Subprocess

In this cookbook recipe, you'll learn how to run external system commands from your Flet app using Python's built-in subprocess module.

This approach is useful when you need to interact with platform tools or system utilities that are not exposed through Flet APIs.

Note

Running external commands is not supported when your app runs in a browser.

Examples

Launching another Android app

Sometimes your Flet app may need to open another Android application. For example, you might want to open the Settings app so the user can adjust system or app settings.

On Android, this can be done using the am start command, which launches activities from the Android shell.

The command requires the component name (-n) of the target activity in the form: package.name/.ActivityName.

For example, the Android Settings app uses:

com.android.settings/.Settings

It is also recommended to pass the --user flag. Without it, the command may fail with an error such as:

Security exception: Permission Denial: startActivityAsUser asks to run as
user -2 but is calling from uid u0a164; this requires android.permission.INTERACT_ACROSS_USERS_FULL

This happens because Android may attempt to launch the activity for a different user than the one your app runs under. Setting --user 0 ensures the command runs for the primary user.

The full command therefore becomes:

am start -n com.android.settings/.Settings --user 0

And can be run from a Flet app like this:

import subprocess
import flet as ft


def main(page: ft.Page):
    def open_settings(e):
        result = subprocess.run(
            ["am", "start", "-n", "com.android.settings/.Settings", "--user", "0"],
            shell=False, # default
            capture_output=True,
            text=True,
        )

        print("STDOUT:", result.stdout)
        print("STDERR:", result.stderr)

    page.add(
        ft.SafeArea(
            content=ft.Button("Open Settings app", on_click=open_settings)
        )
    )


ft.run(main)

Running this app on an Android device, you should see the Settings app launch when the button is clicked.

Common Pitfall

When using subprocess.run(), be careful with the shell argument.

This will not work correctly:

subprocess.run(
    ["am", "start", "-n", "com.android.settings/.Settings", "--user", "0"],
    shell=True
)

Because when shell=True is used, the command must be passed as a single string, not a list.

Correct usage:

subprocess.run(
    "am start -n com.android.settings/.Settings --user 0",
    shell=True
)

However, when possible, prefer shell=False with a list of arguments, as shown in the main example. This is generally safer and avoids shell parsing issues.