fastapi

name	fastapi
description	FastAPI Python web framework. Use when building async APIs with FastAPI, Pydantic v2 models, dependency injection, WebSocket endpoints, or OpenAPI auto-documentation.

fastapi

Modern Python web framework built on Starlette and Pydantic for high-performance async APIs.

Core Concepts

• Async endpoints: define routes with async def, FastAPI handles concurrency via asyncio
• Pydantic v2 models: request/response validation with BaseModel, automatic JSON Schema generation
• Dependency Injection: Depends() resolves reusable components across route handlers
• OpenAPI/Swagger: auto-generated interactive docs at /docs and /redoc from type hints

Common Patterns

• Background tasks: BackgroundTasks for post-response operations like email or logging
• CORS middleware: CORSMiddleware with allow_origins for cross-origin browser requests
• WebSocket: WebSocket endpoint with websocket.accept() for real-time bidirectional communication
• Router modularity: APIRouter with prefix and tags to organize endpoints by domain

Best Practices

• Use Pydantic v2 model_validator over v1 custom validators for cleaner validation logic
• Split routes into versioned routers (v1, v2) for backward-compatible API evolution
• Set response_model on route decorators to control serialization and docs schema
• Use httpx.AsyncClient with TestClient for testing async endpoint chains

Common Code Patterns

• @app.post("/items") with Depends(get_current_user) for auth-gated endpoints
• async with db.session() context manager for transactional database operations