Page decorator not awaiting page functions

[ProbablyBrian]

Description

Just upgraded to 1.4.0 today and am running into the following problems. Code example to reproduce:

from fastapi import FastAPI
from nicegui import ui

app = FastAPI()


@ui.page("/")
async def homepage():
    ui.label("Homepage!")


ui.run_with(app)

starting this with

$ uvicorn test:app

When navigating to the page, I receive a 500 Internal Server Error. The traceback is extensive but there are a few warnings emitted about coroutines not being awaited:

[...snip FastAPI stack...]
  File "/home/brian/portal/venv_new/lib/python3.11/site-packages/fastapi/routing.py", line 191, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/brian/portal/venv_new/lib/python3.11/site-packages/nicegui/page.py", line 102, in decorated
    task = background_tasks.create(wait_for_result())
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/brian/portal/venv_new/lib/python3.11/site-packages/nicegui/background_tasks.py", line 21, in create
    assert core.loop is not None
           ^^^^^^^^^^^^^^^^^^^^^
AssertionError

/usr/lib/python3.11/threading.py:247: RuntimeWarning: coroutine 'page.__call__.<locals>.decorated.<locals>.wait_for_result' was never awaited
  self._release_save = lock._release_save
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
/usr/lib/python3.11/threading.py:247: RuntimeWarning: coroutine 'homepage' was never awaited
  self._release_save = lock._release_save
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

I downgraded back to 1.3.18 and ran the exact code and this works fine in the previous version.

Versions:

• Python 3.11.5
• Nicegui 1.4.0
• Starlette 0.27.0
• FastAPI 0.104.0
• Uvicorn 0.22.0

Other notes:

• The behavior is the same with nicegui.APIRouter's page decorator (the way I initially discovered this issue).
• using ui.run() with no FastAPI Mount works just fine.

Is this something I'm doing incorrectly with the new 1.4.0 version?

[falkoschindler]

Thanks for reporting this issue, @ProbablyBrian!

@rodja I think we broke something when switching to FastAPI's lifespan API: _startup in nicegui.py is only called by our _lifespan, which in turn is only used for creating our core.app. But it doesn't seem to be called when it is attached to an existing app via app.mount(mount_path, core.app) in ui.run_with. This leaves core.loop set to None.

[falkoschindler]

The FastAPI documentation clearly says:

🚨 Have in mind that these lifespan events (startup and shutdown) will only be executed for the main application, not for Sub Applications - Mounts.

[falkoschindler]

This seems to work:

# ui_run_with.py
...
app.mount(mount_path, core.app)
app.on_event('startup')(_startup)
app.on_event('shutdown')(_shutdown)

Or as a workaround for the user code:

from fastapi import FastAPI
from nicegui import ui
from nicegui.nicegui import _shutdown, _startup

app = FastAPI()

@ui.page("/")
async def homepage():
    ui.label("Homepage!")

ui.run_with(app)
app.on_event('startup')(_startup)
app.on_event('shutdown')(_shutdown)

[rodja]

This issue is originating in Starlette (see encode/starlette#649). I think

from nicegui.nicegui import _shutdown, _startup
...
ui.run_with(app)
app.on_event('startup')(_startup)
app.on_event('shutdown')(_shutdown)

will only work if you do not use the lifespan-API in your main application. If you use these, I guess you would need to run

from nicegui.nicegui import _shutdown, _startup
...
ui.run_with(app)
app.lifespan.add_event_handler("startup", _startup)
app.lifespan.add_event_handler("shutdown", _shutdown)

Quite ugly in my opinion. Maybe we can add this magic to ui.run_with, checking if the app has a lifespan object?

[falkoschindler]

@rodja Oh, I see. You mean something like

def lifespan(app):
    print('lifespan start')
    yield
    print('lifespan end')

app = FastAPI(lifespan=lifespan)

This doesn't work indeed.
Unfortunately there is no lifespan attribute in app. But maybe we find another indicator.

[falkoschindler]

I'm pretty lost. I don't find a way to register any startup or shutdown handler if lifespan is set.

Let's ask StackOverflow: https://stackoverflow.com/questions/77362216/

[falkoschindler]

I think I found a solution.

ui_run_with.py:

app.mount(mount_path, core.app)
main_app_lifespan = app.router.lifespan_context

@asynccontextmanager
async def lifespan_wrapper(app):
    _startup()
    async with main_app_lifespan(app) as maybe_state:
        yield maybe_state
    _shutdown()

app.router.lifespan_context = lifespan_wrapper

It seems hacky, but works with and without lifespan on the main app.

[falkoschindler]

I updated PR #1877 accordingly.