Loop is not working with data type test as conditional in Ansible

When I execute ansible playbook with this command and extra vars as

ansible-playbook controller_config/windows_domain_names.yml -e "target_win_host=testserver.testdomain.com"

with one task inside it

- name: Find if the variable target_win_host is a list
  debug:
    msg: "{{ item }} is list data type"
  loop: "{{ target_win_host}}"
  when: target_win_host | type_debug == 'list'

I got the error as

"Invalid data passed to 'loop', it requires a list, got this instead: testserver.testdomain.com. Hint: If you passed a list/dict of just one element, try adding wantlist=True to your lookup invocation or use q/query instead of lookup."

I expect the above block should be skipped as this does not meet the condition that variable is a list and that its a scalar (string).

I tried with different condition as below

- name: Find if the variable target_win_host is a list
  debug:
    msg: "{{ item }} is list data type"
  loop: "{{ target_win_host}}"
  when: target_win_host is not string and target_win_host is iterable and target_win_host is sequence

but same error as above.

Solution

You approach won't work because for the given task, the loop expression is first evaluated, the first item is extracted and only then the when: condition is evaluated (once for each loop iteration).

In other words, for your approach to work, you need to somehow embed the condition in the loop: expression and make it result to a zero length list.

The following task achieves that goal:

- name: Loop over target_win_host if it is a list
  debug:
    msg: "{{ item }} is part of a list of hosts"
  loop: "{{ target_win_host if target_win_host | type_debug == 'list' else [] }}"

Meanwhile if you'd rather play the same task for all hosts whether they are given as a list or as a single host string, this is probably better:

- name: Loop over hosts in target_win_host whether it is a list or a single host string
  debug:
    msg: "{{ item }} was given as one the hosts"
  loop: "{{ [target_win_host] | flatten }}"

This is a full playbook to test both options:

---
- name: Conditional and loops
  hosts: localhost
  gather_facts: false

  vars:
    # default value as a list. Override with extra vars
    target_win_host:
      - host1.example.com
      - host2.example.com

  tasks:
    - name: Loop over target_win_host if it is a list
      debug:
          msg: "{{ item }} is part of a list of hosts"
      loop: "{{ target_win_host if target_win_host | type_debug == 'list' else [] }}"


    - name: Loop over hosts in target_win_host whether it is a list or a single host string
      debug:
        msg: "{{ item }} was given as one of the hosts"
      loop: "{{ [target_win_host] | flatten }}"

Which gives:

$ ansible-playbook test.yml 

PLAY [Conditional and loops] *************************************************************************************************************************************************************************************

TASK [Loop over target_win_host if it is a list] *********************************************************************************************************************************************************************
ok: [localhost] => (item=host1.example.com) => {
    "msg": "host1.example.com is part of a list of hosts"
}
ok: [localhost] => (item=host2.example.com) => {
    "msg": "host2.example.com is part of a list of hosts"
}

TASK [Loop over hosts in target_win_host whether it is a list or a single host string] *****************************************************************************************************************************
ok: [localhost] => (item=host1.example.com) => {
    "msg": "host1.example.com was given as one of the hosts"
}
ok: [localhost] => (item=host2.example.com) => {
    "msg": "host2.example.com was given as one of the hosts"
}

PLAY RECAP ********************************************************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

$ ansible-playbook test.yml -e target_win_host=a.single.host

PLAY [Conditional and loops] *************************************************************************************************************************************************************************************

TASK [Loop over target_win_host if it is a list] *********************************************************************************************************************************************************************
skipping: [localhost]

TASK [Loop over hosts in target_win_host whether it is a list or a single host string] *****************************************************************************************************************************
ok: [localhost] => (item=a.single.host) => {
    "msg": "a.single.host was given as one of the hosts"
}

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