- hosts: localhost
  connection: local

  roles:
  - dummy1
  - dummy2

  tasks:
  - debug: var="{{role_names}}"

ansible-galaxy init roles/dummy1
ansible-galaxy init roles/dummy2
ansible-playbook dummy.yml

[wthames@wthames galaxy]$ ansible-playbook dummy.yml -i localhost,

PLAY [localhost] ************************************************************** 

GATHERING FACTS *************************************************************** 
ok: [localhost]

TASK: [debug var="['dummy1', 'dummy2']"] ************************************** 
ok: [localhost] => {
    "var": {
        "['dummy1', 'dummy2']": "['dummy1', 'dummy2']"
    }
}

PLAY RECAP ******************************************************************** 

[WARNING]: provided hosts list is empty, only localhost is available

PLAY ***************************************************************************

TASK [setup] *******************************************************************
ok: [localhost]

TASK [debug vars={{role_names}}] ***********************************************
fatal: [localhost]: FAILED! => {"failed": true, "msg": "ERROR! 'role_names' is undefined"}

PLAY RECAP *********************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=1