Context manager for patching python date and time functions for unit tests

time_offset.py

# Required because datetime.date is a builtin type that can't be modified
class MockDate(date):
    "A wrapper for date that can be mocked for testing."

    def __new__(cls, *args, **kwargs):
        return date.__new__(date, *args, **kwargs)

# Required because datetime.datetime is a builtin type that can't be modified
class MockDateTime(datetime):
    "A wrapper for datetime that can be mocked for testing."

    def __new__(cls, *args, **kwargs):
        return datetime.__new__(date, *args, **kwargs)

@contextmanager
def time_offset(**kwargs):
    """
    Returns a context manager that changes the return value of date and time
    functions for it's duration. The value will be an offset from the real time,
    specified by kwargs as used by timedelta.

    The following functions are patched:
        time.time, date.today, datetime.today, datetime.now, datetime.utcnow
    """
    time_time = time.time
    date_today = date.today
    datetime_today = datetime.today
    datetime_now = datetime.now
    datetime_utcnow = datetime.utcnow

    delta = timedelta(**kwargs)
    with patch('time.time', lambda: time_time() + delta.total_seconds()), \
         patch('datetime.date', MockDate), \
         patch('datetime.datetime', MockDateTime), \
         patch('datetime.date.today', lambda: date_today() + delta), \
         patch('datetime.datetime.today', lambda: datetime_today() + delta), \
         patch('datetime.datetime.now', lambda: datetime_now() + delta), \
         patch('datetime.datetime.utcnow', lambda: datetime_utcnow() + delta):
        yield