Running a Flet app (Hot Reload) - Flet

import TabItem from '@theme/TabItem'; import Tabs from '@theme/Tabs'; import {Image} from '@site/src/components/crocodocs';

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:

<Tabs groupId="uv--pip"> <TabItem value="uv" label="uv"> ```bash uv run flet run ``` </TabItem> <TabItem value="pip" label="pip"> ```bash flet run ``` </TabItem> </Tabs> 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:

<Tabs groupId="uv--pip"> <TabItem value="uv" label="uv"> ```bash uv run flet run [script] ``` </TabItem> <TabItem value="pip" label="pip"> ```bash flet run [script] ``` </TabItem> </Tabs> 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

<Image src="assets/getting-started/counter-app/macos.png" alt="macOS" width="65%" />
Windows

<Image src="assets/getting-started/counter-app/windows.png" alt="windows" width="65%" />

</div>

Web app

To run Flet app as a web app, use the --web (or -w) option: <Tabs groupId="uv--pip"> <TabItem value="uv" label="uv">

bash

uv run flet run --web [script]  # (1)!

A fixed port can be specified using --port ( or -p) option, followed by the port number. </TabItem>

<TabItem value="pip" label="pip"> ```bash flet run --web [script] # (1)! ```

A fixed port can be specified using --port ( or -p) option, followed by the port number. </TabItem>

</Tabs> A new browser window/tab will be opened and the app will be using a random TCP port: <Image src="assets/getting-started/counter-app/safari.png" alt="Web" width="45%" caption="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:]:

<Tabs groupId="uv--pip"> <TabItem value="uv" label="uv"> ```bash uv run flet run --web [script] -- --dataset big.csv --verbose ``` </TabItem> <TabItem value="pip" label="pip"> ```bash flet run --web [script] -- --dataset big.csv --verbose ``` </TabItem> </Tabs>

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

python

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

:::note[Example]

<Tabs groupId="uv--pip"> <TabItem value="uv" label="uv"> ```bash uv run flet run --recursive [script] ``` </TabItem> <TabItem value="pip" label="pip"> ```bash flet run --recursive [script] ``` </TabItem> </Tabs> :::