test composing variables

inventory_source_1.yml

# add hosts and hostvars to inventory with the yaml inventory plugin
all:
  hosts:
    host1:
      a: 'is_defined'
    host2:

inventory_source_2.yml

# compose variables from the hosts in inventory
plugin: constructed
compose:
  var: a is defined | ternary('is_defined', 'is_not_defined')

output

$ ansible-inventory -i inventory_source_1.yml -i inventory_source_2.yml --list
[WARNING]: You are running the development version of Ansible. You should only run Ansible from "devel" if you are modifying the Ansible engine, or trying out features under development. This is a rapidly
changing source of code and can become unstable at any point.
{
    "_meta": {
        "hostvars": {
            "host1": {
                "a": "is_defined",
                "var": "is_defined"
            },
            "host2": {
                "var": "is_not_defined"
            }
        }
    },
    "all": {
        "children": [
            "ungrouped"
        ]
    },
    "ungrouped": {
        "hosts": [
            "host1",
            "host2"
        ]
    }
}