How to set an answer as LocalDescription in aiortc (python)?

Question

I'm trying to perform signaling using python websocket, then create peer2peer connection using aiortc, to achieve webRTC-based video streaming using pure python.

I have managed to send an offer from a peer to another and setRemoteDescription successfully.

client_offer_request = await websocket.recv()
offer_sdp = RTCSessionDescription(client_offer_request, type='offer')
await pc.setRemoteDescription(offer_sdp)

However, when I create answer and setLocalDescription,

# send answer
answer = await pc.createAnswer()
await pc.setLocalDescription(answer)

I get this exception.

await self.ws_handler(self, path)
  File "/xxxxx/WebRTC/websocket_server.py", line 45, in negotiate
    await pc.setLocalDescription(answer)
  File "/xxxxx/lib/python3.6/site-packages/aiortc/rtcpeerconnection.py", line 768, in setLocalDescription
    t._currentDirection = and_direction(t.direction, t._offerDirection)
  File "/xxxxx/lib/python3.6/site-packages/aiortc/rtcpeerconnection.py", line 247, in and_direction
    return sdp.DIRECTIONS[sdp.DIRECTIONS.index(a) & sdp.DIRECTIONS.index(b)]
ValueError: None is not in list

The code is simply like follows:

from aiortc import RTCIceCandidate, MediaStreamTrack, RTCPeerConnection, RTCSessionDescription
import asyncio, websockets

def negotiate():
    player = MediaPlayer('foo.mp4')
    pc = RTCPeerConnection()
    pc.addTrack(player.video)

    clientOffer = await websocket.recv()
    offer_sdp = RTCSessionDescription(clientOffer, type='offer')
    await pc.setRemoteDescription(offer_sdp)

    # send answer
    answer = await pc.createAnswer()
    await pc.setLocalDescription(answer)


start_server = websockets.serve(negotiate, 'localhost', 8765)
asyncio.get_event_loop().run_until_complete(start_server)
asyncio.get_event_loop().run_forever()

Answer 1

I have resolved the issue. Since the initiative peer sends an offer which is json string like:

{'sdp': 'v=0\r\no=- 3841969261 3841969261 IN IP4 0.0.0.0\r\ns=-\r\nt=0 0\r\na=group:BUNDLE 0\r\na=msid-semantic:WMS *\r\nm=video 34067 UDP/TLS/RTP/SAVPF 97 98 99 100 101 102\r\nc=IN \na=sendrecv\r\na=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid\r\na=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time\r\na=mid:0\r\na=\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=rtcp-mux\r\na=ssrc-group: 124673822\r\na=\r\na=rtpmap:97 VP8/90000\r\na=rtcp-fb:97 nack\r\na=rtcp-fb:97 nack pli\r\na=rtcp-fb:97 goog-remb\r\na=rtpmap:98 rtx/90000\r\na=fmtp:98 apt=97\r\na=rtpmap:99 H264/90000\r\na=rtcp-fb:99 nack\r\na=rtcp-fb:99 nack pli\r\na=rtcp-fb:99 goog-remb\r\na=fmtp:99 packetization-mode=1;level-asymmetry-allowed=1;profile-level-id=42001f\r\na=rtpmap:100 rtx/90000\r\na=fmtp:100 apt=99\r\na=rtpmap:101 H264/90000\r\na=rtcp-fb:101 nack\r\na=rtcp-fb:101 nack pli\r\na=rtcp-fb:101 goog-remb\r\na=fmtp:101 packetization-mode=1;level-asymmetry-allowed=1;profile-level-id=42e01f\r\na=rtpmap:102 rtx/90000\r\na=fmtp:102 apt=101\r\r\n', 'type': 'offer'} 

After receiving such a json string, the receiving peer has to parse it as json and create SDP instance accordingly:

client_offer_request = await websocket.recv()
offer = json.loads(client_offer_request)
offer_sdp = RTCSessionDescription(offer["sdp"], offer["type"])

As such, the issue is orginally from parsing the offer and has nothing to do with setLocalDescription