Agent Skills: Python Backend

Python Backend

Patterns for building production Python backends with asyncio, FastAPI, SQLAlchemy 2.0, and connection pooling. Each category has individual rule files in rules/ loaded on-demand.

Quick Reference

| Category | Rules | Impact | When to Use | |----------|-------|--------|-------------| | Asyncio | 3 | HIGH | TaskGroup, structured concurrency, cancellation handling | | FastAPI | 3 | HIGH | Dependencies, middleware, background tasks | | SQLAlchemy | 3 | HIGH | Async sessions, relationships, migrations | | Pooling | 3 | MEDIUM | Database pools, HTTP sessions, tuning |

Total: 12 rules across 4 categories

Quick Start

# FastAPI + SQLAlchemy async session
async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session_factory() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise

@router.get("/users/{user_id}")
async def get_user(user_id: UUID, db: AsyncSession = Depends(get_db)):
    result = await db.execute(select(User).where(User.id == user_id))
    return result.scalar_one_or_none()

# Asyncio TaskGroup with timeout
async def fetch_all(urls: list[str]) -> list[dict]:
    async with asyncio.timeout(30):
        async with asyncio.TaskGroup() as tg:
            tasks = [tg.create_task(fetch_url(url)) for url in urls]
    return [t.result() for t in tasks]

Asyncio

Modern Python asyncio patterns using structured concurrency, TaskGroup, and Python 3.11+ features.

Key Patterns

• TaskGroup replaces gather() with structured concurrency and auto-cancellation
• asyncio.timeout() context manager for composable timeouts
• Semaphore for concurrency limiting (rate-limit HTTP requests)
• except* with ExceptionGroup for handling multiple task failures
• asyncio.to_thread() for bridging sync code to async

Key Decisions

| Decision | Recommendation | |----------|----------------| | Task spawning | TaskGroup not gather() | | Timeouts | asyncio.timeout() context manager | | Concurrency limit | asyncio.Semaphore | | Sync bridge | asyncio.to_thread() | | Cancellation | Always re-raise CancelledError |

FastAPI

Production-ready FastAPI patterns for lifespan, dependencies, middleware, and settings.

Key Patterns

• Lifespan with asynccontextmanager for startup/shutdown resource management
• Dependency injection with class-based services and Depends()
• Middleware stack: CORS -> RequestID -> Timing -> Logging
• Pydantic Settings with .env and field validation
• Exception handlers with RFC 9457 Problem Details

Key Decisions

| Decision | Recommendation | |----------|----------------| | Lifespan | asynccontextmanager (not events) | | Dependencies | Class-based services with DI | | Settings | Pydantic Settings with .env | | Response | ORJSONResponse for performance | | Health | Check all critical dependencies |

SQLAlchemy

Async database patterns with SQLAlchemy 2.0, AsyncSession, and FastAPI integration.

Key Patterns

• One AsyncSession per request with expire_on_commit=False
• lazy="raise" on relationships to prevent accidental N+1 queries
• selectinload for eager loading collections
• Repository pattern with generic async CRUD
• Bulk inserts chunked 1000-10000 rows for memory management

Key Decisions

| Decision | Recommendation | |----------|----------------| | Session scope | One AsyncSession per request | | Lazy loading | lazy="raise" + explicit loads | | Eager loading | selectinload for collections | | expire_on_commit | False (prevents lazy load errors) | | Pool | pool_pre_ping=True |

Pooling

Database and HTTP connection pooling for high-performance async Python applications.

Key Patterns

• SQLAlchemy pool with pool_size, max_overflow, pool_pre_ping
• Direct asyncpg pool with min_size/max_size and connection lifecycle
• aiohttp session with TCPConnector limits and DNS caching
• FastAPI lifespan creating and closing pools at startup/shutdown
• Pool monitoring with Prometheus metrics

Pool Sizing Formula

pool_size = (concurrent_requests / avg_queries_per_request) * 1.5

Anti-Patterns (FORBIDDEN)

# NEVER use gather() for new code - no structured concurrency
# NEVER swallow CancelledError - breaks TaskGroup and timeout
# NEVER block the event loop with sync calls (time.sleep, requests.get)
# NEVER use global mutable state for db sessions
# NEVER skip dependency injection (create sessions in routes)
# NEVER share AsyncSession across tasks (race condition)
# NEVER use sync Session in async code (blocks event loop)
# NEVER create engine/pool per request
# NEVER forget to close pools on shutdown

Related Skills

• ork:architecture-patterns - Clean architecture and layer separation
• ork:async-jobs - Celery/ARQ for background processing
• streaming-api-patterns - SSE/WebSocket async patterns
• ork:database-patterns - Database schema design