How do I set response headers in Flask

Question

This is my code    app route   hello   methods   POST    def hello        resp   make response       resp headers  Access-Control-Allow-Origin             return resp   However  when I make a request from the browser to my server I get this error   XMLHttpRequest cannot load http   localhost 5000 hello   No  Access-Control-Allow-Origin  header is present on the requested resource    I have also tried this approach  setting the response headers  after  the request    app after request def add header response       response headers  Access-Control-Allow-Origin             return response   No dice  I get the same error  Is there a way to just set the response headers in the route function  Something like this would be ideal    app route   hello   methods   POST        def hello response     is this a thing           response headers  Access-Control-Allow-Origin                 return response   but I cant find anyway to do this  Please help   EDIT  if I curl the url with a POST request like so   curl -iX POST http   localhost 5000 hello   I get this response   HTTP 1 0 500 INTERNAL SERVER ERROR Content-Type  text html Content-Length  291 Server  Werkzeug 0 9 6 Python 2 7 6 Date  Tue  16 Sep 2014 03 58 42 GMT   lt  DOCTYPE HTML PUBLIC  -  W3C  DTD HTML 3 2 Final  EN  gt   lt title gt 500 Internal Server Error lt  title gt   lt h1 gt Internal Server Error lt  h1 gt   lt p gt The server encountered an internal error and was unable to complete your request   Either the server is overloaded or there is an error in the application  lt  p gt    Any ideas

User · Accepted Answer

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    gt  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

User · Answer

We can set the response headers in Python Flask application using Flask application context using flask g  This way of setting response headers in Flask application context using flask g is thread safe and can be used to set custom  amp  dynamic attributes from any file of application  this is especially helpful if we are setting custom dynamic response headers from any helper class  that can also be accessed from any other file   say like middleware  etc   this flask g is global  amp  valid for that request thread only   Say if i want to read the response header from another api http call that is being called from this app  and then extract any  amp  set it as response headers for this app   Sample Code  file  helper py  import flask from flask import request  g from multidict import CIMultiDict from asyncio import TimeoutError as HttpTimeout from aiohttp import ClientSession      def  extract response header response                  extracts response headers from response object        and stores that required response header in flask g app context                 headers   CIMultiDict response headers        if  my response header  not in g          g my response header             g my response header  x-custom-header     headers  x-custom-header         async def call post api post body                   sample method to make post api call using aiohttp clientsession                 try          async with ClientSession   as session            async with session post uri  headers  headers  json post body  as response              responseResult   await response read                extract headers response  responseResult              response text   await response text         except  HttpTimeout  ConnectionError  as ex          raise HttpTimeout exception message    file  middleware py  import flask from flask import request  g  class SimpleMiddleWare object               Simple WSGI middleware              def   init   self  app           self app   app         self  header name    any request header       def   call   self  environ  start response                       middleware to capture request header from incoming http request                     request id header   environ get self  header name          environ self  header name    request id header          def new start response status  response headers  exc info None                               set custom response headers                               set the request header as response header             response headers append  self  header name  request id header                 this is trying to access flask g values set in helper class  amp  set that as response header             values   g get my response header                  if values get  x-custom-header                    response headers append   x-custom-header   values get  x-custom-header                 return start response status  response headers  exc info           return self app environ  new start response    Calling the middleware from main class  file   main py  from flask import Flask import asyncio from gevent pywsgi import WSGIServer from middleware import SimpleMiddleWare      app   Flask   name        app wsgi app   SimpleMiddleWare app wsgi app

User · Answer

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

User · Answer

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

User · Answer

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

[python] How do I set response headers in Flask?

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