Python time machine contextmanager for tests

gistfile1.py

# Except it would not work, because TypeError: can't set attributes of built-in/extension type 'datetime.datetime'
@contextmanager
def time_machine(now=None, utcnow=None, today=None):
    if now is None and utcnow is None and today is None:
        raise ValueError(u"At least one of `now`, `utcnow` or `today` must be specified.")

    real_now = datetime.now()
    real_utcnow = datetime.utcnow()
    tz_delta = real_now - real_utcnow

    if now is None and today is not None:
        now = real_now.replace(year=today.year, month=today.month, day=today.day)
        utcnow = now - tz_delta
    if utcnow is None and today is not None:
        utcnow = real_utcnow.replace(year=today.year, month=today.month, day=today.day)
        now = utcnow + tz_delta
    if today is None:
        today = now.date()

    mock_now, mock_utcnow, mock_today = lambda: now, lambda: utcnow, lambda: today
    (save_now, save_utcnow, save_today, datetime.now, datetime.utcnow, date.today) = (
        datetime.now, datetime.utcnow, date.today, mock_now, mock_utcnow, mock_today)
    yield
    datetime.now, datetime.utcnow, date.today = save_now, save_utcnow, save_today