Twisted, deferred requests and File vs ErrorPage

Question

I'm currently building a http server with python/twisted.

This server must fetch content on another web server, store it locally and send the response back to the client. If it encounters a 404, it must try to serve the local file, or will fail if there's no local file at all.

To achieve this, I'm using differed requests. My code roughly looks like this:

class RemoteFile(Resource):
    isLeaf = True

    def render_GET(self, request):
        # Start downloading the corresponding file on the remote server
        # Will call `self.serve_local_file` when done
        return NOT_DONE_YET

    def serve_local_file(self, request):
        if self.file_exists_for_request(request):
            fileResource = File(filename)
            fileResource.render(request)
        else:
            page = NoResource("No such file")
            html = page.render(request)

            # The next two lines are my problem
            request.write(html)
            request.finish()

I had to add the last two lines (write() and finish()) in order to get the request to actually finish. I don't need these lines for a File resource, because it handles the write/finish by itself, unlike the NoResource.

This is not a problem per se, but it turns out that this page variable is not hardcoded, and it's actually a fallback attribute on my class, which is supposed to be a Resource. It means that I could, in the future, want to replace it with a default File. In which case I'll have to modify my code...

Am I doing something wrong, or is this an inconsistency in the interfaces of File and ErrorPage ?

Answer 1

There's a hint in your question itself:

return NOT_DONE_YET

This is part of how an IResource signals whether it is returning a string which the caller must write to the request before explicitly "finishing" the request or whether the IResource is taking responsibility for doing those things.

Check the return value of whatever you're calling render on and then act accordingly.