Dependency Injection and Partial Mocking

test_xmas.py

class XmasTest(unittest.TestCase):

    @mock.patch('xmas.date')
    def test_days_til_xmas_when_xmas(self, mock_date):
        mock_date.today.return_value = date(2020, 12, 25)
        mock_date.side_effect = lambda *args, **kw: date(*args, **kw)

        days = days_til_xmas()

        self.assertEqual(days, 0)

    @mock.patch('xmas.date')
    def test_days_til_xmas(self, mock_date):
        mock_date.today.return_value = date(2020, 10, 12)
        mock_date.side_effect = lambda *args, **kw: date(*args, **kw)

        days = days_til_xmas()

        self.assertEqual(days, 74)

    @mock.patch('xmas.date')
    def test_days_til_xmas_when_xmas_passed(self, mock_date):
        mock_date.today.return_value = date(2020, 12, 26)
        mock_date.side_effect = lambda *args, **kw: date(*args, **kw)

        days = days_til_xmas()

        self.assertEqual(days, 364)

    def test_days_til_xmas_di_when_xmas(self):
        from_date = date(2020, 12, 25)

        days = days_til_xmas_di(from_date)

        self.assertEqual(days, 0)

    def test_days_til_xmas_di(self):
        from_date = date(2020, 10, 12)

        days = days_til_xmas_di(from_date)

        self.assertEqual(days, 74)

    def test_days_til_xmas_di_when_xmas_passed(self):
        from_date = date(2020, 12, 26)

        days = days_til_xmas_di(from_date)

        self.assertEqual(days, 364)


if __name__ == '__main__':
    unittest.main()

xmas.py

from datetime import date
from dateutil.relativedelta import relativedelta


def days_til_xmas():
    """Returns number of days until xmas.
    Doesn't use dependency injection.
    """
    from_date = date.today()

    xmas = date(from_date.year, 12, 25)

    diff = (xmas - from_date).days
    if diff >= 0:
        return diff

    xmas += relativedelta(years=1)

    return (xmas - from_date).days


def days_til_xmas_di(from_date):
    """Returns number of days until xmas.
    Uses dependency injection.
    """
    xmas = date(from_date.year, 12, 25)

    diff = (xmas - from_date).days
    if diff >= 0:
        return diff

    xmas += relativedelta(years=1)

    return (xmas - from_date).days