FastAPI Patterns

When to use this skill

Use this skill when building Python APIs with FastAPI and you want consistent boundary validation, async correctness, and maintainable dependency injection patterns.

Core patterns

Pydantic at boundaries

• validate request/response models explicitly
• avoid returning raw dicts from endpoints
• keep internal domain models separate from API schemas

Async correctness

• prefer async DB/HTTP libraries when using async endpoints
• avoid blocking calls inside async def

Dependencies (DI)

• use dependencies for auth, DB sessions, feature flags
• keep dependencies small and composable

Error handling

• raise HTTPException for boundary errors
• wrap lower-level errors with context; log server-side details, keep user messages safe

Testing

• unit tests for pure logic
• integration tests with TestClient / httpx
• test error paths explicitly

Atlarix tool notes

• Explore: map routers and dependencies; locate schema models.
• Build: run pytest; keep endpoint changes small and typed.
• Debug: reproduce with a single request; inspect validation errors first.
• Review: check schema correctness, authz, and async blocking.

Common mistakes to avoid

• Mixing domain logic into endpoint functions
• Returning raw dicts everywhere
• Blocking IO in async endpoints

Mini-checklist

• Typed schemas at boundaries
• DI used intentionally
• Error paths tested
• Async is consistent