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group_names is one of magic variables from Ansible [1] which is a list of all groups the current host is in, using this syntax will be more readable and avoid assuming that specific group is already defined in inventory. Another use case is if we want to run a task only if the host does not belong to a certain group. This also makes consitent with some usages in current code [2]. [1] http://docs.ansible.com/ansible/latest/playbooks_variables.html#magic-variables-and-how-to-access-information-about-other-hosts [2] https://github.com/openstack/openstack-ansible-os_cinder/blob/master/tasks/main.yml#L70 Change-Id: I7f85e18829cd3c9835f592ceb74f4e6460208ca6