Solve Cross Origin Resource Sharing with Flask

I struggled a lot with something similar. Try the following:

1. Use some sort of browser plugin which can display the HTML headers.
2. Enter the URL to your service, and view the returned header values.
3. Make sure Access-Control-Allow-Origin is set to one and only one domain, which should be the request origin. Do not set Access-Control-Allow-Origin to *.

If this doesn’t help, take a look at this article. It’s on PHP, but it describes exactly which headers must be set to which values for CORS to work.

CORS That Works In IE, Firefox, Chrome And Safari

Note that setting the Access-Control-Allow-Origin header in the Flask response object is fine in many cases (such as this one), but it has no effect when serving static assets (in a production setup, at least). That’s because static assets are served directly by the front-facing web server (usually Nginx or Apache). So, in that case, you have to set the response header at the web server level, not in Flask.

For more details, see this article that I wrote a while back, explaining how to set the headers (in my case, I was trying to do cross-domain serving of Font Awesome assets).

Also, as @Satu said, you may need to allow access only for a specific domain, in the case of JS AJAX requests. For requesting static assets (like font files), I think the rules are less strict, and allowing access for any domain is more accepted.

You can get the results with a simple:

@app.route('your route', methods=['GET'])
def yourMethod(params):
    response = flask.jsonify({'some': 'data'})
    response.headers.add('Access-Control-Allow-Origin', '*')
    return response

Might as well make this an answer. I had the same issue today and it was more of a non-issue than expected. After adding the CORS functionality, you must restart your Flask server (ctrl + c -> python manage.py runserver, or whichever method you use)) in order for the change to take effect, even if the code is correct. Otherwise the CORS will not work in the active instance.

Here’s how it looks like for me and it works (Python 3.6.1, Flask 0.12):

factory.py:

from flask import Flask
from flask_cors import CORS  # This is the magic


def create_app(register_stuffs=True):
    """Configure the app and views"""
    app = Flask(__name__)
    CORS(app)  # This makes the CORS feature cover all routes in the app

    if register_stuffs:
        register_views(app)
    return app


def register_views(app):
    """Setup the base routes for various features."""
    from backend.apps.api.views import ApiView
    ApiView.register(app, route_base="/api/v1.0/")

views.py:

from flask import jsonify
from flask_classy import FlaskView, route


class ApiView(FlaskView):
    @route("/", methods=["GET"])
    def index(self):
        return "API v1.0"

    @route("/stuff", methods=["GET", "POST"])
    def news(self):
        return jsonify({
            "stuff": "Here be stuff"
        })

In my React app console.log:

Sending request:
GET /stuff
With parameters:
null
bundle.js:17316 Received data from Api:
{"stuff": "Here be stuff"}

Well, I faced the same issue. For new users who may land at this page.
Just follow their official documentation.

Install flask-cors

pip install -U flask-cors

then after app initialization, initialize flask-cors with default arguments:

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app)

@app.route("/")
def helloWorld():
   return "Hello, cross-origin-world!"

I used decorator given by Armin Ronacher with little modifications (due to different headers that are requested by the client).And that worked for me. (where I use angular as the requester requesting application/json type).

The code is slightly modified at below places,

from flask import jsonify

@app.route('/my_service', methods=['POST', 'GET','OPTIONS'])
@crossdomain(origin='*',headers=['access-control-allow-origin','Content-Type'])
def my_service():
    return jsonify(foo='cross domain ftw')

jsonify will send a application/json type, else it will be text/html.
headers are added as the client in my case request for those headers

 const httpOptions = {
      headers: new HttpHeaders({
        'Content-Type': 'application/json',
        'Access-Control-Allow-Origin':'*'
      })
    };
    return this.http.post<any>(url, item,httpOptions)

Note: The placement of cross_origin should be right and dependencies are installed.
On the client side, ensure to specify kind of data server is consuming. For example application/json or text/html

For me the code written below did magic

from flask import Flask,request,jsonify
from flask_cors import CORS,cross_origin
app=Flask(__name__)
CORS(app, support_credentials=True)
@app.route('/api/test', 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":"GET"})


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

I might be a late on this question but below steps fixed the issue

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app)

This is a pretty old and well-covered question already, but figured I’d contribute what I think is the simplest solution of the lot:

@app.after_request
def after_request(response: Response) -> Response:
    response.access_control_allow_origin = "*"
    return response

I think the problem is with preflighted requests.
It seems there’s no effective way to disable the automatic answer to OPTIONS request if you’re using @route @put @patch @delete decorators.
my workaround is the following snippet at the start of the api (before any decorated function)

def option_todo(id): 
  return '', 204 
 
app.add_url_rule('/', view_func=option_todo, provide_automatic_options=False, methods=['OPTIONS']) 
app.add_url_rule(r'/<path:path>', view_func=option_todo, provide_automatic_options=False, methods=['OPTIONS']) 

@app.after_request
def after_request(response):
  response.headers['Access-Control-Allow-Methods']='*'
  response.headers['Access-Control-Allow-Origin']='*'
  response.headers['Vary']='Origin'
  return response

I encountered this issue today, and none of the answers worked for me.

I set the cross_origin() decorator as follows:

@app.route('/', methods = ['POST'])
@cross_origin()
def index():

Then I did:

1. Open the Flask Python file in the IDE.
2. Right mouse-click on the file.
3. Click: Run Python File in Terminal

The problem for me was running the Flask Python file with CTRL+ALT+N. The "Run Code" button in VSCode, I shouldn’t have ran the Flask server that way. Because of that I assume CORS didn’t load properly.

Also, my print()s didn’t always appear, because of that so it was also hard to debug stuff. This happened because by running the app via CTRL+ALT+N, VSCode focusses on the OUTPUT window instead of the TERMINAL window.

Also, print()s that did appear in the OUTPUT window, don’t support emojis like the TERMINAL window does. So my app crashed for the longest time until I figured it all out.

Stupid mistake on my part though, should’ve known better. Hope this helps others!

My issue was a preflight one, but I had to add to app.py

...

@app.after_request
def after_request(response: Response) -> Response:
    response.access_control_allow_credentials = True
    return response

My fix was

@app.after_request
def handle_options(response):
    response.headers["Access-Control-Allow-Origin"] = "*"
    response.headers["Access-Control-Allow-Methods"] = "GET, POST, PUT, DELETE, OPTIONS"
    response.headers["Access-Control-Allow-Headers"] = "Content-Type, X-Requested-With"

    return response