CORS issues for Flask API call from React both in localhost

Question

I am calling a Flask API from React (both running on localhost) and I am running into CORS problems. When the request is made, the browser (Chrome) gives the following error

Access to XMLHttpRequest at 'http://localhost:5000/sas' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: Redirect is not allowed for a preflight request.

Setup

The Flask setup is the following

from flask_cors import CORS
from flask import Flask, request, Response
from flask_restful import Api, Resource
import json

class SAS(Resource):
    def get(self):
        content = request.json
        js = json.dumps(
            {
                "Response": "Some response"
            }
        )
        resp = Response(js, status=200, mimetype='application/json')
        return resp

app = Flask(__name__)
cors = CORS(app)
api = Api(app)
api.add_resource(SAS, '/sas/')

The call using axios in React is the following

  buttonPressed = async event => {
    event.preventDefault();
    const response = await axios.get(`http://localhost:5000/sas`, {
      data: {
        user: "user",
        file_name: "user",
        directory_name: "user"
      }
    });
  };

NOTE: Running the request from Postman works and shows the header Access-Control-Allow-Origin = *

Any idea on how to solve the problem?

Also, if I was to run the React frontend and API as a dockerized application, I should not see this problem anymore correct? In this case I would also consider circumnavigating the problem while I am developing

Answer 1

Please try using the cross_origin decorator like in this minimal example:

from flask import Flask, request, jsonify
from flask_cors import CORS, cross_origin
app = Flask(__name__)
CORS(app, support_credentials=True)

@app.route('/sas', methods=['POST', 'GET', 'OPTIONS'])
@cross_origin(supports_credentials=True)
def index():
    if(request.method=='POST'):
     some_json = request.get_json()
     return jsonify({"key": some_json})
    else:
        return jsonify({"GET": "Nice Get Request"})

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

If this works then we'll move to implement the same decorator in your code.