At the moment ControlEvent.data (or precisely Event.data) is of type string and defaults to an empty string. This is misleading as one could think that's an actual data which was passed, whereas, seeing None will be a good indication that nothing was passed.

Test Code

import flet as ft


def main(page: ft.Page):
    page.add(
        ft.ElevatedButton("Hello", on_click=lambda e: print(f"Button clicked: {e.data!r}")), # same with on_focus, on_blur etc
    )


ft.app(main)