Fastapi Testing

FastAPI Testing

Patterns for testing FastAPI applications.

Core Concepts

Test Clients

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

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

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

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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/>