Project Layout

How the file tree compares

A NestJS or Express project has its own conventions. FastAPI has none enforced by the framework — but the community has settled on a pattern that will feel familiar.

# Express / NestJS project
my-api/
├── src/
│   ├── main.ts          # bootstrap
│   ├── app.module.ts    # root module (NestJS)
│   ├── items/
│   │   ├── items.controller.ts
│   │   ├── items.service.ts
│   │   └── items.dto.ts
│   └── database/
│       └── database.module.ts
├── test/
├── package.json
└── tsconfig.json

# FastAPI project
my-api/
├── app/
│   ├── main.py          # FastAPI() instance + lifespan
│   ├── routers/
│   │   └── items.py     # APIRouter — like a controller
│   ├── models/
│   │   └── item.py      # SQLAlchemy ORM models
│   ├── schemas/
│   │   └── item.py      # Pydantic schemas (DTOs)
│   ├── dependencies.py  # shared Depends() factories
│   └── database.py      # engine + Session
├── tests/
├── pyproject.toml       # like package.json
└── .env

The mapping is almost 1:1:

controller → routers/ (uses APIRouter)
service → you can add a services/ layer — FastAPI does not enforce it
dto → schemas/ (Pydantic models)
entity → models/ (SQLAlchemy models)
app.module.ts → main.py

main.py — the entry point

// Express src/main.ts
import express from 'express';
import { itemsRouter } from './items/items.router';

const app = express();
app.use(express.json());
app.use('/items', itemsRouter);

app.listen(3000, () => console.log('listening on 3000'));

# app/main.py
from contextlib import asynccontextmanager

from fastapi import FastAPI
from app.routers import items


# Startup/shutdown via lifespan: code before yield runs at startup,
# code after yield runs at shutdown (e.g. open/close a DB pool).
@asynccontextmanager
async def lifespan(app: FastAPI):
    print("Server is starting up")
    yield
    print("Server is shutting down")


app = FastAPI(title="Items API", version="1.0.0", lifespan=lifespan)

app.include_router(items.router, prefix="/items", tags=["items"])

# Run: uvicorn app.main:app --reload

pyproject.toml — the package.json equivalent

// package.json
{
  "name": "my-api",
  "version": "1.0.0",
  "scripts": {
    "start": "node dist/main.js",
    "dev": "ts-node-dev src/main.ts",
    "build": "tsc"
  },
  "dependencies": {
    "express": "^4.18.0",
    "zod": "^3.22.0"
  },
  "devDependencies": {
    "typescript": "^5.0.0",
    "@types/express": "^4.17.0"
  }
}

# pyproject.toml  (PEP 517/518 — used by pip / poetry / uv)
[project]
name = "my-api"
version = "1.0.0"
requires-python = ">=3.11"
dependencies = [
    "fastapi>=0.115",
    "uvicorn[standard]>=0.29",
    "pydantic>=2.0",
    "sqlalchemy>=2.0",
]

[project.optional-dependencies]
dev = ["pytest", "httpx", "mypy"]

[tool.uvicorn]
# Start: uvicorn app.main:app --reload
# or:    python -m uvicorn app.main:app --reload

Run this locally — needs FastAPI + a server. Create the layout above, install deps with pip install -e ".[dev]" or uv sync, then uvicorn app.main:app --reload.