class HttpHandler(object):
    def __call__(self, request, start_response):
        data = 'Hello, World!\n'
        status = '200 OK'
        response_headers = [
            ('Content-type','text/plain'),
            ('Content-Length', str(len(data)))
        ]
        start_response(status, response_headers)
        return iter([data])

app = HttpHandler()