gistfile1.py

def add_role(target, role):
    target_grains = client().cmd(target, 'grains.items')
    for host, grains in target_grains.items():
        roles = grains.get('roles', [])
        if role not in roles:
            roles.append(role)
            result = client().cmd(
                host, 'grains.setval', ['roles', json.dumps(roles)])
            print result
        else:
            print {host: {'roles': roles}}