I get CORS error in python flask RESTApi when sending a request using angular

I have a python script which is a flask RESTApi:

from flask import Flask from flask_cors import CORS api = Flask(__name__) CORS(api) @api.route("/api/stopService", methods=['POST']) def startService() return { "MSG": "OK" } if __name__ == '__main__': api.run(port=8000) 

Now I want to make a request with angular using this code:

this.http.post('http://localhost:8000/api/stopService', {}).subscribe((res: any) => { }); 

It's just a simple button that when I click on, it triggers the request. but I get this error:

And if I click on the button again It's OK and I have the response.

Here is the flask log:

First line is when I get the CORS error, and the second and the third one is the OK response.

Edit 1
I changed my flask code to this but still no luck:

from flask import Flask api = Flask(__name__) @api.route("/api/stopService", methods=['POST']) def startService() return { "MSG": "OK" } @api.after_request def addHeaders(response): response.headers.add('Access-Control-Allow-Origin', '*') response.headers.add('Access-Control-Allow-Headers', 'Content-Type') return response if __name__ == '__main__': api.run(port=8000)