Testing

Unit tests

Unit testing service objects is meant to be easy as the container does not interfere in any way with the underlying classes.

Classes can be instantiated as usual in tests, and you need to pass dependencies such as services or parameters to them yourself.

To specify custom behavior for tests, provide a custom implementation or a subclass that returns test data as a dependency instead of mocks.

It is also possible to use the container to build a part of your dependencies by calling container.get(ThingService) which will return a ThingService instance.

Overriding

Sometimes you need to be able to swap a service object on the fly for a different one such as a mock.

The container.override property provides access to a number of useful methods and context managers which help with overriding dependencies (See override manager).

Good to know

• Overriding only applies to future autowire calls.
• Once a singleton service has been instantiated, it is not possible to directly replace any of its direct or transitive dependencies via overriding as the object is already in memory.
  • You will need to call container.clear_initialized_objects() and then override the desired service. This will make the container use the override when the new copy of the service is being built.
• When using injecting interfaces and/or qualifiers, override the interface and/or qualifier rather than the implementation that will be injected.

Examples

Context Manager

random_mock = MagicMock()
# Chosen by fair dice roll. Guaranteed to be random.
random_mock.get_random.return_value = 4

with self.container.override.service(target=RandomService, new=random_mock):
    # Assuming in the context of a web app:
    # /random endpoint has a dependency on RandomService
    # requests to inject RandomService during the lifetime
    # of this context manager will result in random_mock being injected instead.
    response = client.get("/random")

Python unittest

Use the setup method to replace a service with a mock for the duration of the test.

class SomeEndpointTest(unittest.TestCase):
    def setUp(self) -> None:
        self.db_service = MagicMock()

        # Drop references to initialized objects.
        # Services or autowire targets requesting DBService
        # will get the mocked object instead.
        container.clear_initialized_objects()
        container.override.service(DbService, new=self.db_service)

Pytest

Similar to the above example but this uses pytest's autouse to achieve the same result.

@pytest.fixture(autouse=True)
def setup_container(db_service_mock: MagicMock) -> None:
    container.clear_initialized_objects()
    container.override.service(DbService, new=db_service_mock)

def test_something_with_mocked_db_service(client: TestClient, db_service_mock: MagicMock):
    # Set up the db service mock
    db_service_mock.get_things.return_value = ...
    response = client.get("/some/path")

    # Assert response and mock calls.