dt, firstday='monday'):
    if firstday == 'monday':
        return dt.date() - timedelta(days=(dt.weekday() + 0)%7)
    elif firstday == 'sunday':
        return dt.date() - timedelta(days=(dt.weekday() + 1)%7)

class TestWeekMath(TestCase):
    def test_week_start_day(self):
        # sunday
        d = get_week_start_day(datetime(2017, 1, 1, 5, 5, 5), firstday='monday')
        self.assertEqual(d, date(2016, 12, 26))

        # monday
        d = get_week_start_day(datetime(2017, 1, 2, 5, 5, 5), firstday='monday')
        self.assertEqual(d, date(2017, 1, 2))

        # sunday
        d = get_week_start_day(datetime(2017, 1, 1, 5, 5, 5), firstday='sunday')
        self.assertEqual(d, date(2017, 1, 1))

        # monday
        d = get_week_start_day(datetime(2017, 1, 2, 5, 5, 5), firstday='sunday')
        self.assertEqual(d, date(2017, 1, 1))