python-fastapi-ddd-skill

name	python-fastapi-ddd-skill
description	Guides FastAPI backend design using Domain-Driven Design (DDD) and Onion Architecture in Python. Use when structuring a FastAPI app (routes/handlers, Pydantic schemas, Depends-based DI), modeling domain Entities/Value Objects, defining repository interfaces, implementing SQLAlchemy infrastructure adapters, or writing use cases, based on the dddpy reference.
license	Apache-2.0
metadata	{"author":"Takahiro Ikeuchi","version":"1.0.0"}

FastAPI + Python DDD & Onion Architecture Design Guide

Guides FastAPI backend design using DDD principles and Onion Architecture, based on the dddpy reference implementation (FastAPI + SQLAlchemy + Python 3.13+).

Architecture Overview

Four concentric layers with dependencies pointing inward:

Presentation  →  UseCase  →  Infrastructure  →  Domain (innermost)

Key rule: Inner layers never depend on outer layers. The Domain layer has zero external dependencies.

Layer	Responsibility	Examples
Domain	Core business logic, no framework deps	Entities, Value Objects, Repository interfaces, Exceptions
Infrastructure	External integrations	DB repos, DTOs, DI config, SQLAlchemy models