Normalize the URL to create a safe key for the cache

def _urlnorm(cls, uri):
        """Normalize the URL to create a safe key for the cache"""
        (scheme, authority, path, query, fragment) = parse_uri(uri)
        if not scheme or not authority:
            raise Exception("Only absolute URIs are allowed. uri = %s" % uri)

        scheme = scheme.lower()
        authority = authority.lower()

        if not path:
            path = "/"

        # Could do syntax based normalization of the URI before
        # computing the digest. See Section 6.2.2 of Std 66.
        request_uri = query and "?".join([path, query]) or path
        defrag_uri = scheme + "://" + authority + request_uri

        return defrag_uri