Python context manager for mocking out datetime.now() in unit tests.

mocknow.py

import contextlib
import datetime


@contextlib.contextmanager
def mock_now(dt_value):
    """Context manager for mocking out datetime.now() in unit tests.

    Example:
    with mock_now(datetime.datetime(2011, 2, 3, 10, 11)):
        assert datetime.datetime.now() == datetime.datetime(2011, 2, 3, 10, 11)

    """

    class MockDateTime(datetime.datetime):
        @classmethod
        def now(cls):
            # Create a copy of dt_value.
            return datetime.datetime(
                dt_value.year, dt_value.month, dt_value.day,
                dt_value.hour, dt_value.minute, dt_value.second, dt_value.microsecond,
                dt_value.tzinfo
            )
    real_datetime = datetime.datetime
    datetime.datetime = MockDateTime
    try:
        yield datetime.datetime
    finally:
        datetime.datetime = real_datetime