FastAPI Testing
Patterns for testing FastAPI applications.
Core Concepts
Test Clients
| Client | Use Case | Async Support |
|---|---|---|
| TestClient | Sync tests (most cases) | No |
| AsyncClient (httpx) | Async endpoints, lifespan events | Yes |
Key concept: TestClient wraps the ASGI app - no server needed.
Dependency Override Pattern
FastAPI's killer feature for testing: replace any dependency.
How it works:
- Define test version of dependency
- Add to
app.dependency_overrides[original] = test_version - Run tests
- Clear overrides after test
Common overrides:
- Database session → test database or mock
- Auth/current user → test user or bypassed auth
- External services → mocked responses
Test Patterns
CRUD Testing Checklist
| Operation | Happy Path | Error Cases |
|---|---|---|
| Create | Returns 201, correct data | 400 validation, 409 duplicate |
| Read | Returns 200, correct data | 404 not found |
| Update | Returns 200, updated data | 404, 400 validation |
| Delete | Returns 204 | 404 not found |
Validation Testing
Use parametrize for boundary conditions:
- Valid inputs → expected success
- Invalid email format → 422
- Missing required fields → 422
- Too short/long values → 422
Database Testing Strategies
| Strategy | Pros | Cons |
|---|---|---|
| In-memory SQLite | Fast, isolated | Different from prod DB |
| Test database | Realistic | Slower, needs setup |
| Transactions + rollback | Fast, realistic | Complex setup |
| Mocked repository | Fastest | Less integration coverage |
Key concept: Use function-scoped fixtures to ensure test isolation.
Project Structure
tests/
├── conftest.py # Shared fixtures (client, db, auth)
├── test_users.py # Endpoint tests
├── test_auth.py # Auth-specific tests
└── factories/ # Test data factoriesCommon Pitfalls
| Pitfall | Solution |
|---|---|
| Shared state between tests | Use function-scoped fixtures |
| Forgetting to clear overrides | Use fixture with cleanup |
| Testing implementation not behavior | Focus on HTTP responses |
| Missing async marks | Add @pytest.mark.asyncio |
| SQLite vs Postgres differences | Use same DB type for important tests |
Quick Reference
Test client fixture pattern: Create client, set overrides, yield, clear overrides
Protected endpoint testing: Override get_current_user dependency
File upload testing: Use files={"file": (name, content, mimetype)}
Resources
- FastAPI Testing: https://fastapi.tiangolo.com/tutorial/testing/
- HTTPX AsyncClient: https://www.python-httpx.org/