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name: FastAPI Testing description: This skill should be used when the user asks about "testing FastAPI", "FastAPI tests", "API testing", "test FastAPI endpoints", "mock FastAPI dependencies", "TestClient", "async API tests" version: 1.0.0

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/