How do you write tests for Flask and FastAPI applications?

Flask testing: use the built-in test client: def test_home(client): response = client.get('/'); assert response.status_code == 200. Create a pytest fixture: @pytest.fixture def client(): app = create_app('testing'); with app.test_client() as c: yield c. FastAPI testing: use Starlette's TestClient (wraps httpx): from fastapi.testclient import TestClient; client = TestClient(app); def test_read(): response = client.get('/users/1'); assert response.status_code == 200. For async tests with httpx: async with AsyncClient(app=app, base_url="http://test") as ac: response = await ac.get('/'). Both frameworks support overriding dependencies in tests: FastAPI uses app.dependency_overrides to inject mocks. Always use a test database — configure via environment variables or fixtures.