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From Fastify to Nexy

In Nexy a Fastify route becomes a Python file under src/routes/: JSON schemas become Pydantic models, plugins with a prefix become folders, and hooks become dependencies.py and middleware.py files.

Each section below shows a Fastify habit, then the same thing in Nexy.

A route

server.js
import Fastify from "fastify"; const app = Fastify({ logger: true }); app.get("/api/hello", async () => ({ message: "Hello" })); await app.listen({ port: 3000 });

The file path is the URL, and the function name is the method:

src/routes/api/hello.py
async def GET():    return {"message": "Hello"}

Every request is logged with its method, path, status and time, with no logger to set up.

Params and querystring schemas

server.js
app.get(  "/api/users/:id",  {    schema: {      params: { type: "object", properties: { id: { type: "integer" } } },      querystring: { type: "object", properties: { fields: { type: "string" } } },    },  },  async (request) => ({ id: request.params.id, fields: request.query.fields }),);

The schema is the signature: [id] in the file name, types on the parameters:

src/routes/api/users/[id].py
async def GET(id: int, fields: str | None = None):    return {"id": id, "fields": fields}

Body schemas

server.js
app.post(  "/api/items",  {    schema: {      body: {        type: "object",        required: ["name", "price"],        properties: { name: { type: "string" }, price: { type: "number", exclusiveMinimum: 0 } },      },    },  },  async (request, reply) => reply.code(201).send(request.body),);
src/routes/api/items.py
from pydantic import BaseModel, Field from nexy.http import JSONResponse  class NewItem(BaseModel):    name: str    price: float = Field(gt=0)  async def POST(item: NewItem):    return JSONResponse(item.model_dump(), status_code=201)

A body that fails answers 422, where Fastify answers 400.

Response schemas

server.js
app.get(  "/api/me",  {    schema: {      response: {        200: { type: "object", properties: { name: { type: "string" } } },      },    },  },  async () => ({ name: "Ada", passwordHash: "x" }),);

The return type does what the response schema does: fields it does not declare are left out.

src/routes/api/me.py
from pydantic import BaseModel  class PublicUser(BaseModel):    name: str  async def GET() -> PublicUser:    return {"name": "Ada", "password_hash": "x"}

Plugins with a prefix

routes/admin.js
export default async function admin(app) {  app.get("/stats", async () => ({ users: 3 }));}
server.js
app.register(admin, { prefix: "/admin" });

The folder is the prefix, and nothing is registered:

src/routes/admin/stats.py
async def GET():    return {"users": 3}

preHandler and onRequest hooks

routes/admin.js
export default async function admin(app) {  app.addHook("preHandler", async (request, reply) => {    if (request.headers["x-role"] !== "admin") {      return reply.code(403).send({ error: "Admins only" });    }  });}

A hook scoped to a plugin is a dependencies.py in its folder:

src/routes/admin/dependencies.py
from nexy.http import Header, HTTPException  def require_admin(x_role: str = Header("")) -> None:    if x_role != "admin":        raise HTTPException(status_code=403, detail="Admins only")  dependencies = [require_admin]

onSend and onResponse hooks

server.js
app.addHook("onSend", async (request, reply, payload) => {  reply.header("x-app", "demo");  return payload;});

Code after call_next in a middleware.py runs on the way out:

src/routes/middleware.py
from nexy.http import Request  async def middleware(request: Request, call_next):    response = await call_next(request)    response.headers["x-app"] = "demo"    return response

decorate and decorateRequest

server.js
app.decorate("db", { users: () => [{ name: "Ada" }] }); app.get("/api/users", async function () {  return this.db.users();});

A shared object is a function that returns it, and a handler asks for it with Depends:

src/db.py
class Db:    def users(self) -> list[dict]:        return [{"name": "Ada"}]  db = Db()  def get_db() -> Db:    return db
src/routes/api/users/index.py
from nexy.http import Depends from src.db import Db, get_db  async def GET(db: Db = Depends(get_db)):    return db.users()

A test swaps it with app.dependency_overrides[get_db].

@fastify/cors, @fastify/compress, @fastify/cookie

server.js
await app.register(import("@fastify/cors"), { origin: "https://example.com" });await app.register(import("@fastify/compress"));
nexyconfig.py
from nexy.core.models import NexyConfigModel  class NexyConfig(NexyConfigModel):    useCORS = {"allow_origins": ["https://example.com"]}    useGZip = {"minimum_size": 1000}

A cookie needs no plugin: read it with a Cookie parameter, set it with response.set_cookie.

src/routes/api/theme.py
from nexy.http import Cookie  async def GET(theme: str = Cookie("light")):    return {"theme": theme}

setErrorHandler and setNotFoundHandler

server.js
app.setNotFoundHandler((request, reply) => reply.code(404).type("text/html").send("<h1>Not found</h1>"));

Browsers get notfound.nexy, API clients {"detail": "Not Found"}:

src/routes/notfound.nexy
<h1>Not found</h1>

A handler that knows what went wrong raises it:

src/routes/api/orders/[id].py
from nexy.http import HTTPException  async def GET(id: int):    if id > 100:        raise HTTPException(status_code=404, detail="No such order")    return {"id": id}

@fastify/static

server.js
await app.register(import("@fastify/static"), { root: path.join(__dirname, "public") });

Nothing to write: public/ is served at /.

@fastify/websocket

server.js
await app.register(import("@fastify/websocket")); app.get("/ws/echo", { websocket: true }, (socket) => {  socket.on("message", (message) => socket.send(message));});
src/routes/ws/echo.py
from fastapi import WebSocket  async def SOCKET(websocket: WebSocket):    await websocket.accept()    await websocket.send_text(await websocket.receive_text())

@fastify/swagger

server.js
await app.register(import("@fastify/swagger"));await app.register(import("@fastify/swagger-ui"), { routePrefix: "/docs" });

Nothing to register: the OpenAPI schema is built from the type annotations and served at /docs.

Tests with inject

server.test.js
const response = await app.inject({ method: "GET", url: "/api/hello" });expect(response.json()).toEqual({ message: "Hello" });

TestClient calls the application in memory too, with no port:

tests/test_hello.py
from fastapi.testclient import TestClient from nexy.app import app  def test_hello():    assert TestClient(app).get("/api/hello").json() == {"message": "Hello"}

Running

Fastify Nexy
npm init fastify nx new
fastify start -w app.js nx dev
node server.js in production nx build, then nx start
npm test nx test

Next steps

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