Testing

Quickstart

We have to use the service provider to resolve the services we want to test.

FastAPIConsole application

We have to import services from main.py, and create a fixture to inject the service provider.

from main import services

@pytest.fixture
async def service_provider() -> AsyncGenerator[ServiceProvider]:
    async with services.build_service_provider() as service_provider:
        yield service_provider

Note

If you're thinking about FastAPI's TestClient, it doesn't work well with async, and AsyncClient doesn't execute lifespan events.

We can't import services directly from main.py because it's created inside the main function. We have to create the function configure_services (or move the services variable outside the main function).

def configure_services() -> ServiceCollection:
    services = ServiceCollection()
    ...

    return services

The next step is creating a fixture to inject the service provider.

from main import configure_services

@pytest.fixture
async def service_provider() -> AsyncGenerator[ServiceProvider]:
    services = configure_services()

    async with services.build_service_provider() as service_provider:
        yield service_provider

And then we can inject it into our tests and resolve the services.

async def test_create_user(service_provider: ServiceProvider) -> None:
    user_service = await service_provider.get_required_service(UserService)

    await user_service.create_user()

Override services

UserService could have the dependency EmailService, that sends real emails. During testing, we want to replace it with a mock implementation that doesn't send real emails.

To replace a service during testing, we can use the override_service and override_keyed_service methods provided by ServiceProvider. This allows us to temporarily replace a service for the duration of context manager block.

async def test_create_user(service_provider: ServiceProvider, mocker: MockerFixture) -> None:
    email_service_mock = mocker.create_autospec(EmailService, instance=True)

    with service_provider.override_service(EmailService, email_service_mock):
        user_service = await service_provider.get_required_service(UserService)

        await user_service.create_user()

Globally override services

We can also override a service for all tests by modifying the fixture that provides the ServiceProvider instance. This is useful when we want to use a mock for a service across multiple tests, or all tests.

@pytest.fixture
async def service_provider(mocker: MockerFixture) -> AsyncGenerator[ServiceProvider]:
    async with services.build_service_provider() as service_provider:
        email_service_mock = mocker.create_autospec(EmailService, instance=True)

        with service_provider.override_service(EmailService, email_service_mock):
            yield service_provider

ServiceCollection registration

The context manager approach is straightforward, but if we want to test a more complex scenario, we can directly register the mock implementation in the ServiceCollection before building the ServiceProvider. This way, the mock will be used whenever EmailService is resolved.

@pytest.fixture
async def service_provider(mocker: MockerFixture) -> AsyncGenerator[ServiceProvider]:
    services = configure_services()
    email_service_mock = mocker.create_autospec(EmailService, instance=True)
    services.add_singleton(EmailService, email_service_mock)

    async with services.build_service_provider() as service_provider:
        yield service_provider

Remember that if EmailService is already registered, registering it again means the last registration will be used when resolving the service.

Note: Another strategy could be not to register EmailService in local, and register it depending on the environment.