⚡ Section 8 — Mid-Level Masterclass
Dependency Injection & Testable Architecture
Decouple your services from concrete instantiation. Use FastAPI's Depends() to inject database sessions, repositories, and authentication contexts.
👁️ Views: 374 Page Views
💡 Mindset Shift
Hardcoded Instantiation vs FastAPI Dependency Injection
Hardcoding database connections inside functions prevents mock testing. Dependency injection turns dependencies into configurable parameters.
❌ Hardcoded Database Instantiation (Untestable)
# ❌ Creates real DB connection inside handler
@router.post("/users")
def create_user(data: UserCreate):
db = SessionLocal() # Hardcoded dependency!
repo = SqlAlchemyUserRepository(db)
service = UserService(repo)
return service.create_user(data)
✅ FastAPI Depends() Injection (100% Testable)
# ✅ Injected automatically via Depends() provider
@router.post("/users")
def create_user(
data: UserCreate,
service: UserService = Depends(get_user_service)
):
return service.create_user(data)⚡ The FastAPI Provider Chain
1. get_db()
➔
2. get_user_repository(db=Depends())
➔
3. get_user_service(repo=Depends())
➔
4. @router.post(service=Depends())
💻 Production Blueprint: Provider Factories & Pytest Overrides
Observe how `app.dependency_overrides` lets you swap out real database providers for mock implementations during automated testing.
# ─── 1. DEPENDENCY PROVIDER FACTORY (api/dependencies.py) ─────────────────
def get_user_service(repo: UserRepository = Depends(get_user_repository)) -> UserService:
return UserService(repository=repo)
# ─── 2. FASTAPI ROUTE CONSUMPTION (api/routes.py) ──────────────────────────
@router.post("/users", status_code=201)
def create_user(data: UserCreate, service: UserService = Depends(get_user_service)):
return service.create_user(data)
# ─── 3. PYTEST OVERRIDE IN MOCK TEST (tests/test_users.py) ─────────────────
def get_mock_user_service() -> UserService:
mock_repo = InMemoryUserRepository()
return UserService(repository=mock_repo)
# Override real database service with mock during test execution
app.dependency_overrides[get_user_service] = get_mock_user_service
def test_create_user_fast_mock():
client = TestClient(app)
res = client.post("/users", json={"email": "test@test.com", "password": "pass"})
assert res.status_code == 201