Checking whether nested variable is defined in the when statement - ansible

Question

Nested data structures can be accessed either by indexing using the key name or dot syntax with the key name - https://docs.ansible.com/ansible/latest/user_guide/playbooks_variables.html#accessing-complex-variable-data
But I'm not able to access nested data structure from variables in the when statement. Here is a minimal sample to demonstrate the issue
# cat play.yml
- hosts: localhost
vars:
parent:
child1: true tasks:
- name: "Check if variable is defined"
fail:
msg: "mandatory variable {{ item }} not passed as extra args when invoking playbook"
when: item not in vars
loop:
- parent
- parent.child1
- parent.child2
Here is the sample output
ansible-playbook play.yml (livingstone-dev/monitoring)
[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
PLAY [localhost] *********************************************************************************************************************************************
TASK [Gathering Facts] ***************************************************************************************************************************************
ok: [localhost]
TASK [Check if variable is defined] **************************************************************************************************************************
skipping: [localhost] => (item=parent)
failed: [localhost] (item=parent.child1) => {"ansible_loop_var": "item", "changed": false, "item": "parent.child1", "msg": "mandatory variable parent.child1 not passed as extra args when invoking playbook"}
failed: [localhost] (item=parent.child2) => {"ansible_loop_var": "item", "changed": false, "item": "parent.child2", "msg": "mandatory variable parent.child2 not passed as extra args when invoking playbook"}
PLAY RECAP ***************************************************************************************************************************************************
localhost : ok=1 changed=0 unreachable=0 failed=1 skipped=0 rescued=0 ignored=0
Any idea how I can have nested variables in ansible when statements. Here are some things which I've already tried and didn't work:
Using dotted notation for accessing variable
Changing the when statement to "item is not defined" without the quotes

Answer 1

Below code worked for me. You have to provide the values in loop within "{{}}"
- name: "Check if variable is defined"
fail:
msg: "mandatory variable {{ item }} not passed as extra args when invoking playbook"
loop:
- "{{parent.child1}}"
- "{{parent}}"
- "{{parent.child2}}"
when: item is not defined

Answer 2

To get the same result without warning change
when: "{{item}} is not defined"
to
when: not vars|json_query(item)

Answer 3

Here is the working playbook. Thanks to #smily for pointing me in the right direction. Since the loop variable item needs to be evaluated before the when condition is evaluated, I had to expand the variable item. I've done it by encompassing the entire when condition in double quotes and just expanding the item loop variable.
$ cat play.yml (livingstone-dev/monitoring)
- hosts: localhost
vars:
parent:
child1: true
tasks:
- name: show variable
debug:
var: parent
- name: "Check if variable is defined"
fail:
msg: "mandatory variable item not passed as extra args when invoking playbook"
when: "{{item}} is not defined"
loop:
- parent
- parent.child1
- parent.child2
Here is the playbook output.
$ ansible-playbook play.yml (livingstone-dev/monitoring)
[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
PLAY [localhost] *********************************************************************************************************************************************
TASK [Gathering Facts] ***************************************************************************************************************************************
ok: [localhost]
TASK [show variable] *****************************************************************************************************************************************
ok: [localhost] => {
"parent": {
"child1": true
}
}
TASK [Check if variable is defined] **************************************************************************************************************************
[WARNING]: conditional statements should not include jinja2 templating delimiters such as {{ }} or {% %}. Found: {{item}} is not defined
skipping: [localhost] => (item=parent)
skipping: [localhost] => (item=parent.child1)
failed: [localhost] (item=parent.child2) => {"ansible_loop_var": "item", "changed": false, "item": "parent.child2", "msg": "mandatory variable item not passed as extra args when invoking playbook"}
PLAY RECAP ***************************************************************************************************************************************************
localhost : ok=2 changed=0 unreachable=0 failed=1 skipped=0 rescued=0 ignored=0