[Python] Mocking datetime in unit tests

mock_datetime.py

# Taken from https://solidgeargroup.com/mocking-the-time
# (unlike many answers on StackOverflow, this one actually works and is easy to use)

import datetime
import mock


def mock_datetime(target, datetime_module):
    real_datetime_class = datetime.datetime

    class DatetimeSubclassMeta(type):
        @classmethod
        def __instancecheck__(mcs, obj):
            return isinstance(obj, real_datetime_class)

    class BaseMockedDatetime(real_datetime_class):
        @classmethod
        def now(cls, tz=None):
            return target.replace(tzinfo=tz)

        @classmethod
        def utcnow(cls):
            return target

        @classmethod
        def today(cls):
            return target

    # Python2 & Python3-compatible metaclass
    mocked_datetime = DatetimeSubclassMeta('datetime', (BaseMockedDatetime,), {})
    return mock.patch.object(datetime_module, 'datetime', mocked_datetime)
    
    
# usage:

import mock_datetime from mock_datetime
    …
    def test_is_new_years_eve_positive(self):
    “””
    Test that it’s the 25 December
    “””
    target = datetime.datetime(2014, 12, 31)

    with mock_datetime(target, datetime):
        is_new_years_eve = date_helpers.is_new_years_eve()
        self.assertTrue(is_new_years_eve)

    target = datetime.datetime(2014, 11, 31)

    with mock_datetime(target, datetime):
        is_new_years_eve = date_helpers.is_new_years_eve()
        self.assertFalse(is_new_years_eve)

    #date_helpers.py
    #def is_new_years_eve(self):
    #    today = datetime.datetime.today()
    #    return today.month is 12 and today.day is 31