After setting up a lightweight API using Flask, I tried making a request from a different server and ran into the following error:

XMLHttpRequest cannot load <Flask server url>. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '<My client server>' is therefore not allowed access.  

Luckily, I found a function written by Armin Ronachere which allows cross-orgin resource sharing using Flask.

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


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('/')
@crossdomain(origin='*')
def landing():  
    return jsonify(i_am_a='cross domain resource!')

if __name__ == '__main__':  
    app.run(host='0.0.0.0', port=8080)

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