Testing with Lagom

Since lagom leaves most code unmodified testing can be done on the plain objects without needing to make any changes at all. One of the goals of lagom is to enable this style of testing and avoid any patching.

For example the following class:

class UserPremiumService:

    # The DB here will be injected by lagom at runtime. But we don't need to modify this
    # class at at all to allow this.
    def __init__(self, db: DB):
        pass

    def upgrade_user_to_premium(self, user):
        pass

this can be tested like this:

def test_user_can_be_made_premium():
    mock_db = MockDBOfSomeKind()
    service = UserPremiumService(mock_db)
    test_user = SomeUser()

    service.make_user_premium(test_user)

    assert_mock_db_has_premium_user(mock_db, test_user)

Your tests now have no dependency on global state, patching or lagom itself.

Using a container in tests

Although the testing style above is the goal of lagom you may want some tests with a copy of the production setup but with certain key dependencies mocked out. Once a container has been cloned definitions can be altered.

Taking container from the example here we can create a fixture like this which patches out how communication works:

def container_fixture():
    from my_app.prod_container import container
    test_container = container.clone() # Cloning enables overwriting deps
    test_container[DiceClient] = StubbedResponseClient()
    return test_container

and then it can be used like this:

def test_something(container_fixture: Container):
    container_fixture[DiceClient] = FakeDice(always_roll=6)
    game_to_test = container_fixture[Game]
    # TODO: act & assert on something