/fastapi-pro

name: fastapi-pro
type: reference
description: "Production FastAPI patterns — async endpoints, SQLAlchemy 2.0 async, Pydantic V2, dependency injection, JWT auth, testing. Use for Python 3.11+ FastAPI backends. NOT for Django (→ `django-patterns`) or Node.js (→ `backend-patterns`)."
paths: ["**/*.py", "**/requirements*.txt", "**/pyproject.toml", "**/main.py", "**/app/**/*.py"]
effort: 3
allowed-tools: Read, Glob, Grep, Write, Edit, Bash
user-invocable: true
when_to_use: "When building async FastAPI APIs with SQLAlchemy 2.0 async + Pydantic V2 — endpoints, auth, DB session handling, testing, deployment"

FastAPI Production Patterns

Critical rules (non-obvious)

• **`async def` endpoint blocking sync DB call** → blocks entire event loop. Either use `async` DB driver (asyncpg/aiomysql) throughout OR switch endpoint to plain `def` (FastAPI runs it in threadpool).
• **Pydantic V2 `model_config = ConfigDict(...)` replaces V1 `class Config`**. Forgetting this silently loses settings like `from_attributes=True` needed for ORM → DTO conversion.
• **`Depends()` caches per-request**: same dependency called twice in one request returns same instance. Don't rely on this for cross-request state — use app state / Redis instead.
• **SQLAlchemy 2.0 async session must not leak across requests**: always scope via `Depends` with `async with AsyncSession(...)` — raw module-level session causes `GreenletError` under load.
• **`BackgroundTasks` runs AFTER response sent in the same worker process**: if worker dies mid-task the work is lost. For durable background jobs use Celery / Dramatiq / ARQ.
• **Uvicorn `--workers N` forks processes — can't share in-memory state**. Use Redis or DB for any shared state (rate-limit counters, cache).

Project layout

app/
├── main.py               # FastAPI() instance + lifespan
├── api/
│   ├── deps.py           # shared Depends (get_db, get_current_user)
│   └── v1/
│       ├── users.py      # APIRouter
│       └── products.py
├── core/
│   ├── config.py         # Pydantic Settings
│   ├── security.py       # JWT encode/decode, password hashing
│   └── db.py             # engine + AsyncSession factory
├── models/               # SQLAlchemy ORM models
├── schemas/              # Pydantic DTOs (Request/Response)
├── services/             # business logic (no framework coupling)
└── tests/

Pydantic V2 settings + config

# app/core/config.py
from pydantic import Field
from pydantic_settings import BaseSettings, SettingsConfigDict

class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_file=".env", env_prefix="APP_")

    database_url: str = Field(..., description="postgresql+asyncpg://...")
    jwt_secret: str = Field(..., min_length=32)
    jwt_algorithm: str = "HS256"
    jwt_exp_minutes: int = 30
    cors_origins: list[str] = Field(default_factory=list)

settings = Settings()  # fails fast at import if required vars missing

SQLAlchemy 2.0 async session (per-request)

# app/core/db.py
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker

engine = create_async_engine(
    settings.database_url,
    pool_size=20,
    max_overflow=10,
    pool_pre_ping=True,          # reconnect on stale conns (LB idle timeout)
    echo=False,
)
SessionLocal = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)

# app/api/deps.py
from typing import AsyncIterator
from fastapi import Depends

async def get_db() -> AsyncIterator[AsyncSession]:
    async with SessionLocal() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise
        # close is automatic via `async with`

Lifespan + startup/shutdown

# app/main.py
from contextlib import asynccontextmanager
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup: warm caches, test DB
    async with engine.begin() as conn:
        await conn.execute(text("SELECT 1"))
    yield
    # Shutdown: drain connections
    await engine.dispose()

app = FastAPI(title="My API", lifespan=lifespan)

JWT auth with OAuth2PasswordBearer

# app/core/security.py
from datetime import datetime, timedelta, timezone
from jose import jwt, JWTError
from passlib.context import CryptContext

pwd = CryptContext(schemes=["bcrypt"], deprecated="auto")

def hash_password(p: str) -> str: return pwd.hash(p)
def verify_password(p: str, h: str) -> bool: return pwd.verify(p, h)

def create_access_token(sub: str) -> str:
    exp = datetime.now(timezone.utc) + timedelta(minutes=settings.jwt_exp_minutes)
    return jwt.encode({"sub": sub, "exp": exp}, settings.jwt_secret, settings.jwt_algorithm)

# app/api/deps.py
from fastapi import HTTPException, status
from fastapi.security import OAuth2PasswordBearer

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/v1/auth/login")

async def get_current_user(
    token: str = Depends(oauth2_scheme),
    db: AsyncSession = Depends(get_db),
) -> User:
    try:
        payload = jwt.decode(token, settings.jwt_secret, algorithms=[settings.jwt_algorithm])
        user_id: str = payload.get("sub")
    except JWTError:
        raise HTTPException(status.HTTP_401_UNAUTHORIZED, "Invalid token")
    user = await db.get(User, user_id)
    if not user:
        raise HTTPException(status.HTTP_401_UNAUTHORIZED, "User not found")
    return user

Endpoint pattern (thin controller, service below)

# app/api/v1/users.py
from fastapi import APIRouter, Depends, HTTPException, status

router = APIRouter(prefix="/users", tags=["users"])

@router.post("", response_model=UserOut, status_code=status.HTTP_201_CREATED)
async def create_user(
    payload: UserCreate,
    db: AsyncSession = Depends(get_db),
) -> UserOut:
    try:
        user = await user_service.create(db, payload)
    except DuplicateEmailError:
        raise HTTPException(sta