Solve Cross Origin Resource Sharing with Flask

javascript python ajax flask cors

218,940

Solution 1

It worked like a champ, after bit modification to your code

# initialization
app = Flask(__name__)
app.config['SECRET_KEY'] = 'the quick brown fox jumps over the lazy   dog'
app.config['CORS_HEADERS'] = 'Content-Type'

cors = CORS(app, resources={r"/foo": {"origins": "http://localhost:port"}})

@app.route('/foo', methods=['POST'])
@cross_origin(origin='localhost',headers=['Content- Type','Authorization'])
def foo():
    return request.json['inputVar']

if __name__ == '__main__':
   app.run()

I replaced * by localhost. Since as I read in many blogs and posts, you should allow access for specific domain

Solution 2

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

Solution 3

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!"

Solution 4

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"}

Solution 5

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.

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218,940

Author by

Matteo

I'm a PhD student in Computer Science.

Updated on May 08, 2022

Comments

Matteo about 2 years

For the following ajax post request for Flask (how can I use data posted from ajax in flask?):

$.ajax({
    url: "http://127.0.0.1:5000/foo", 
    type: "POST",
    contentType: "application/json",
    data: JSON.stringify({'inputVar': 1}),
    success: function( data ) { 
        alert( "success" + data );
    }   
});

I get a Cross Origin Resource Sharing (CORS) error:

No 'Access-Control-Allow-Origin' header is present on the requested resource. 
Origin 'null' is therefore not allowed access. 
The response had HTTP status code 500.

I tried solving it in the two following ways, but none seems to work.

Using Flask-CORS

This is a Flask extension for handling CORS that should make cross-origin AJAX possible.

My pythonServer.py using this solution:

from flask import Flask
from flask.ext.cors import CORS, cross_origin

app = Flask(__name__)
cors = CORS(app, resources={r"/foo": {"origins": "*"}})
app.config['CORS_HEADERS'] = 'Content-Type'

@app.route('/foo', methods=['POST','OPTIONS'])
@cross_origin(origin='*',headers=['Content-Type','Authorization'])
def foo():
    return request.json['inputVar']

if __name__ == '__main__':
    app.run()

Using specific Flask Decorator

This is an official Flask code snippet defining a decorator that should allow CORS on the functions it decorates.

My pythonServer.py using this solution:

from flask import Flask, make_response, request, current_app
from datetime import timedelta
from functools import update_wrapper

app = Flask(__name__)

def crossdomain(origin=None, methods=None, headers=None,
                max_age=21600, attach_to_all=True,
                automatic_options=True):
    if methods is not None:
        methods = ', '.join(sorted(x.upper() for x in methods))
    if headers is not None and not isinstance(headers, basestring):
        headers = ', '.join(x.upper() for x in headers)
    if not isinstance(origin, basestring):
        origin = ', '.join(origin)
    if isinstance(max_age, timedelta):
        max_age = max_age.total_seconds()

    def get_methods():
        if methods is not None:
            return methods

        options_resp = current_app.make_default_options_response()
        return options_resp.headers['allow']

    def decorator(f):
        def wrapped_function(*args, **kwargs):
            if automatic_options and request.method == 'OPTIONS':
                resp = current_app.make_default_options_response()
            else:
                resp = make_response(f(*args, **kwargs))
            if not attach_to_all and request.method != 'OPTIONS':
                return resp

            h = resp.headers

            h['Access-Control-Allow-Origin'] = origin
            h['Access-Control-Allow-Methods'] = get_methods()
            h['Access-Control-Max-Age'] = str(max_age)
            if headers is not None:
                h['Access-Control-Allow-Headers'] = headers
            return resp

        f.provide_automatic_options = False
        return update_wrapper(wrapped_function, f)
    return decorator

@app.route('/foo', methods=['GET','POST','OPTIONS'])
@crossdomain(origin="*")
def foo():
    return request.json['inputVar']

if __name__ == '__main__':
    app.run()

Can you please give some some indication of why that is?

shao over 9 years

did you figure out? I run into the exact same issue :(
Juha Untinen about 7 years

This is an old question, but just to be sure: Did you restart your Flask server? I was also wondering why I got the same error even thought everything was exactly as they should. Turns out, you have to restart the server in order for it to really take effect

Cătălin Florescu over 7 years

Better that adding crossdomain implementation!
Anthony over 7 years

Simple and effective
PALEN almost 7 years

Brilliant! I agree, simple and effective. IMO Should be accepted answer
Matteo almost 7 years

@Salvador Dali - Do you know what is proper way of allowing cross origin if I am rendering a template instead of just a json object? i.e. last line of code in yourMethod is: return render_template('template.html',some_var = response)
W.Leto over 5 years

Could this be used in a post method? I tried for a file upload method and I failed.
humblePilgrim almost 5 years

This gives the following error for me Access to XMLHttpRequest at 'my_domain' from origin '127.0.0.1:5000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: The 'Access-Control-Allow-Origin' header contains multiple values '127.0.0.1:5000, *', but only one is allowed.
Eziz Durdyyev over 4 years

why do you do the same thing(support_credentials=True) in cross_origin?
W.P. McNeill over 4 years

Doesn't work for me. stackoverflow.com/questions/59652099/…
Jose about 4 years

Works for me too Tks!
BLang about 4 years

In case anyone is using blueprints, you need to add the CORS() to each blueprint, for example: my_blueprint = Blueprint('my_bp_name', name, url_prefix="/my-prefix") CORS(my_blueprint)
José Tomás Tocino almost 4 years

The link to the decorator is dead
DragonKnight over 3 years

how to set Access-Control-Max-Age for flask_cors?
Anime no Sekai over 3 years

1) Headers are from HTTP connections, not HTML documents 2) You can just open up your devtools on any modern browser to display the headers of the connection under the 'Network' tab without installing any extension 3) You might need to use * in Access-Control-Allow-Origin if you don't want to to set a custom header for every request and you want for example to create a Public API
mattyb about 3 years

Worked for me!! Thank you. This should be the accepted answer!
mattyb about 3 years

See the one-liner below as well: stackoverflow.com/a/46637194/3559330
oeter almost 3 years

@Matteo It's been 4 years, but you can do something like this: response = make_response(render_template('index.html', foo=42)), response.headers.add('Access-Control-Allow-Origin', '*')
Matteo almost 3 years

@oeter - Thanks for the tip!
Alaa M. almost 3 years

Adding my 2 cents... I still got a CORS error, but when I ran heroku logs --tail I saw a module import error. Then I added Flask-Cors==3.0.10 to the requirements.txt file, and it worked. I did not have to restart the dynos. Also you can run pip freeze > requirements.txt to get all the module requirements automatically.
Noname about 2 years

doesn't seem to work with POST requests
jeff about 2 years

Please note that the wildcard should be avoided, i.e. setting Access-Control-Allow-Origin to '*'. As per Mozilla docs: Access-Control-Allow-Origin specifies either a single origin which tells browsers to allow that origin to access the resource; or else — for requests without credentials — the " * " wildcard tells browsers to allow any origin to access the resource. You wouldn't want any origin from accessing your endpoints.