Mock datetime.datetime.now

mock_now.py

import datetime


def mock_now(now):
    from datetime import datetime

    class FakeDateType(type):
        def __instancecheck__(self, instance):
            return isinstance(instance, datetime)

    class FakeDate(datetime, metaclass=FakeDateType):
        def __new__(cls, *args, **kwargs):
            return datetime.__new__(datetime, *args, **kwargs)

        @staticmethod
        def now(timezone=None):
            return now
    return mock.patch('datetime.datetime', FakeDate)


with mock_now(datetime.datetime(2013, 11, 22, 8, 0, 0)):
    assert datetime.datetime.now().strftime('%Y-%m-%d') == '2013-11-22'