python keystoneclient AttributeError: 'RoleManager' object has no attribute 'add'

Using python shell, I execute this script:

keystone.roles.add_user_role(user='aji2', project='project.aji', role='role.aji')

and the result is this:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'RoleManager' object has no attribute 'add'

I think 'add' exists according to this documentation: role add

openstack role add --domain | --project [--project-domain ] --user [--user-domain ] | --group [--group-domain ] --role-domain --inherited

beg for a solution. thanks.