After I tried others suggestions and answers. Here's what I use, which works.

Steps:

1. pip install flask flask-cors

2. Copy and paste this in app.py file

Code

from flask import Flask, jsonify
from flask_cors import CORS, cross_origin

app = Flask(__name__)
CORS(app, support_credentials=True)

@app.route("/login")
@cross_origin(supports_credentials=True)
def login():
  return jsonify({'success': 'ok'})

if __name__ == "__main__":
  app.run(host='0.0.0.0', port=8000, debug=True)

3. python app.py

Note: be sure in your client's ajax configuration has the following:

$.ajaxSetup({
    type: "POST",
    data: {},
    dataType: 'json',
    xhrFields: {
       withCredentials: true
    },
    crossDomain: true,
    contentType: 'application/json; charset=utf-8'
});

If one wonders, support_credentials=True just means it sends cookies along the payload back and forth.

Flask has the flask-cors module. Following is the code snippet as well as the procedure.

1. pip install -U flask-cors

2. Add this lines in your flask application:

   from flask import Flask
   from flask_cors import CORS, cross_origin
   
   app = Flask(__name__)
   CORS(app)
   
   @app.route("/")
   def helloWorld():
       return "Hello world"
   

See more by clicking on this link

Here is how to get your hand dirty by handling the CORS detail all by yourself:

handle_result = {'result': True, 'msg': 'success'}

try:
    # origin, where does this request come from, like www.amazon.com
    origin = flask.request.environ['HTTP_ORIGIN']
except KeyError:
    origin = None

# only accept CORS request from amazon.com
if origin and origin.find('.amazon.com') > -1:
    resp = flask.make_response(str(handle_result))
    resp.headers['Content-Type'] = 'application/json'

    h = resp.headers
    # prepare headers for CORS authentication
    h['Access-Control-Allow-Origin'] = origin
    h['Access-Control-Allow-Methods'] = 'GET'
    h['Access-Control-Allow-Headers'] = 'X-Requested-With'

    resp.headers = h
    return resp

return flask.abort(403)

use the cors decorator after the route decorator.

here's a snippet from the documentation...

@app.route("/")
@cross_origin() # allow all origins all methods.
def helloWorld():
  return "Hello, cross-origin-world!"

now, it appears you are using json, if that's the case, you should likely just read the documentation as it specifically mentions this use case, and what cors_headers to set... it's below the fold, but this documentation is well written and easy to understand.

http://flask-cors.readthedocs.org/en/latest/#using-json-with-cors

Please use @cross_origin(origin='*') in your python file

from flask import Flask, jsonify
from flask_cors import CORS, cross_origin

app = Flask(__name__)

@app.route("/login", methods = ['GET'])
@cross_origin(origin='*')
def login():
  return jsonify({'success': 'ok'})

if __name__ == "__main__":
  app.run(host='0.0.0.0', port=8000, debug=True)

The below solution worked for me. I included a method that will add the headers necessary for you and then raise the HTTP response. Ex:

def some_method(response_data, status_code):
    response_data = //here you can manipulate the data, JSONify, convert arrays into objects or vice versa
    headers = {
        "Content-Type": "application/json",
        "Access-Control-Allow-Origin": '*',
        "Access-Control-Allow-Methods": 'PUT, GET, POST, DELETE, OPTIONS',
        "Access-Control-Allow-Headers": 'Origin, Accept, Content-Type, X-Requested-With, X-CSRF-Token'
        }
    //THEN RAISE HTTPResponse
    raise HTTPResponse(status, headers, body)

Note: The above method is not python compiling so you may have to edit it.

Try this:

@app.after_request
def add_headers(response):
    response.headers.add('Access-Control-Allow-Origin', '*')
    response.headers.add('Access-Control-Allow-Headers', 'Content-Type,Authorization')

I've tried @cross_origin tutorial on the Flask website,however, it did not work for me.

But it seems like you can add headers to your response later.

Here is my remaining code that I think it may be useful.

from flask import Flask, request
from sklearn.externals import joblib

app = Flask(__name__)

Based on GyuHyeon Choi's response, but with added return response and an extra Access-Control-Expose-Headers worked for me.

@app.after_request
def add_headers(response):
    response.headers.add('Content-Type', 'application/json')
    response.headers.add('Access-Control-Allow-Origin', '*')
    response.headers.add('Access-Control-Allow-Methods', 'PUT, GET, POST, DELETE, OPTIONS')
    response.headers.add('Access-Control-Allow-Headers', 'Content-Type,Authorization')
    response.headers.add('Access-Control-Expose-Headers', 'Content-Type,Content-Length,Authorization,X-Pagination')
    return response

On client side you only need to ensure that kind of data your server is dealing with. For example form data or json.

Note: The placement of cross_origin should be right.

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 have installed flask using the following command and using the declaration like this:

pips3.6 install --user flask-cors

  from flask_cors import CORS   
    app = Flask(__name__)
    CORS(app)