# do not use mutable as default argument
# http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments
def checkCredentials(self, creds, operation, xrns=None,
# do not use mutable as default argument
# http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments
def checkCredentials(self, creds, operation, xrns=None,