examples/web/async.py: "no current event loop ..." right after start

[otaku42]

When I connect to http://localhost:8000/ right after starting the app with uvicorn async:app, the browser only shows the message Internal Server Error.

The console shows:

(v4l2py-testing) ┌─[✗]─[mrenzmann@parrot]─[~/src/v4l2py-testing/examples/web]
└──╼ $uvicorn async:app
INFO:     Started server process [44996]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
AnyIO worker thread 2023-05-19 22:21:39,288 INFO  v4l2py.device.video0: opening /dev/video0
AnyIO worker thread 2023-05-19 22:21:39,293 INFO  v4l2py.device.video0: opened /dev/video0 (Integrated IR Camera: Integrate)
AnyIO worker thread 2023-05-19 22:21:39,293 INFO  v4l2py.device.video0: closing /dev/video0 (Integrated IR Camera: Integrate)
AnyIO worker thread 2023-05-19 22:21:39,293 INFO  v4l2py.device.video0: closed /dev/video0 (Integrated IR Camera: Integrate)
INFO:     127.0.0.1:60972 - "GET / HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/protocols/http/h11_impl.py", line 428, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/middleware/proxy_headers.py", line 78, in __call__
    return await self.app(scope, receive, send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/fastapi/applications.py", line 276, in __call__
    await super().__call__(scope, receive, send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/applications.py", line 122, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/middleware/errors.py", line 184, in __call__
    raise exc
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/middleware/errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
    raise exc
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
    await self.app(scope, receive, sender)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/fastapi/middleware/asyncexitstack.py", line 21, in __call__
    raise e
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/routing.py", line 718, in __call__
    await route.handle(scope, receive, send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/routing.py", line 276, in handle
    await self.app(scope, receive, send)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/routing.py", line 66, in app
    response = await func(request)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/fastapi/routing.py", line 237, in app
    raw_response = await run_endpoint_function(
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/fastapi/routing.py", line 165, in run_endpoint_function
    return await run_in_threadpool(dependant.call, **values)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/starlette/concurrency.py", line 41, in run_in_threadpool
    return await anyio.to_thread.run_sync(func, *args)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/anyio/to_thread.py", line 31, in run_sync
    return await get_asynclib().run_sync_in_worker_thread(
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 937, in run_sync_in_worker_thread
    return await future
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 867, in run
    result = context.run(func, *args)
  File "/home/mrenzmann/src/v4l2py-testing/examples/web/async.py", line 107, in index
    "index.html", {"request": request, "cameras": cameras()}
  File "/home/mrenzmann/src/v4l2py-testing/examples/web/async.py", line 94, in cameras
    cameras[device.index] = Camera(device)
  File "/home/mrenzmann/src/v4l2py-testing/examples/web/async.py", line 39, in __init__
    self.clients: asyncio.Queue = asyncio.Queue()
  File "/usr/lib/python3.9/asyncio/queues.py", line 35, in __init__
    self._loop = events.get_event_loop()
  File "/usr/lib/python3.9/asyncio/events.py", line 642, in get_event_loop
    raise RuntimeError('There is no current event loop in thread %r.'
RuntimeError: There is no current event loop in thread 'AnyIO worker thread'.

The error goes away when calling http://localhost:8000/camera/0/stream, then heading back to http://localhost:8000/. This can serve as a work-around, but it's still annoying.

The duck found another project with a similar error. There it has been fixed already; check how they fixed it and see if their fix works for us, too.

[otaku42]

See also here.