Javascript - No 'Access-Control-Allow-Origin' header is present on the requested resource

Question

I need to send data through XmlHttpRequest from JavaScript to Python server. Because I\'m using localhost, I need to use CORS. I\'m using the Flask framework an

Answer 1

I got Javascript working with Flask by using this decorator, and adding "OPTIONS" to my list of acceptable methods. The decorator should be used beneath your route decorator, like this:

@app.route('/login', methods=['POST', 'OPTIONS'])
@crossdomain(origin='*')
def login()
    ...

Edit: Link appears to be broken. Here's the decorator I used.

from datetime import timedelta
from flask import 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):
    """Decorator function that allows crossdomain requests.
      Courtesy of
      https://blog.skyred.fi/articles/better-crossdomain-snippet-for-flask.html
    """
    if methods is not None:
        methods = ', '.join(sorted(x.upper() for x in methods))
    # use str instead of basestring if using Python 3.x
    if headers is not None and not isinstance(headers, basestring):
        headers = ', '.join(x.upper() for x in headers)
    # use str instead of basestring if using Python 3.x
    if not isinstance(origin, basestring):
        origin = ', '.join(origin)
    if isinstance(max_age, timedelta):
        max_age = max_age.total_seconds()

    def get_methods():
        """ Determines which methods are allowed
        """
        if methods is not None:
            return methods

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

    def decorator(f):
        """The decorator function
        """
        def wrapped_function(*args, **kwargs):
            """Caries out the actual cross domain code
            """
            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)
            h['Access-Control-Allow-Credentials'] = 'true'
            h['Access-Control-Allow-Headers'] = \
                "Origin, X-Requested-With, Content-Type, Accept, Authorization"
            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