ansible - example of using default var or result from nested dict

output

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

TASK [debug] *******************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "a exists!"
}

PLAY [this should work, it doesn't] ********************************************************************************************************************************************************************************************************************************************

TASK [debug] *******************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": ""
}

PLAY [this works - but uses unoffically exposed 'vars'] ************************************************************************************************************************************************************************************************************************

TASK [debug] *******************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "bcde exists!"
}

PLAY [this works] **************************************************************************************************************************************************************************************************************************************************************

TASK [set_fact] ****************************************************************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [debug] *******************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "bcde exists!"
}

PLAY RECAP *********************************************************************************************************************************************************************************************************************************************************************
localhost                  : ok=5    changed=0    unreachable=0    failed=0

play.yml

---
- name: testing for a
  hosts: localhost
  gather_facts: False
  connection: local
  vars:
    a: a exists!
  tasks:
    - debug:
        msg: "{{ a|default(inventory_hostname|extract(hostvars, ['b', 'c', 'd', 'e'])) }}"

- name: testing for b: this should work, it doesn't
  hosts: localhost
  gather_facts: False
  connection: local
  vars:
    b: 
      c:
        d:
          e: 'bcde exists!'
  tasks:
    - debug:
        msg: "{{ a|default(inventory_hostname|extract(hostvars, ['b', 'c', 'd', 'e'])) }}"

- name: testing for b: this works - but uses unoffically exposed 'vars'
  hosts: localhost
  gather_facts: False
  connection: local
  vars:
    b: 
      c:
        d:
          e: 'bcde exists!'
  tasks:
    - debug:
        msg: "{{ a|default('b'|extract(vars, ['c', 'd', 'e'])) }}"

- name: testing for b: this works
  hosts: localhost
  gather_facts: False
  connection: local
  tasks:
    - set_fact:
        b: 
          c:
            d:
              e: 'bcde exists!'

    - debug:
        msg: "{{ a|default(inventory_hostname|extract(hostvars, ['b', 'c', 'd', 'e'])) }}"