fastapi-expert

SKILL.md

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

Analyze requirements — Identify endpoints, data models, auth needs

Design schemas — Create Pydantic V2 models for validation

Implement — Write async endpoints with proper dependency injection

Secure — Add authentication, authorization, rate limiting

Test — Write async tests with pytest and httpx; run pytest after each endpoint group and verify OpenAPI docs at /docs

Checkpoint after each step: confirm schemas validate correctly, endpoints return expected HTTP status codes, and /docs reflects the intended API surface before proceeding.

Minimal Complete Example

Schema + endpoint + dependency injection in one cohesive unit:

# schemas.py
from pydantic import BaseModel, EmailStr, field_validator, model_config

class UserCreate(BaseModel):
    model_config = model_config(str_strip_whitespace=True)

    email: EmailStr
    password: str
    name: str | None = None

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

class UserResponse(BaseModel):
    model_config = model_config(from_attributes=True)

    id: int
    email: EmailStr
    name: str | None = None

# routers/users.py
from fastapi import APIRouter, Depends, HTTPException, status
from sqlalchemy.ext.asyncio import AsyncSession
from typing import Annotated

from app.database import get_db
from app.schemas import UserCreate, UserResponse
from app import crud

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

DbDep = Annotated[AsyncSession, Depends(get_db)]

@router.post("/", response_model=UserResponse, status_code=status.HTTP_201_CREATED)
async def create_user(payload: UserCreate, db: DbDep) -> UserResponse:
    existing = await crud.get_user_by_email(db, payload.email)
    if existing:
        raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail="Email already registered")
    return await crud.create_user(db, payload)

# crud.py
from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession
from app.models import User
from app.schemas import UserCreate
from app.security import hash_password

async def get_user_by_email(db: AsyncSession, email: str) -> User | None:
    result = await db.execute(select(User).where(User.email == email))
    return result.scalar_one_or_none()

async def create_user(db: AsyncSession, payload: UserCreate) -> User:
    user = User(email=payload.email, hashed_password=hash_password(payload.password), name=payload.name)
    db.add(user)
    await db.commit()
    await db.refresh(user)
    return user

JWT Authentication Snippet

# security.py
from datetime import datetime, timedelta, timezone
from jose import JWTError, jwt
from fastapi import Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer
from typing import Annotated

SECRET_KEY = "read-from-env"  # use os.environ / settings
ALGORITHM = "HS256"
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/auth/token")

def create_access_token(subject: str, expires_delta: timedelta = timedelta(minutes=30)) -> str:
    payload = {"sub": subject, "exp": datetime.now(timezone.utc) + expires_delta}
    return jwt.encode(payload, SECRET_KEY, algorithm=ALGORITHM)

async def get_current_user(token: Annotated[str, Depends(oauth2_scheme)]) -> str:
    try:
        data = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        subject: str | None = data.get("sub")
        if subject is None:
            raise ValueError
        return subject
    except (JWTError, ValueError):
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")

CurrentUser = Annotated[str, Depends(get_current_user)]

Reference Guide

Load detailed guidance based on context:

Topic
Reference
Load When

Pydantic V2
references/pydantic-v2.md
Creating schemas, validation, model_config

SQLAlchemy
references/async-sqlalchemy.md
Async database, models, CRUD operations

Endpoints
references/endpoints-routing.md
APIRouter, dependencies, routing

Authentication
references/authentication.md
JWT, OAuth2, get_current_user

Testing
references/testing-async.md
pytest-asyncio, httpx, fixtures

Django Migration
references/migration-from-django.md
Migrating from Django/DRF to FastAPI

Constraints

MUST DO

Use type hints everywhere (FastAPI requires them)

Use Pydantic V2 syntax (field_validator, model_validator, model_config)

Use Annotated pattern for dependency injection

Use async/await for all I/O operations

Use X | None instead of Optional[X]

Return proper HTTP status codes

Document endpoints (auto-generated OpenAPI)

MUST NOT DO

Use synchronous database operations

Skip Pydantic validation

Store passwords in plain text

Expose sensitive data in responses

Use Pydantic V1 syntax (@validator, class Config)

Mix sync and async code improperly

Hardcode configuration values

Output Templates

When implementing FastAPI features, provide:

Schema file (Pydantic models)

Endpoint file (router with endpoints)

CRUD operations if database involved

Brief explanation of key decisions

Knowledge Reference

FastAPI, Pydantic V2, async SQLAlchemy, Alembic migrations, JWT/OAuth2, pytest-asyncio, httpx, BackgroundTasks, WebSockets, dependency injection, OpenAPI/Swagger

Documentation