Dependency Injection

DI Standard

Use FastAPI Depends only. No custom container, no global registries, no service locators.

Dependency Chain Pattern

# src/racerapi/modules/users/deps.py

def get_user_repo(db: Session = Depends(get_db)) -> UserRepo:
    return UserRepo(db)


def get_user_service(repo: UserRepo = Depends(get_user_repo)) -> UserService:
    return UserService(repo)

This chain gives: - explicit construction graph - easy test overrides - deterministic runtime wiring

Endpoint Injection Example

@router.get("/{user_id}")
def get_user(user_id: int, service: UserService = Depends(get_user_service)):
    return UserRead.model_validate(service.get_user(user_id))

DO

• Put DI providers in each module deps.py.
• Keep provider functions small and pure.
• Reuse core.get_db for DB session ownership.

DON'T

• Instantiate repos/services directly in route functions.
• Create module-level singleton services.
• Hide dependencies in globals.

Common Mistakes

• Writing service = UserService(UserRepo(SessionLocal())) in API handlers.
• Passing FastAPI Request down into service layer.
• Mixing DI and manual construction in same endpoint.

When to Break the Rule

In tests only, when overriding dependencies:

app.dependency_overrides[get_user_service] = lambda: FakeUserService()

Do not carry test wiring into production code.