How do I set response headers in Flask?

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You can do this pretty easily:

@app.route("https://stackoverflow.com/")
def home():
    resp = flask.Response("Foo bar baz")
    resp.headers['Access-Control-Allow-Origin'] = '*'
    return resp

Look at flask.Response and flask.make_response()

But something tells me you have another problem, because the after_request should have handled it correctly too.

EDIT
I just noticed you are already using make_response which is one of the ways to do it. Like I said before, after_request should have worked as well. Try hitting the endpoint via curl and see what the headers are:

curl -i http://127.0.0.1:5000/your/endpoint

You should see 

> curl -i 'http://127.0.0.1:5000/'
HTTP/1.0 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 11
Access-Control-Allow-Origin: *
Server: Werkzeug/0.8.3 Python/2.7.5
Date: Tue, 16 Sep 2014 03:47:13 GMT

Noting the Access-Control-Allow-Origin header.

EDIT 2
As I suspected, you are getting a 500 so you are not setting the header like you thought. Try adding app.debug = True before you start the app and try again. You should get some output showing you the root cause of the problem.

For example:

@app.route("https://stackoverflow.com/")
def home():
    resp = flask.Response("Foo bar baz")
    user.weapon = boomerang
    resp.headers['Access-Control-Allow-Origin'] = '*'
    return resp

Gives a nicely formatted html error page, with this at the bottom (helpful for curl command)

Traceback (most recent call last):
...
  File "/private/tmp/min.py", line 8, in home
    user.weapon = boomerang
NameError: global name 'boomerang' is not defined

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