I ran into a problem while trying to use selector (a very cool little library; one of Luke Arno’s hacks) under wsgiref’s CGIHandler. Selector assumes environ['PATH_INFO'] is set, but CGIHandler doesn’t set it, so everything blows up.

Fortunately, Python makes this sort of thing incredibly easy to work around. First, let’s define a shim class which’ll frob environ if necessary:

class SelectorShim:
    """Wraps a Selector so that we can use it under CGIHandler."""
    def __init__(self, urls):
        self.selector = urls
    def __call__(self, environ, start_response):
        if 'PATH_INFO' not in environ:
            # Either PATH_INFO or SCRIPT_NAME *must* be defined
            environ['PATH_INFO'] = environ['SCRIPT_NAME']
        return self.selector(environ, start_response)

Using the shim is just a matter of running your selector through it on its way to CGIHandler: just change

CGIHandler().run(urls)

to

CGIHandler().run(SelectorShim(urls))

I love how easy this was to do. I ran into this problem and fixed it while on the bus this morning — I was able to fix it while offline and unable to view any of the relevant documentation. Yet Another reason to love Python.