CLI pattern if __name__ == "__main__": main() does not work with misleading message

[nazarewk]

related to #143

One of the patterns for running CLI programs is:

from nicegui import ui

def main():
    ui.label('Hello NiceGUI!')
    ui.button('BUTTON', on_click=lambda: ui.notify('button was pressed'))
    ui.run(port=8082)

if __name__ == "__main__":
    main()

This does not work with reload=True, because the reloadable __name__ is __mp_main__ instead.

So I made it work using second entry:

from nicegui import ui

def main():
    ui.label('Hello NiceGUI!')
    ui.button('BUTTON', on_click=lambda: ui.notify('button was pressed'))
    ui.run(port=8082)

if __name__ in ("__main__", "__mp_main__"):
    main()

Any idea what causes this and/or whether it can be fixed in nicegui itself?

[falkoschindler]

When Uvicorn is reloading the app (which happens for the first time right after starting the script), it re-evaluates main.py while searching for the nicegui:app object. In contrast to the original process, this "reloader" process has __name__ "__mp_main__" with "mp" indicating "multi-processing". The main guard (successfully...) prevents the new process from evaluating main(), running ui.run and initializing globals.config. This is not what you want in this case.

Technically I don't see an easy solution for supporting the default main guard if __name__ == '__main__': for auto-reloading NiceGUI apps. Sure, you can extend the guard with __mp_main__. The question is, what is you motivation for the guard in the first place - besides it being a common pattern?

If you want to avoid the code in main() being evaluated a second time when starting with reload=True, you can wrap it in a page function:

from nicegui import ui

@ui.page('/')
def main_page():
    ui.label('Hello NiceGUI!')
    ui.button('BUTTON', on_click=lambda: ui.notify('button was pressed'))

ui.run(port=8082)

Personally I'm not sure if you really need a main guard. Ideally your main script is short and concise, so that it does not need to be imported from anywhere.

[nazarewk]

I want my main script to serve as a CLI primarily and be able to spin up a web gui as an option.

[falkoschindler]

I'll close this issue, since I don't see how we can avoid the conflict with the main guard. Using NiceGUI as a secondary mode in an otherwise headless CLI application in combination with reload=True seems to be a rather tricky setup. But if including __mp_main__ in the main guard helps, I'd leave it this way.

[denravonska]

Is this supposed to work when you invoke your code as a module? I get the exact same error message when using a __main__.py consisting of only

from nicegui import ui

ui.run()

$ python -m apto
ERROR:    Traceback (most recent call last):
  File "env/lib/python3.10/site-packages/starlette/routing.py", line 677, in lifespan
    async with self.lifespan_context(app) as maybe_state:
  File "env/lib/python3.10/site-packages/starlette/routing.py", line 566, in __aenter__
    await self._router.startup()
  File "env/lib/python3.10/site-packages/starlette/routing.py", line 656, in startup
    handler()
  File "env/lib/python3.10/site-packages/nicegui/nicegui.py", line 62, in handle_startup
    raise RuntimeError('\n\n'
RuntimeError: 

You must call ui.run() to start the server.
If ui.run() is behind a main guard
   if __name__ == "__main__":
remove the guard or replace it with
   if __name__ in {"__main__", "__mp_main__"}:
to allow for multiprocessing.

ERROR:    Application startup failed. Exiting.

[falkoschindler]

Hi @denravonska!
Can you, please, open a new discussion for this matter? And can you give a bit more information about your file and directory structure so that we can reproduce the issue? Thanks!

[denravonska]

@falkoschindler Done, #1111 :)