Fastapi Skill

FastAPI Development

Expert guidance for building modern FastAPI backends.

Quick Start

Minimal App

from fastapi import FastAPI

app = FastAPI(title="Todo API", version="1.0.0")

@app.get("/")
async def root():
    return {"message": "Hello World"}

@app.get("/health")
async def health_check():
    return {"status": "healthy"}

Run with Uvicorn

uvicorn main:app --reload --host 0.0.0.0 --port 8000

Request/Response Models

from pydantic import BaseModel, Field
from datetime import datetime

class TaskCreate(BaseModel):
    title: str = Field(..., min_length=1, max_length=200)
    description: str | None = None

class TaskResponse(BaseModel):
    id: str
    title: str
    description: str | None
    completed: bool
    created_at: datetime

    model_config = {"from_attributes": True}

@app.post("/tasks", response_model=TaskResponse, status_code=201)
async def create_task(task: TaskCreate) -> TaskResponse:
    # task is already validated
    new_task = await save_task(task)
    return new_task

Dependency Injection

from fastapi import Depends
from typing import Annotated

async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session() as session:
        yield session

DB = Annotated[AsyncSession, Depends(get_db)]

@app.get("/tasks")
async def list_tasks(db: DB) -> list[TaskResponse]:
    tasks = await db.execute(select(Task))
    return tasks.scalars().all()

CORS for Frontend

from fastapi.middleware.cors import CORSMiddleware

app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000"],  # Next.js dev
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

Project Structure

src/
├── main.py              # FastAPI app creation
├── config.py            # Settings with pydantic-settings
├── routers/
│   ├── __init__.py
│   ├── tasks.py         # /tasks endpoints
│   └── users.py         # /users endpoints
├── models/
│   ├── __init__.py
│   ├── task.py          # SQLModel Task
│   └── user.py          # SQLModel User
├── schemas/
│   ├── __init__.py
│   ├── task.py          # Pydantic request/response
│   └── user.py
├── services/
│   ├── __init__.py
│   └── task_service.py  # Business logic
├── dependencies/
│   ├── __init__.py
│   ├── database.py      # DB session
│   └── auth.py          # Auth dependencies
└── database.py          # SQLModel engine setup

Reference Guides

For detailed patterns, see:

• App Structure: See references/app-structure.md for routing, routers, and middleware
• Pydantic Models: See references/pydantic-models.md for validation and serialization
• Dependency Injection: See references/dependency-injection.md for Depends() patterns
• CORS Config: See references/cors-config.md for frontend integration
• Async Handlers: See references/async-handlers.md for async/await patterns
• SQLModel: See references/sqlmodel-patterns.md for database operations