Fastapi patterns

Skill Mattakushi432/Claude-Code-Skills-Custom-DevTools-Pack/plugins/devtools-pack/skills/fastapi-patterns

A curated pack of custom Claude Code skills for developers — installable as a Claude Code plugin marketplace.

Install

npx -y skills add Mattakushi432/Claude-Code-Skills-Custom-DevTools-Pack --skill fastapi-patterns

Assembled from the repository path, not quoted from the project. Check it against their README if it does not work.

One thing to look at

0 stars0 stars. Stars are a popularity signal and not a quality one, but at this level it is likely that nobody has read this closely except its author, and you would be relying on your own review.

What its author says it does

Copied from the file, not written here

When to activate: FastAPI, Pydantic v2, dependency injection, async endpoints, middleware, background tasks, OpenAPI

SKILL.md

3.9 KB, 842 tokens by cl100k_base, as published. Nobody here has run it

FastAPI Patterns

Application Structure

app/
├── main.py              # FastAPI() instance, startup/shutdown events
├── api/
│   ├── deps.py          # Shared dependencies (db, auth, settings)
│   └── v1/
│       ├── router.py    # APIRouter for v1
│       └── users.py     # Endpoint handlers
├── models/
│   ├── user.py          # SQLAlchemy models
│   └── base.py          # Base, metadata
├── schemas/
│   ├── user.py          # Pydantic request/response schemas
│   └── common.py        # Shared schemas (pagination, errors)
├── services/
│   └── user_service.py  # Business logic (not in endpoints)
└── core/
    ├── config.py        # Settings (pydantic-settings)
    └── security.py      # Auth helpers

Dependency Injection

from fastapi import Depends
from sqlalchemy.ext.asyncio import AsyncSession

# Good: generator dependency with cleanup
async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session_maker() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise

# Type-annotated dependency
CurrentUser = Annotated[User, Depends(get_current_user)]

@router.get("/me", response_model=UserSchema)
async def get_me(user: CurrentUser) -> User:
    return user

Pydantic v2 Schemas

from pydantic import BaseModel, EmailStr, field_validator, model_validator, ConfigDict

class UserCreate(BaseModel):
    email: EmailStr
    password: str
    name: str

    @field_validator("password")
    @classmethod
    def validate_password(cls, v: str) -> str:
        if len(v) < 8:
            raise ValueError("Password must be at least 8 characters")
        return v

class UserResponse(BaseModel):
    model_config = ConfigDict(from_attributes=True)  # replaces orm_mode
    
    id: int
    email: EmailStr
    name: str
    created_at: datetime

Response Models and Error Handling

from fastapi import HTTPException, status
from fastapi.responses import JSONResponse

# Always specify response_model
@router.post("/users", response_model=UserResponse, status_code=status.HTTP_201_CREATED)
async def create_user(body: UserCreate, db: AsyncSession = Depends(get_db)) -> User:
    if await user_service.email_exists(db, body.email):
        raise HTTPException(
            status_code=status.HTTP_409_CONFLICT,
            detail={"code": "email_taken", "message": "Email already registered"},
        )
    return await user_service.create(db, body)

# Global exception handler
@app.exception_handler(AppError)
async def app_error_handler(request: Request, exc: AppError) -> JSONResponse:
    return JSONResponse(status_code=400, content={"error": str(exc)})

Background Tasks

from fastapi import BackgroundTasks

@router.post("/reports")
async def generate_report(
    params: ReportParams,
    background_tasks: BackgroundTasks,
    db: AsyncSession = Depends(get_db),
) -> dict:
    report_id = uuid4()
    # Don't fire-and-forget without error capture
    background_tasks.add_task(report_service.generate, db, report_id, params)
    return {"report_id": str(report_id), "status": "processing"}

Anti-Patterns

Using Session (sync) inside async def endpoints
Missing response_model on endpoints
Business logic inside endpoint handlers (use service layer)
async def without any actual await (just use def)
Mutable defaults in Pydantic fields: tags: list = [] → tags: list = Field(default_factory=list)