[Python] How to create simple webrtc server to browser stream example in python?

I'm trying to write simple python server to browser video streamer using aiortc? For simplicity the server and the browser are in one local network.

The python code:

import asyncio
from aiortc import RTCPeerConnection, RTCSessionDescription, VideoStreamTrack
from av import VideoFrame
import numpy as np
import uuid
import cv2
from aiohttp import web

class BouncingBallTrack(VideoStreamTrack):

def __init__(self):
super().__init__()
self.width = 640
self.height = 480
[...]

async def recv(self):
[...]
return new_frame


async def offer(request):
params = await request.json()
offer = RTCSessionDescription(sdp=params["sdp"], type=params["type"])

pc = RTCPeerConnection()

# Add local media track
pc.addTrack(BouncingBallTrack())

await pc.setRemoteDescription(offer)
answer = await pc.createAnswer()
await pc.setLocalDescription(answer)

return {
"sdp": pc.localDescription.sdp,
"type": pc.localDescription.type
}

if __name__ == "__main__":

app = web.Application()

app.router.add_post("/offer", offer)
app.router.add_static('/', path='static', name='static')

web.run_app(app, port=8080)


The JavaScript code:

pc = new RTCPeerConnection({
iceServers: [] // No ICE servers needed for local network
});

// When remote adds a track, show it in our video element
pc.ontrack = event => {
if (event.track.kind === 'video') {
video.srcObject = event.streams[0];
}
};

// Create an offer to send to the server
const offer = await pc.createOffer();
await pc.setLocalDescription(offer);

// Send the offer to the server and get the answer
const response = await fetch('/offer', {
method: 'POST',
headers: {
'Content-Type': 'application/json'
},
body: JSON.stringify({
sdp: pc.localDescription.sdp,
type: pc.localDescription.type
})
});

const answer = await response.json();
await pc.setRemoteDescription(new RTCSessionDescription(answer));



When a client sends the offer, the server failes with ValueError: None is not in list on line await pc.setLocalDescription(answer). Actually sdp in the offer and in the answer are so strange that it cannot work. The browser and the server sdp do not contain any information about stream tracks, neither contains any local network address to connect. What I'm missing?

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