fastapi-patterns

FastAPI Patterns

Production FastAPI patterns. Covers the 20% of FastAPI that handles 80% of real use cases.

When to Use

• Building a new FastAPI service
• Reviewing FastAPI code for quality or performance issues
• Adding auth, background tasks, or streaming to an existing service
• Debugging FastAPI errors you don't understand

Project Structure

app/
├── main.py              # create_app(), lifespan
├── config.py            # Settings (pydantic-settings)
├── dependencies.py      # Shared dependencies (DB, auth)
├── routers/
│   ├── jobs.py
│   └── users.py
└── models/
    ├── requests.py      # Pydantic request models
    └── responses.py     # Pydantic response models

App Setup with Lifespan

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

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup: init DB pools, load models, etc.
    app.state.db = await create_db_pool()
    app.state.model = load_ml_model()
    yield
    # Shutdown: cleanup
    await app.state.db.close()

app = FastAPI(title="My API", lifespan=lifespan)
app.include_router(jobs.router, prefix="/jobs", tags=["jobs"])
app.include_router(users.router, prefix="/users", tags=["users"])

Dependency Injection

FastAPI's Depends is the right way to share DB connections, auth, and config:

# dependencies.py
from fastapi import Depends, HTTPException, status
from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials

security = HTTPBearer()

async def get_db(request: Request) -> AsyncGenerator:
    async with request.app.state.db.acquire() as conn:
        yield conn  # connection returned to pool after request

async def get_current_user(
    credentials: HTTPAuthorizationCredentials = Depends(security),
    db = Depends(get_db),
) -> User:
    token = credentials.credentials
    user = await verify_token(token, db)
    if not user:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid or expired token",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user

# routers/jobs.py — inject where needed
@router.get("/", response_model=list[JobResponse])
async def list_jobs(
    current_user: User = Depends(get_current_user),
    db = Depends(get_db),
    limit: int = Query(default=20, ge=1, le=100),
):
    return await db.fetch_jobs(user_id=current_user.id, limit=limit)