Connect Python websocket library with Plotly Dash - python-3.x

Question

I haven't been able to find an example that connects Python Websocket client to Plotly Dash and am wondering what the best way to design this it. My Websocket server that listens to a stock trade stream implementation looks like this:
class StockSocket:
def __init__(self):
self.underlying_symbol = underlying_symbol
self.last_underlying_price = ''
def on_message(self, ws, message):
m = json.loads(message)
self.last_underlying_price = str(m['last'])
# I want to send this message to client here
def on_error(self, ws, error):
print(error)
def on_close(self, ws, close_status_code, close_msg):
print("### closed ###")
def on_open(self, ws):
payload = json.dumps({"symbol": self.underlying_symbol, "sessionid": self.sessionId})
ws.send(payload)
def run_forever(self):
websocket.WebSocketApp("<market-feed-url>",on_open=self.on_open, on_message=self.on_message, on_error=self.on_error, on_close=self.on_close).run_forever()
def runWebsocket(sessionId, underlying_symbol):
ws = StockSocket(sessionId, underlying_symbol)
ws.run_forever()
I plan on running this via Python Flask server and then have Dash Websocket listen in, but am not quite sure about the implementation.
For client side code I was thinking something like this:
import dash_core_components as dcc
import dash_html_components as html
import dash_extensions as de
from dash import Dash
from dash.dependencies import Input, Output
# Create example app.
app = Dash(prevent_initial_callbacks=True)
app.layout = html.Div([
dcc.Input(
id="input", autoComplete="off"
),
html.Div(id="msg"),
de.WebSocket(url="ws://localhost:8000/", id="ws")
])
# Send input value using websocket.
send = "function(value){return value;}"
app.clientside_callback(
send,
Output("ws", "send"),
[Input("input", "value")],
)
# Update div using websocket.
receive = "function(msg){console.log(msg.data); return \"Response from websocket: \" + msg.data;}"
app.clientside_callback(
receive,
Output("msg", "children"),
[Input("ws", "message")]
)
if __name__ == '__main__':
app.run_server(host='0.0.0.0', port=8000, debug=True, use_reloader=False)
My two main questions are:
How do I send message from server to client using Python Websocket library (documentation is sparse)
How does client poll the server?
Thanks!

Answer 1

The generic websocket in python is this:
import asyncio
from websockets import connect
async def hello(uri):
async with connect(uri) as websocket:
await websocket.send("Hello world!")
await websocket.recv()
asyncio.run(hello("ws://localhost:8765"))
But you can get everything that you need from here:
https://pypi.org/project/websockets/
and then here:
https://websockets.readthedocs.io/en/stable/intro/index.html