--reload flag gives me an OSError on Windows

Hi, I’m trying a basic example with uvicorn : https://github.com/Celeborn2BeAlive/python_uvicorn_example It’s working but no the reload function: when I modify a file I get an OSError and the server seems to block when I refresh my page in my browser. After that I have to kill the server and start it again. Here is my complete output from uvicorn:

$ uvicorn.exe example:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [18104]
INFO:     Started server process [8440]
INFO:     Waiting for application startup.
INFO:     ASGI 'lifespan' protocol appears unsupported.
INFO:     Application startup complete.
INFO:     127.0.0.1:59375 - "GET / HTTP/1.1" 200
INFO:     127.0.0.1:59375 - "GET /favicon.ico HTTP/1.1" 200
WARNING:  Detected file change in 'example.py'. Reloading...
INFO:     Started server process [15148]
ERROR:    Accept failed on a socket
socket: <asyncio.TransportSocket fd=812, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 8000)>
Traceback (most recent call last):
  File "c:\python38\Lib\asyncio\proactor_events.py", line 817, in loop
    f = self._proactor.accept(sock)
  File "c:\python38\Lib\asyncio\windows_events.py", line 545, in accept
    self._register_with_iocp(listener)
  File "c:\python38\Lib\asyncio\windows_events.py", line 714, in _register_with_iocp
    _overlapped.CreateIoCompletionPort(obj.fileno(), self._iocp, 0, 0)
OSError: [WinError 87] Paramètre incorrect
INFO:     Waiting for application startup.
INFO:     ASGI 'lifespan' protocol appears unsupported.
INFO:     Application startup complete.

Here is some details about my system:

• python 3.8.0
• uvicorn 0.10.3 (installed from pip)
• Windows 10 Professional x64 1903 18362.418

Note that I also tried with python 3.7.x and uvicorn 0.9.x and I had the same results.

Thanks for your work on uvicorn 👍

[tomchristie]

“The asyncio work-around does work, though”

Hrm. That’s interesting. Could you open a fresh issue to track this, so we’ve got a cleaner history on the issue (there’s a few bits and pieces mashed together on this one.)

I think we probably want to default to asyncio on all platforms at the moment, since the IOCP variant seems more flaky.

You’d be welcome to make a PR onto it as well if you fancy…

https://github.com/encode/uvicorn/blob/ad4ea90568a42eaa24cc6c597783246de5f35915/uvicorn/loops/auto.py#L8

This is where we’re auto-selecting which event loop to use. We probably just want to get rid of the if sys.platform == "win32": branch entirely.

[tomchristie]

Thanks for bringing this up!