How do I set response headers in Flask?

This work for me

from flask import Flask
from flask import Response

app = Flask(__name__)

@app.route("/")
def home():
    return Response(headers={'Access-Control-Allow-Origin':'*'})

if __name__ == "__main__":
    app.run()

You can do this pretty easily:

@app.route("/")
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("/")
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

This was how added my headers in my flask application and it worked perfectly

@app.after_request
def add_header(response):
    response.headers['X-Content-Type-Options'] = 'nosniff'
    return response

Use make_response of Flask something like

@app.route("/")
def home():
    resp = make_response("hello") #here you could use make_response(render_template(...)) too
    resp.headers['Access-Control-Allow-Origin'] = '*'
    return resp

From flask docs,

flask.make_response(*args)

Sometimes it is necessary to set additional headers in a view. Because views do not have to return response objects but can return a value that is converted into a response object by Flask itself, it becomes tricky to add headers to it. This function can be called instead of using a return and you will get a response object which you can use to attach headers.