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From FastAPI to Nexy

Nexy is FastAPI with the route written as a file: a function that answers under @app.get("/x") answers in src/routes/x.py as GET, and the rest of your code stays the same.

What stays, what changes

Nexy serves your handlers with FastAPI itself, so everything inside a handler works unchanged:

Stays the same	Changes
Pydantic models, request bodies and responses	`@app.get("/x")` becomes `src/routes/x.py` with `async def GET()`
`Depends`, `Query`, `Header`, `Cookie`, `Form`, `File`	`{id}` in the path becomes a `[id].py` file or `[id]/` folder
`HTTPException`, `status`, `JSONResponse`, `RedirectResponse`	`APIRouter(prefix="/users")` becomes the folder `users/`
`Request`, `BackgroundTasks`, `UploadFile`, WebSockets	`dependencies=[...]` on a router becomes a `dependencies.py` file
Middleware classes, `/docs`, `TestClient`	`app.add_middleware(...)` becomes a field of `nexyconfig.py`: `useCORS`, `useGZip`, `useMiddlewares`

Import them from fastapi as before, or from nexy.http, which re-exports the same objects.

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

An endpoint

FastAPI registers a function on an app with a decorator:

main.py

from fastapi import FastAPI app = FastAPI()  @app.get("/api/hello")async def hello():    return {"message": "Hello"}

Nexy reads the URL from the file path and the method from the function name:

src/routes/api/hello.py

async def GET():    return {"message": "Hello"}

Path and query parameters

main.py

@app.get("/api/users/{id}")async def get_user(id: int, fields: str | None = None):    return {"id": id, "fields": fields}

The path parameter becomes a [id] segment of the file name. The query parameter does not move:

src/routes/api/users/[id].py

async def GET(id: int, fields: str | None = None):    return {"id": id, "fields": fields}

/api/users/abc answers 404, because abc is not an int.

A request body

main.py

from pydantic import BaseModel  class NewItem(BaseModel):    name: str    price: float  @app.post("/api/items", status_code=201)async def create_item(item: NewItem):    return item

The Pydantic model is the same. The status code moves into the response:

src/routes/api/items.py

from pydantic import BaseModel from nexy.http import JSONResponse  class NewItem(BaseModel):    name: str    price: float  async def POST(item: NewItem):    return JSONResponse(item.model_dump(), status_code=201)

A body that does not match the model answers 422, as in FastAPI.

Several methods on one URL

main.py

@app.get("/api/notes/{id}")async def read_note(id: int): ...  @app.delete("/api/notes/{id}")async def delete_note(id: int): ...

One file holds every method of a URL:

src/routes/api/notes/[id].py

async def GET(id: int):    return {"id": id}  async def DELETE(id: int):    return {"deleted": id}

Routers and prefixes

main.py

from fastapi import APIRouter admin = APIRouter(prefix="/admin")  @admin.get("/stats")async def stats():    return {"users": 3}  app.include_router(admin)

The folder is the prefix, and there is nothing to include:

src/routes/admin/stats.py

async def GET():    return {"users": 3}

Dependencies on a group of routes

main.py

from fastapi import Depends, Header, HTTPException  def require_admin(x_role: str = Header(...)) -> None:    if x_role != "admin":        raise HTTPException(status_code=403, detail="Admins only")  admin = APIRouter(prefix="/admin", dependencies=[Depends(require_admin)])

A dependencies.py in the folder runs before every route below it:

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]

A dependency for one handler is still Depends, in its signature:

src/routes/api/me.py

from nexy.http import Depends, Header  def current_user(authorization: str = Header(...)) -> str:    return authorization.removeprefix("Bearer ")  async def GET(user: str = Depends(current_user)):    return {"user": user}

Middleware

main.py

from fastapi import Request  @app.middleware("http")async def add_header(request: Request, call_next):    response = await call_next(request)    response.headers["x-app"] = "demo"    return response

A middleware.py wraps the routes of its folder, the root one wraps them all:

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

CORS, GZip and other app-wide middleware

main.py

from fastapi.middleware.cors import CORSMiddlewarefrom fastapi.middleware.gzip import GZipMiddleware app.add_middleware(CORSMiddleware, allow_origins=["https://example.com"])app.add_middleware(GZipMiddleware, minimum_size=1000)

Each one is a field of nexyconfig.py, with the same arguments:

nexyconfig.py

from nexy.core.models import NexyConfigModel  class NexyConfig(NexyConfigModel):    useCORS = {"allow_origins": ["https://example.com"]}    useGZip = {"minimum_size": 1000}

HTML pages

main.py

from fastapi import Requestfrom fastapi.templating import Jinja2Templates templates = Jinja2Templates(directory="templates")  @app.get("/users")async def users_page(request: Request):    users = [{"name": "Ada"}, {"name": "Alan"}]    return templates.TemplateResponse(request, "users.html", {"users": users})

A .nexy page holds the Python above the --- line and the Jinja2 below it, in one file:

src/routes/users.nexy

---users = [{"name": "Ada"}, {"name": "Alan"}]---<ul>  {% for user in users %}  <li>{{ user.name }}</li>  {% endfor %}</ul>

Template inheritance

templates/base.html

<html>  <body>{% block content %}{% endblock %}</body></html>

A layout.nexy wraps every page of its folder, with no {% extends %} in the pages:

src/routes/layout.nexy

---children: prop[NexyElement]---<!DOCTYPE html><html>  <body>{{ children | safe }}</body></html>

Static files

main.py

from fastapi.staticfiles import StaticFiles app.mount("/static", StaticFiles(directory="static"), name="static")

Files in public/ are served from the root: public/robots.txt is /robots.txt.

public/

robots.txt/robots.txt

logo.png/logo.png

Not-found pages

main.py

from fastapi import Requestfrom fastapi.responses import HTMLResponsefrom starlette.exceptions import HTTPException as StarletteHTTPException  @app.exception_handler(StarletteHTTPException)async def not_found(request: Request, exc: StarletteHTTPException):    return HTMLResponse("<h1>Not found</h1>", status_code=exc.status_code)

A notfound.nexy answers browsers, and API clients still get {"detail": ...}:

src/routes/notfound.nexy

<h1>Not found</h1><p><a href="/">Back home</a></p>

WebSockets

main.py

from fastapi import WebSocket  @app.websocket("/ws/echo")async def echo(websocket: WebSocket):    await websocket.accept()    await websocket.send_text(await websocket.receive_text())

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())

Tests

tests/test_main.py

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

The client is the same; the app comes from Nexy:

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

FastAPI	Nexy
`uvicorn main:app --reload`	`nx dev`
`uvicorn main:app --workers 4`	`nx build`, then `nx start`
`/docs` from `FastAPI()`	`/docs`, listing every `.py` route

Keeping an existing app

You do not have to move every route at once. An APIRouter you already have mounts as it is:

nexyconfig.py

from nexy.core.models import NexyConfigModel from legacy.api import router  class NexyConfig(NexyConfigModel):    useRouter = router

Your old routes and the files of src/routes/ then answer side by side. While useRouter is set, .nexy pages are not wrapped by layout.nexy: move the API first, then the pages, then remove the field. See Custom router integration.

Next steps

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