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I'm having issues on my application that has been deployed to Heroku. When I was developing it on localhost, I didn't get this error, but once I deployed it, it spontaneously throws this error:

Access to fetch at 
'https://frontend.herokuapp.com' from origin 
'https://backend.herokuapp.com' has been blocked 
by CORS policy: No 'Access-Control-Allow-Origin' header is present 
on the requested resource. If an opaque response serves your needs, 
set the request's mode to 'no-cors' to fetch the resource with CORS 
disabled.

Simplified version of my Flask server:

from flask import Flask, request, make_response, jsonify, url_for, 
redirect, render_template
from flask_cors import CORS, cross_origin
import flask
import json

app = Flask(__name__)
CORS(app)

@app.route('/api/action', methods=['GET', 'POST'])
@cross_origin()
def prescription(response={}):

    # POST request
    if request.method == 'POST':
         print('score POST request')
         response = request.get_json()
         data = json.loads(response.get('data'))
         rating = a_super_cool_feature
         return jsonify(rating), 200

     # GET request
    else:
        print('GET request')
        return getScore(response)

@app.route('/api/score', methods=['GET', 'POST'])
@cross_origin()
def scoreHandler(response={}):

    # POST request
    if request.method == 'POST':
         print('Incoming..')
        response = request.get_json()
        parseResponse(response)
        return str(getScore(response)), 200


    # GET request
    else:
        return getScore(response)


@app.route('/test')
def test_page():
    # look inside `templates` and serve `index.html`
    return render_template('index.html')

if __name__ == "__main__":
     app.debug=True
     app.run()

My front-end is a React application. These two servers run on separate Heroku servers and I'm using the free version so I only get one dyno at a time (in case this could cause this error).

The interesting thing is this:

About half the time, when these POST requests are called on the deployed Heroku app, the error is thrown. Otherwise, it works no problem. Any ideas on fixing this?

Sean Kearney
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  • Possible duplicate of [How to enable CORS in flask and heroku](https://stackoverflow.com/questions/25594893/how-to-enable-cors-in-flask-and-heroku) – Selcuk Aug 20 '19 at 04:38

3 Answers3

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You could use this snippet to enable cors app wide.

from flask import Flask
from flask_cors import CORS
app = Flask(__name__) CORS(app)
Vishnudev Krishnadas
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Really weird psuedo fix. Basically it was giving me this error about 40% of the time I made the POST request to the Flask backend, then I added 

 app.config['CORS_HEADERS'] = 'Content-Type'

Under the:

app = Flask(__name__)
CORS(app)

And this did not fix the issue completely. However; now this error is only thrown about 5% of the time when requests are made to the back-end. See other answer for actual fix.

Sean Kearney
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  • What do you mean by the percentages? Are you making it from localhost or some client like postman where it will work by default but not with production application. – Vishnudev Krishnadas Aug 20 '19 at 19:19
  • In postman it works. Here's what I mean with the percentages: I'll deploy the application, click the button in the app that sends a POST request, look at the console in inspect elements, and most of the time it works just fine. However, when I refresh the page and click the button again, about 5% of the time I would do this the console gave me the 'No Access-Control-Allow-Origin' headers error. – Sean Kearney Aug 21 '19 at 20:48
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Okay now I actually fixed it. I bought the "Hobby" dyno type in Heroku and I never get the error now. I think it just has to do with the other server not being up and running when I make the request because I can only have one dyno running at a time with the free version. Thus it somehow timed out and gives that error.

With the Hobby dyno type my backend is always running so when I make a request it no longer times out and gives that error.

Sean Kearney
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