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.

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Dependency Override Pattern

FastAPI's killer feature for testing: replace any dependency.

How it works:

1. Define test version of dependency
2. Add to app.dependency_overrides[original] = test_version
3. Run tests
4. Clear overrides after test

Common overrides:

• Database session → test database or mock
• Auth/current user → test user or bypassed auth
• External services → mocked responses

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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

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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.

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Project Structure

tests/
├── conftest.py       # Shared fixtures (client, db, auth)
├── test_users.py     # Endpoint tests
├── test_auth.py      # Auth-specific tests
└── factories/        # Test data factories

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Common 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 |

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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/