Read output of a Python subprocess

Question

I'm running a WSS server from a Python script to get certain data from the client. I'd like to receive that data in Blender and use it. I run the process like:

import bpy
from subprocess import Popen, PIPE
import sys

server = Popen([sys.executable, "wss.py"], stdout=PIPE)

def update(scene):
    print(server.stdout.readline())

bpy.app.handlers.frame_change_pre.append(update)

Problem is, attempting to access stdout in any way (stdout.read() or for line in stdout:) results in Blender waiting for the server process to finish running and freezing. Are there any workarounds for this?

wss.py is as follows, server works fine, but requires an asyncio loop and the process to be running:

import asyncio
import SSL
import websockets
import json
import sys

CONNECTION = None

async def onconnect(websocket, path):
    global CONNECTION
    CONNECTION = websocket
    asyncio.get_event_loop().create_task(messages(websocket))
    try:
        await websocket.wait_closed()
    finally:
        CONNECTION = None

async def messages(websocket):
    while True:
        msg = await websocket.recv()
        sys.stdout.write(msg)
        sys.stdout.flush()

ssl_context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
ssl_context.load_cert_chain('host.pem', 'host.key')

server = websockets.serve(onconnect, '0.0.0.0', 8000, ssl=ssl_context)

asyncio.get_event_loop().run_until_complete(server)
asyncio.get_event_loop().run_forever()

Answer 1

I have a very similar issue. A workaround for me is to append to a text file in the subprocess, and then read from that in the main loop. This most likely has large drawbacks, but it is functional.