Python Flask cross site HTTP POST - doesn't work for specific allowed origins

Question

I'm trying to get Flask to handle cross-site scripting properly. I've taken the crossdomain decorator snippet from here: http://flask.pocoo.org/snippets/56/

In the code below, I've put the decorator snippet and the basic flask server.

I'm calling the decorator with headers='Content-Type' because otherwise I was getting "Request header field Content-Type is not allowed by Access-Control-Allow-Headers." in the browser.

So here is my question: As-is, the code below works. But when I want to restrict to only a specific server like so:

@crossdomain(origin='myserver.com', headers='Content-Type')

I get the browser error

"Origin http://myserver.com is not allowed by Access-Control-Allow-Origin."

I can't get it working for anything other than origin='*'.

Does anybody have any ideas?

Here is the complete code:

from datetime import timedelta
from flask import make_response, request, current_app, Flask, jsonify
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 = Flask(__name__)

@app.route('/my_service', methods=['POST', 'OPTIONS'])
@crossdomain(origin='*', headers='Content-Type')
def my_service():
    return jsonify(foo='cross domain ftw')

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

For reference my python version is 2.7.2 Flask version is 0.7.2

Nate Nate · Accepted Answer · 2013-04-27T17:15:12

I just tried the same code with python version 2.7.3 and Flask version 0.8.

With these versions, it fails with

@crossdomain(origin='myserver.com', headers='Content-Type')

but it works with

@crossdomain(origin='http://myserver.com', headers='Content-Type')

Perhaps it just doesn't work with Flask 0.7.2? (despite what it says on the snippet page).

EDIT: After playing with this a lot more (and upgrading to Flask 0.9) it seems that the real problem (or yet another problem) might be related to having multiple allowed origins in a list. In other words, using the above code like this:

@crossdomain(origin=['http://myserver.com', 'http://myserver2.com'], headers='Content-Type')

doesn't work.

To fix this problem I tweaked the decorator. See code here: http://chopapp.com/#351l7gc3

This code returns only the domain of the requesting site if it is in the list. Kinda quirky, but at least for me, problem solved :)

Python Flask cross site HTTP POST - doesn't work for specific allowed origins

2 Answers