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:
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Define test version of dependency
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Add to app.dependency_overrides[original] = test_version
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Run tests
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Clear overrides after test
Common overrides:
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Database session → test database or mock
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Auth/current user → test user or bypassed auth
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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:
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Valid inputs → expected success
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Invalid email format → 422
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Missing required fields → 422
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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 factories
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
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
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FastAPI Testing: https://fastapi.tiangolo.com/tutorial/testing/
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HTTPX AsyncClient: https://www.python-httpx.org/