playbook, inventory, and group_names

So let's say I have an inventory like this:

[cars]
honda
volvo
saab

[motos]
aprilla
moto-guzzi

and in my playbook I am referring to motorcycles only

- hosts: motos

From what I understand, group_names returns the groups defined in the

inventory. How can I get a list of the host groups I am using in the
playbook?

You can't ref this thread
https://groups.google.com/forum/#!msg/ansible-project/WntLXpN_gtg/HQ7PnjLZBAAJ