Qfastapi-expert

FastAPI Expert

Deep expertise in async Python, Pydantic V2, and production-grade API development with FastAPI.

When to Use This Skill

• Building REST APIs with FastAPI
• Implementing Pydantic V2 validation schemas
• Setting up async database operations
• Implementing JWT authentication/authorization
• Creating WebSocket endpoints
• Optimizing API performance

Core Workflow

1. Analyze requirements — Identify endpoints, data models, auth needs
2. Design schemas — Create Pydantic V2 models for validation
3. Implement — Write async endpoints with proper dependency injection
4. Secure — Add authentication, authorization, rate limiting
5. Test — Run pytest after each endpoint group; verify /docs before proceeding

Code Patterns

Basic: Route with Pydantic Model + Docstring

@router.get("/users/{user_id}", response_model=UserResponse)
async def get_user(user_id: int, db: DbDep) -> UserResponse:
    """Retrieve a user by ID. Returns 404 if not found."""
    user = await crud.get_user(db, user_id)
    if not user:
        raise HTTPException(status_code=404, detail="User not found")
    return user

Error Handling: Custom Exception + HTTPException

class DuplicateEmailError(Exception):
    pass

@router.exception_handler(DuplicateEmailError)
async def duplicate_email_handler(req, exc):
    return JSONResponse(status_code=409, content={"detail": "Email already exists"})

Advanced: Dependency Injection + Async SQLAlchemy

async def get_current_user(token: Annotated[str, Depends(oauth2_scheme)], db: DbDep) -> User:
    """Verify JWT and return authenticated user."""
    payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
    user = await crud.get_user(db, int(payload["sub"]))
    if not user:
        raise HTTPException(status_code=401, detail="Invalid token")
    return user

Comment Template

Use Google-style Python docstrings:

def process_payment(amount: float, user_id: int) -> dict[str, str]:
    """Process a payment transaction.
    
    Args:
        amount: Payment amount in USD.
        user_id: ID of the user making payment.
        
    Returns:
        Transaction confirmation with status and ID.
        
    Raises:
        ValueError: If amount is negative.
        HTTPException: If user not found (status 404).
    """