fastapi-backend

name	fastapi-backend
description	FastAPI best practices for building production Python backends. Use when building REST APIs, async endpoints, Pydantic models, dependency injection, middleware, or any FastAPI server.

FastAPI Backend Patterns

Project Structure

app/
  main.py          # FastAPI app, lifespan, middleware
  config.py        # Pydantic Settings, env vars
  models/          # SQLAlchemy ORM models
  schemas/         # Pydantic request/response schemas
  routers/         # APIRouter per domain (auth, users, posts)
  services/        # Business logic (no DB or HTTP here)
  deps.py          # Shared dependencies (get_db, get_current_user)
  database.py      # Engine, SessionLocal, Base

Core Patterns

App setup with lifespan

from contextlib import asynccontextmanager
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    # startup
    await database.connect()
    yield
    # shutdown
    await database.disconnect()

app = FastAPI(lifespan=lifespan)

Dependency injection

# deps.py
async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with AsyncSessionLocal() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise

async def get_current_user(
    token: str = Depends(oauth2_scheme),
    db: AsyncSession = Depends(get_db)
) -> User:
    ...

Router pattern

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

@router.get("/{user_id}", response_model=UserResponse)
async def get_user(user_id: int, db: AsyncSession = Depends(get_db)):
    user = await db.get(User, user_id)
    if not user:
        raise HTTPException(status_code=404, detail="User not found")
    return user

Error handling

from fastapi import Request
from fastapi.responses import JSONResponse

@app.exception_handler(ValueError)
async def value_error_handler(request: Request, exc: ValueError):
    return JSONResponse(status_code=400, content={"detail": str(exc)})

Pydantic schemas

class UserCreate(BaseModel):
    email: EmailStr
    password: str = Field(min_length=8)
    name: str = Field(max_length=100)

class UserResponse(BaseModel):
    id: int
    email: str
    name: str
    model_config = ConfigDict(from_attributes=True)

Rules

• Always use async def for endpoints
• Validate ALL input with Pydantic (never trust raw dicts)
• Use HTTPException with specific status codes (not 500 for everything)
• Never put business logic in route handlers — use service layer
• Use response_model to control what's returned (never expose passwords)
• Add CORS middleware for browser clients
• Use Depends() for database sessions — never create sessions manually