How do you test FastAPI applications?

Summary

Use TestClient (sync, based on httpx) or httpx.AsyncClient with ASGITransport for async tests, override dependencies (DB, auth, settings) with app.dependency_overrides, and use pytest fixtures for setup.

Longer Answer

For DB tests, run each test in a transaction that is rolled back, or use a disposable test database (e.g., Testcontainers). Wrap TestClient(app) in a with block to trigger lifespan events. Test both happy paths and validation errors (422), and use pytest-asyncio or anyio for async tests.

Code Example

from fastapi.testclient import TestClient

def fake_user():
    return User(id=1, email="[email protected]", role="admin")

def test_reports_requires_admin():
    app.dependency_overrides[get_current_user] = fake_user
    with TestClient(app) as client:
        r = client.get("/reports")
    assert r.status_code == 200
    app.dependency_overrides.clear()

Key Points

• TestClient(app) in a with block runs lifespan
• dependency_overrides to swap DB/auth
• httpx.AsyncClient(transport=ASGITransport(app)) for async
• Isolated DB per test