Solve Cross Origin Resource Sharing with Flask

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

        url: "", 
        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.

  1. Using Flask-CORS

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

My 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'])
    def foo():
        return request.json['inputVar']

    if __name__ == '__main__':
  1. Using specific Flask Decorator

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

My 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,
        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()
                    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'])
    def foo():
        return request.json['inputVar']

    if __name__ == '__main__':

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

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__':

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