python-backend

SKILL.md

python-backend

Production-ready Python backend patterns for FastAPI, SQLAlchemy, and Upstash.

When to Use This Skill

Building REST APIs with FastAPI

Implementing JWT/OAuth2 authentication

Setting up SQLAlchemy async databases

Integrating Redis/Upstash caching and rate limiting

Refactoring AI-generated Python code

Designing API patterns and project structure

Core Principles

Async-first - Use async/await for I/O operations

Type everything - Pydantic models for validation

Dependency injection - Use FastAPI's Depends()

Fail fast - Validate early, use HTTPException

Security by default - Never trust user input

Quick Patterns

Project Structure

src/
├── auth/
│   ├── router.py      # endpoints
│   ├── schemas.py     # pydantic models
│   ├── models.py      # db models
│   ├── service.py     # business logic
│   └── dependencies.py
├── posts/
│   └── ...
├── config.py
├── database.py
└── main.py

Async Routes

# BAD - blocks event loop
@router.get("/")
async def bad():
    time.sleep(10)  # Blocking!

# GOOD - runs in threadpool
@router.get("/")
def good():
    time.sleep(10)  # OK in sync function

# BEST - non-blocking
@router.get("/")
async def best():
    await asyncio.sleep(10)  # Non-blocking

Pydantic Validation

from pydantic import BaseModel, EmailStr, Field

class UserCreate(BaseModel):
    email: EmailStr
    username: str = Field(min_length=3, max_length=50, pattern="^[a-zA-Z0-9_]+$")
    age: int = Field(ge=18)

Dependency Injection

async def get_current_user(token: str = Depends(oauth2_scheme)) -> User:
    payload = decode_token(token)
    user = await get_user(payload["sub"])
    if not user:
        raise HTTPException(401, "User not found")
    return user

@router.get("/me")
async def get_me(user: User = Depends(get_current_user)):
    return user

SQLAlchemy Async

from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine

engine = create_async_engine(DATABASE_URL, pool_pre_ping=True)
SessionLocal = async_sessionmaker(engine, expire_on_commit=False)

async def get_session() -> AsyncGenerator[AsyncSession, None]:
    async with SessionLocal() as session:
        yield session

Redis Caching

from upstash_redis import Redis

redis = Redis.from_env()

@app.get("/data/{id}")
def get_data(id: str):
    cached = redis.get(f"data:{id}")
    if cached:
        return cached
    data = fetch_from_db(id)
    redis.setex(f"data:{id}", 600, data)
    return data

Rate Limiting

from upstash_ratelimit import Ratelimit, SlidingWindow

ratelimit = Ratelimit(
    redis=Redis.from_env(),
    limiter=SlidingWindow(max_requests=10, window=60),
)

@app.get("/api/resource")
def protected(request: Request):
    result = ratelimit.limit(request.client.host)
    if not result.allowed:
        raise HTTPException(429, "Rate limit exceeded")
    return {"data": "..."}

Reference Documents

For detailed patterns, see:

Document
Content

references/fastapi_patterns.md
Project structure, async, Pydantic, dependencies, testing

references/security_patterns.md
JWT, OAuth2, password hashing, CORS, API keys

references/database_patterns.md
SQLAlchemy async, transactions, eager loading, migrations

references/upstash_patterns.md
Redis, rate limiting, QStash background jobs

Resources

FastAPI Documentation

SQLAlchemy 2.0 Documentation

Upstash Documentation

Pydantic Documentation