Question: Trying to create a socketio link between flask server and reactjs client. It shows this error 

"Access to XMLHttpRequest at '<http://127.0.0.1:5000/socket.io/?EIO=3&transport=polling&t=MrcruFC>' from origin '<http://localhost:3000>' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource."

I have tried including CORS from the flask cors documentation but it still doesnot work.

Server:

```
from flask import Flask, Response
from flask_cors import CORS
from flask_socketio import SocketIO

app = Flask(__name__)
cors = CORS(app)
socketio=SocketIO(app)

@socketio.on('connection')
def handle_my_custom_event():
    socket.emit('outgoing data',{num: '10'})

@app.route("/")
def hello():
    return 'Hello'

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

```

Answer: 
You can add an option for creating SocketIO.

```
socketio = SocketIO(app=app, cors_allowed_origins='*')

```
You can allow the CORS using the following headers:

```py
header = response.headers
header['Access-Control-Allow-Origin'] = '*'

```