Flet - a better UI for FastAPI

Flet for FastAPI allows adding interactive real-time dashboards to your FastAPI app as well as host any Flet web app inside FastAPI with production-grade reliability.

Installation

pip install flet-fastapi

First app

Create counter.py with the following content:

import flet as ft
import flet_fastapi

async def main(page: ft.Page):
    counter = ft.Text("0", size=50, data=0)

    async def add_click(e):
        counter.data += 1
        counter.value = str(counter.data)
        counter.update()

    page.floating_action_button = ft.FloatingActionButton(
        icon=ft.icons.ADD, on_click=add_click
    )
    page.add(
        ft.Container(counter, alignment=ft.Alignment.CENTER, expand=True)
    )

app = flet_fastapi.app(main)

That's a simple app displaying a counter and a button at the right bottom to increment that counter.

flet_fastapi.app() configures a single Flet app at the root of FastAPI app with main() sessions handler and the following endpoints:

/ws (WS) - WebSocket handler for the Flet app.

/upload (PUT) - file uploads handler.

/oauth_callback (GET) - OAuth flow callback handler.

/ (GET) - Flet app static files with SPA catch-all handler.

Running the app locally

Install Uvicorn web server:

pip install uvicorn

Start uvicorn with:

uvicorn counter:app

Open the browser and navigate to http://127.0.0.1:8000 to see the app running.

Hosting multiple Flet apps under the same domain

import flet as ft
import flet_fastapi


async def root_main(page: ft.Page):
    page.add(ft.Text("This is root app!"))


async def sub_main(page: ft.Page):
    page.add(ft.Text("This is sub app!"))


app = flet_fastapi.FastAPI()


app.mount("/sub-app", flet_fastapi.app(sub_main))
app.mount("/", flet_fastapi.app(root_main))

Sub-apps must be mapped before the root Flet app as it configures catch-all index.html for SPA.

Run the app with uvicorn and visit http://127.0.0.1:8000 and then http://127.0.0.1:8000/sub-app/ to see both Flet apps running. Notice the trailing slash in /sub-app/ URL - without the slash the request will be routed to a root app.

Adding Flet to the existing FastAPI app

from contextlib import asynccontextmanager

import flet as ft
import flet_fastapi
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    await flet_fastapi.app_manager.start()
    yield
    await flet_fastapi.app_manager.shutdown()

app = FastAPI(lifespan=lifespan)

async def main(page: ft.Page):
    page.add(ft.Text("Hello, Flet!"))

app.mount("/flet-app", flet_fastapi.app(main))

When adding Flet app to the existing FastAPI app you need to call flet_fastapi.app_manager.start() on app start and flet_fastapi.app_manager.shutdown() on shutdown. Use the way that best suites you: lifespan (in the example above) or app events.

app_manager.start() method starts background tasks cleaning up expired sessions and OAuth flow states.

app_manager.shutdown() method removes any temporary files created by a Flet app.

Running the app in production

It is recommended to run FastAPI in production with Hypercorn which is ASGI web server, but it is also possible to run FastAPI apps with Gunicorn which is a WSGI server, but has more features, like passing proxy headers.

To install Gunicorn:

pip install gunicorn

Start gunicorn with:

gunicorn -k uvicorn.workers.UvicornWorker counter:app

Reference

Environment variables

FLET_SECRET_KEY - secret key to sign upload requests. Must be set if upload directory is configured.

FLET_SESSION_TIMEOUT - the number of seconds to keep session alive after user has disconnected. Default is 3,600 seconds.

FLET_OAUTH_STATE_TIMEOUT - OAuth state lifetime, in seconds, which is a maximum allowed time between starting OAuth flow and redirecting to OAuth callback URL. Default is 600 seconds.

FLET_MAX_UPLOAD_SIZE - max allowed size of an uploaded file, bytes.