keys.yaml

---
- name: thing
  hosts: localhost
  gather_facts: false
  vars:
    foo:
      keys:
        - 1
        - 2
        - 3

  tasks:
    - name: thing
      debug:
        var: foo.keys

    - name: thing2
      debug:
        var: foo['keys']
        
        
> ansible-playbook -i mastery-hosts keys.yaml -vv

PLAY [thing] ******************************************************************

TASK: [thing] *****************************************************************
ok: [localhost] => {
    "var": {
        "foo.keys": "<built-in method keys of dict object at 0x7f9dc29ea360>"
    }
}

TASK: [thing2] ****************************************************************
ok: [localhost] => {
    "var": {
        "foo['keys']": [
            1,
            2,
            3
        ]
    }
}