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Flask, AngularJS and CORS

I am currently developing a website that will entirely be a Single Page Application (SPA). I started building the back-end in D and the front-end in AngularJS. I decided after a couple of weeks that Jeff Atwood was right in saying “Storage is cheap, programmers are expensive.” Though D is a useful language, it was taking me too long to produce useful RESTful API endpoints, so I decided to make the switch to Flask given my familiarity with it.

In the first iteration of the project I had a simple index.html being served from the same application that the API was running, accessing all my scripts, styles, images, etc… from a CDN. This was a fairly simple CORS setup and a little bit of tweaking in Angular to make it all work.

When I made the switch to Flask I changed the structure of the hosting so that the static content was being served from www.example.com and the API was accessed from api.example.com. I figured this wouldn’t cause too many issues and it should again just be a simple CORS setup. Unfortunately there was a little more to it than that. I found that when making requests with Angular’s $http provider cookies that were being set were immediately being discareded. I ended up routing all my requests through nginx so that the API was hosted at www.example.com/api and the cookies were being retained correctly.

The solution was simple in the end. All I needed to do was add the following code to my Angular module .config() method.

$httpProvider.defaults.withCredentials = true;

On the server-side initially I was using Flask-CORS but it was frustrating to have to decorate all my REST endpoints (essentially every single route in my application) with @cross_domain(), so I ended up writing my own basic Flask context processor.

@app.after_request
def add_cors_headers(response):
    response.headers.add('Access-Control-Allow-Origin', 'www.example.com')
    response.headers.add('Access-Control-Allow-Credentials', 'true')
    response.headers.add('Access-Control-Allow-Headers', 'Content-Type')
    return response

The Access-Control-Allow-Credentials is necessary when specifying withCredentials on $http requests.