app.on_exception and async

[kleynjan]

Description

In #445, Falko gives a minimal usage example of app.on_exception, but it doesn't work with async:

from nicegui import app, ui

app.on_exception(ui.notify)

async def impossible():
    print(1 / 0)

ui.button("Do something impossible", on_click=impossible)

ui.run()

Apparently, the get_client() in globals fails:

Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 63, in uvloop.loop.Handle._run
  File "...lib/python3.11/site-packages/nicegui/background_tasks.py", line 59, in _handle_task_result
    globals.handle_exception(e)
  File ".../lib/python3.11/site-packages/nicegui/globals.py", line 114, in handle_exception
    result = handler() if not inspect.signature(handler).parameters else handler(exception)
                                                                         ^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.11/site-packages/nicegui/functions/notify.py", line 35, in notify
    outbox.enqueue_message('notify', options, globals.get_client().id)
                                              ^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.11/site-packages/nicegui/globals.py", line 101, in get_client
    return get_slot().parent.client
           ^^^^^^^^^^
  File ".../lib/python3.11/site-packages/nicegui/globals.py", line 97, in get_slot
    return get_slot_stack()[-1]
           ~~~~~~~~~~~~~~~~^^^^
IndexError: list index out of range

Going by the docs it's hard to say if this is a bug or a feature request. If the latter, as @groucho86 already suggested in #445, it would be nice to have a general mechanism to catch exceptions that works in async apps too. For now I am back to using the decorators that @smojef shared in #289.

Cheers, -Peter

[falkoschindler]

Thanks for reporting this bug, @kleynjan!
It took some time to get to the bottom of this issue. But apparently we didn't handle the exception for async callbacks in the right place, so the UI context got lost. The ui.timer already did it correctly. It should be fix with commit 9a4082b.

[kleynjan]

Thanks, Falko!

[rodja]

v1.3.9 has just been released!