For python-socketio v5, there is an error when trying to enter a client into a room when the namespace parameter is non-default.

Following are the server and client code used to isolate this situation:

Server

import socketio
from aiohttp import web

sio = socketio.AsyncServer(async_mode='aiohttp')
app = web.Application()
sio.attach(app)

@sio.event
async def connect(sid, environ):
    print("connect ", sid)
    sio.enter_room(sid=sid, room='test_room', namespace='/test_ns')
    await sio.emit('message', data='ping', room='test_room', namespace='/test_ns')

if __name__ == '__main__':
    web.run_app(app=app, port=6000)

Client

import asyncio
import socketio

sio = socketio.AsyncClient()

@sio.event
async def connect():
    print('connection established')

@sio.event(namespace='/test_ns')
async def message(data):
    print(data)

async def main():
    await sio.connect('http://localhost:6000')
    await sio.wait()

if __name__ == '__main__':
    asyncio.run(main())

Traceback eventually ends up here:
File "[redacted]\venv\lib\site-packages\socketio\base_manager.py", line 115, in enter_room eio_sid = self.rooms[namespace][None][sid] KeyError: None

Probable Cause
I believe the error is caused by two functions.

1. The enter_room() function in socketio/server.py (line 389 as of time of writing), which calls enter_room() in base_manager, does not have an input for eio_sid. This causes eio_sid in the enter_room() function in base_manager to always default to None.
2. This means that lines 114-115 is always triggered:

if eio_sid is None:
    eio_sid = self.rooms[namespace][None][sid]

In the example, the namespace to be accessed would be '/test_ns', which does not exist self.rooms, thus causing the KeyError.

The Fix
I believe that the namespace to be accessed when eio_sid is None should be '/'. Replacing eio_sid = self.rooms[namespace][None][sid] with eio_sid = self.rooms['/'][None][sid] resolved the issue.