'Get the key for a particular namespaced argument'

def getKey(self, namespace, ns_key):

    namespace = self._fixNS(namespace)
    if (namespace == BARE_NS):
        return ns_key
    ns_alias = self.namespaces.getAlias(namespace)
    if (ns_alias is None):
        return None
    if (ns_alias == NULL_NAMESPACE):
        tail = ns_key
    else:
        tail = ('%s.%s' % (ns_alias, ns_key))
    return ('openid.' + tail)
