CORS (cross origin resource charing) using cherrypy

Cherrypy allows to define functions to be so called „tools“ (cherrypy docs). These helper functions might be „hooked“ at various processing points within cherrypys serving process: on_start_resource, before_request_body, before_handler, before_finalize, on_end_resource, before_error_response, after_error_response, on_end_request.

Making the tool is working straight forward. According to the working draft cross origin access allowance needs to be declared within the response header (see RFC for further information on CORS headers). The following example will add a response header to allow CORS to any („*“) client; change the ‚*‘ to an uri-scheme have provide more granular scaling.

import cherrypy

def CORS():
…cherrypy.response.headers[„Access-Control-Allow-Origin“] = „*“ #

attaching the function to a process stage hook needs to be done in the main proc:

if __name__==“__main__“:
… = cherrypy.Tool(‚before_finalize‘, CORS)

# insert tool declaration within def main() or even directly
# before cherrypy.tree.mount command to prevent Error like

If you try to declare the toll earlier an TypeError starting with:
The ‚CORS‘ Tool does not accept positional arguments … is likely to be thrown.

Like with other cherrypy tools you might use them globally by adding „cherrypy.tool.CORS.on : True“ to the general.configs or by using the decorator right after the exposure decorator of a
page/wsgi def.