Very simple WSGI question

[Matthew Wilson]

I want to write some middleware to notice when the inner app returns a
500 status code.  I'm sure there are already sophisticated loggers that
do this sort of thing, but I'm using this as a learning exercise.

Right now, I wrapped the start_response callable.  So when the WSGI
application calls the start response callable, I look at the first arg
passed in and do my test.

What's the right way to do this?

Matt