The sendfile example application does not actually result in a call to sendfile. I verified this by adding code to http/wsgi.py and also looking at the output of strace.

It works if I remove the @validator decorator on the app method in sendfile.py.

I guess this is because the worker is checking for an instance of the write type, but the type is not the same with the validator wrapper. workers/sync.py handle_request() says

if isinstance(respiter, environ['wsgi.file_wrapper']):