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Running a Flet app (Hot Reload)

Flet apps can be executed as either desktop or web applications using the flet run command. Doing so will start the app in a native OS window or a web browser, respectively, with hot reload enabled to view changes in real-time.

Desktop app​

To run Flet app as a desktop app, use the following command:

uv run flet run

When you run the command without any arguments, main.py script in the current directory will be executed, by default.

If you need to provide a different path, use the following command:

uv run flet run [script]

Where [script] is a relative (ex: counter.py) or absolute (ex: /Users/john/projects/flet-app/main.py) path to the Python script you want to run.

The app will be started in a native OS window:

  • macOS

    macOS

  • Windows

    windows

Web app​

To run Flet app as a web app, use the --web (or -w) option:

uv run flet run --web [script]

A new browser window/tab will be opened and the app will be using a random TCP port:

Web
Running Flet app as a web app

Passing arguments to your app​

Anything after a -- separator is passed through to your app script instead of being interpreted by Flet, and shows up there as sys.argv[1:]:

uv run flet run --web [script] -- --dataset big.csv --verbose

Your app reads them as it would when started with python:

main.py
import argparse
import sys

import flet as ft

parser = argparse.ArgumentParser()
parser.add_argument("--dataset", default="small.csv")
parser.add_argument("--verbose", action="store_true")
options = parser.parse_args(sys.argv[1:])


def main(page: ft.Page):
    page.add(ft.Text(f"Dataset: {options.dataset}"))


ft.run(main)

The -- separator is what keeps your app's arguments from being read as Flet's own - without it, flet run main.py --verbose fails because --verbose is not a flet run option. Arguments that don't start with a - can be passed without a separator, e.g. flet run main.py big.csv.

note

The same arguments are re-applied every time the app is hot reloaded, and they are passed through in all run modes: desktop, --web, --ios and --android.

Watching for changes​

By default, Flet will watch the script file that was run and reload the app whenever the contents of this file are modified+saved, but will not watch for changes in other files.

To modify this behavior, you can use one or more of these flet run options:

  • -d or --directory to watch for changes in the [script]s directory only
  • -r or --recursive to watch for changes in the [script]s directory and all sub-directories recursively
Example
uv run flet run --recursive [script]