How to test FastAPI routes without hitting the database using dependency overrides

FastAPI's dependency_overrides feature allows developers to replace database and authentication dependencies during testing without using Python's patch utility. Unlike patch, which fails because FastAPI resolves dependencies at import time, dependency_overrides works directly with the function object registered on the app. Developers can override get_db with an in-memory test session, and override get_current_user with a mock user to test protected routes without generating real JWTs. A critical teardown step—calling app.dependency_overrides.clear()—is necessary after each test, since the app is a module-level singleton and leftover overrides can silently affect the entire test suite. The technique is suited for testing endpoint wiring, but not a substitute for dedicated database tests that verify constraints, migrations, and transaction behavior.