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