asyncio not using ProactorEventLoop with reload enabled

[lausfl]

Description

Version information:

• OS: Windows 10.0.19045
• Python: 3.11.4
• NiceGUI: 2.3.0

I was unable to find any documentation or existing issues about this and I am not sure if I am doing something wrong here.

I want to create an application that uses NiceGUI and has a separate process running from which I can parse the output on stdout and do something with it.
The problem I encounter is that I am unable to start the process when the reload argument of ui.run() is set to True (a NotImplementedError exception is thrown by asyncio).
On further investigation I found that the asyncio event loop is set to _WindowsSelectorEventLoop when reload is enabled and to ProactorEventLoop when it's disabled.

I found the following issue, which seems to be related: encode/uvicorn#1220

So I tried setting the event loop policy to try and force it to use a ProactorEventLoop, but to no avail.

Here's the MRE:

import asyncio

asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
from nicegui import ui, app

def on_packet(packet):
    print('packet received')

async def start_ping():
    print(type(asyncio.get_event_loop()).__name__)
    cmd = ['ping', '127.0.0.1']
    proc = await asyncio.create_subprocess_exec(*cmd, stdout=asyncio.subprocess.PIPE)
    async for line in proc.stdout:
        print(line)

ui.label('Hello NiceGUI!')

app.on_startup(start_ping)
ui.run(reload=True)

When I change the above code to ui.run(reload=False), it works as intended.

[rodja]

Have you tried using a main guard? The subprocess evaluates the main file again, but should not start another NiceGUI application.

[lausfl]

@rodja do you mean like this?

import asyncio

asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
from nicegui import ui, app

def on_packet(packet):
    print('packet received')

async def start_ping():
    print(type(asyncio.get_event_loop()).__name__)
    cmd = ['ping', '127.0.0.1']
    proc = await asyncio.create_subprocess_exec(*cmd, stdout=asyncio.subprocess.PIPE)
    async for line in proc.stdout:
        print(line)

if __name__ == '__mp_main__':
    ui.label('Hello NiceGUI!')

app.on_startup(start_ping)
ui.run(reload=True)

I also tried putting app.on_startup(start_ping) inside the guard. Either way I get the same behaviour.

[rodja]

The ui.run() must be guarded:

...
if __name__ in ('__mp_main__', '__main__'):
    ui.run()

[lausfl]

It's still using the _WindowsSelectorEventLoop with the guard. I also tried putting all three lines inside the guard, but nothing changes.
Is this only happening on my end or can you reproduce it?

import asyncio

asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
from nicegui import ui, app

def on_packet(packet):
    print('packet received')

async def start_ping():
    print(type(asyncio.get_event_loop()).__name__)
    cmd = ['ping', '127.0.0.1']
    proc = await asyncio.create_subprocess_exec(*cmd, stdout=asyncio.subprocess.PIPE)
    async for line in proc.stdout:
        print(line)

ui.label('Hello NiceGUI!')
app.on_startup(start_ping)

if __name__ in ('__mp_main__', '__main__'):
    ui.run()

[rodja]

We do not use Windows. Maybe someone from the community?

[falkoschindler]

@lausfl Is the problem still persisting? Can anyone else reproduce it?

[evnchn]

It is basically a matter-of-fact that when uvicorn is run with reload=True (yes, I should remind you that the hot-reload functionality comes from uvicorn, not us), that reload=True ==> SelectorEventLoop, reload=False ==> ProactorEventLoop.

Force the loop policy to SelectorEventLoop on Windows with 3.8+
encode/uvicorn#529 (comment)
https://github.com/encode/uvicorn/pull/535/files

---

So, you can either:

• Mess with Uvicorn to un-force the loop policy to SelectorEventLoop
• Accept the fact and use reload=False

---

Also: I can't seem to be able to use WinLoop for this. I am still investigating why.

[evnchn]

@lausfl Check this out. WinLoop to the rescue, and now we force uvicorn to not set the event loop in any way by passing loop="none".

Source code:

import asyncio
from nicegui import ui, app
from fastapi import FastAPI
import winloop

fapp = FastAPI()

@ui.page("/")
def main_page():
    ui.label(str(type(asyncio.get_event_loop())))

async def start_ping():
    print(str(type(asyncio.get_event_loop())))
    cmd = ['ping', '127.0.0.1']
    proc = await asyncio.create_subprocess_exec(*cmd, stdout=asyncio.subprocess.PIPE)
    async for line in proc.stdout:
        print(line)

app.on_startup(start_ping)

asyncio.set_event_loop(winloop.new_event_loop())
asyncio.set_event_loop_policy(winloop.EventLoopPolicy())

ui.run(loop="none", port=9000) # let us configure the loop ourselves

---

Regarding #4474: I may have to update my code snippet to fully use WinLoop. Apparently just doing winloop.install() (which is asyncio.set_event_loop_policy(winloop.EventLoopPolicy()) in disguise) may not be enough in fully setting everything to use WinLoop.

[lausfl]

@falkoschindler yes, I just tested it with the latest version of nicegui and it still occurs.

[evnchn]

Also test the winloop implementation?

[lausfl]

@evnchn thanks a lot for the alternative solution. I am closing the issue, as my problem can be solved by using WinLoop.

[lausfl]

@evnchn WinLoop works as expected, thanks!