Flask:XMLHttpRequest at '...' from origin has been blocked by CORS policy No 'Access-Control-Allow-Origin' header is present on the requested resource

Question

I am currently trying to get a small server running on my RaspberryPi and accessing it from my PC within the LAN.

I get the following error when accessing the Raspberry, where the index.html is displayed to me just fine. Apparently it seems there are problems with my "resetList" endpoint.

I also tried the solution Solve Cross Origin Resource Sharing with Flask but it did not work for me. Thank you in advance.

I have the following Python code:

gevent.monkey.patch_all()
from flask import Flask, render_template
from flask_socketio import SocketIO
import dbfunctions as db
import json
from flask_cors import CORS


app = Flask(__name__, template_folder='./build', static_folder='./build/static')
CORS(app)
socket_io = SocketIO(app)


@app.route('/', methods=['GET', 'POST', 'PUT'])
def index():
    return render_template('index.html')


# receiving messages
@socket_io.on('resetList')
def reset_list():
    database = r"data.db"
    conn = db.create_connection(database)
    with conn:
        data = db.get_data_without_initial(conn)
        fields = ["id", "name", "profit", "sum", "high", "maxProfit", "last_update"]
        json_list = []
        for i in range(len(data)):
            item_to_add = {fields[j]: data[i][j] for j in range(len(fields))}
            json_list.append(item_to_add)
        json.dumps(json_list)
        socket_io.emit('receivedData', json_list)


if __name__ == '__main__':
    socket_io.run(app, host='0.0.0.0', port=5000, debug=True)

Answer 1

i found a solution, the problem was not my python code it was my java code :/ a simple add of " transports: ["polling"] " solve the problem

import io from "socket.io-client";
let endPoint = "http://localhost:5000";
let socket = io.connect(`${endPoint}`, { transports: ["polling"] });

function receiveData(cb) {
  socket.on("receivedData", (resp) => cb(resp));
}

export { receiveData };