This is a simple middleware for adding Access-Control-* headers to your application, allowing Cross-Origin Resource Sharing -- basically meaning that you can use XMLHttpRequest from one domain to access resources on another domain.
The middleware is located in wsgiaccesscontrol.accesscontrol.AccessControl.
This is the code that we use to make the Firefox Sync web service available across origins (domains):
application = AccessControlMiddleware( application, allow_origin='*', allow_methods=('GET','POST','PUT','DELETE','OPTIONS'), allow_headers=('X-Authorization', 'Authorization', 'X-If-Modified-Since', 'X-If-Unmodified-Since', 'Content-Type'), expose_headers=('X-Weave-Timestamp', 'X-Weave-Backoff', 'X-Weave-Alert', 'X-Weave-Records'))
Note that there is a little workaround for a Chrome bug that turns * into a dynamic Access-Control-Allow-Origin header (copying the Origin request header). A simple future feature is to allow a whitelist of origins (instead of just a single domain or wildcard), which also requires a dynamic Allow-Origin header (because the header is only allowed to include one value).