Testing

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.

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 injections.
• 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.
• When injecting interfaces and/or qualifiers, override the interface and/or qualifier rather than the implementation that will be injected.

Tip

If you're using an integration to get the container instance you can use the wireup.integration.xxx.get_app_container method. This returns the container associated with your application.

Context Manager

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

with 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")

Pytest

app.py

def create_app():
    app = ...

    container = wireup.create_container(...)
    # Example shows FastAPI but any integration will work the same.
    wireup.integration.fastapi.setup(container, app)

    return app

conftest.py

# This is a function scoped fixture which means 
# you'll get a fresh copy of the application and container every time.
@pytest.fixture
def app():
    return create_app()

some_test_file.py

from wireup.integration.fastapi import get_app_container

def test_something_with_mocked_db_service(client: TestClient, app):
    with get_app_container(app).override.service(DBService, new=...):
        response = client.get("/some/path")

    # Assert response and mock calls.