Ansible: How to set_fact from Template- / YAML-file?

Question

We typically load a file of YAML code into our playbook using include_vars.

Now there are YAML blocks in the YAML file that we would like to generate using Jinja2.

The YAML file then looks simplified like this:

my_list_from_template:
  {% filter from_yaml %}
  {% for my_os_stage in all_os_stage -%}
  - name: "Template for {{ my_os_stage }}"
  {% endfor %}
  {% endfilter %}

Our playbook looks like this

---
- name: "read tmp/test.yml"
  hosts: localhost
  connection: local
  remote_user: opsadm
  gather_facts: False
  become: yes

  #-----------------------------------------------
  vars:
    all_os_stage:
      - TST
      - DEV
      - INT
      - PRD

  #-----------------------------------------------
  tasks:
    - name: "read tmp/test.yml as template"
      ansible.builtin.set_fact: "{{ lookup('ansible.builtin.template', 'tmp/test.yml') }}"

    - debug:
        msg: "{{ my_list_from_template }}"

However, the debug fails with this error message:

TASK [read tmp/test.yml as template] 
task path: test.yml:19
File lookup using tmp/test.yml as file
ok: [localhost] => {
    "ansible_facts": {
        "_raw_params": "my_list_from_template:\n  [{'name': 'Template for TST'}, {'name': 'Template for DEV'}, {'name': 'Template for INT'}, {'name': 'Template for PRD'}]\n"
    },
    "changed": false
}

TASK [debug]
task path: test.yml:22
fatal: [localhost]: FAILED! => {
    "msg": "The task includes an option with an undefined variable. The error was: 'my_list_from_template' is undefined. 'my_list_from_template' is undefined\n\nThe error appears to be in '/home/oizkrwa/ANS_GIT/LINUX_project_controller_2.4.0/test.yml': line 22, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n    - debug:\n      ^ here\n"
}

How can I solve the problem?

Answer 1

For a Jinja2 Template called templates/my_list.j2 as input and with content of

my_list_from_template:
  {% for my_os_stage in all_os_stage -%}
  - name: "Template for {{ my_os_stage }}"
  {% endfor %}

processing with a minimal example playbook

---
- hosts: localhost
  become: false
  gather_facts: false

  vars:

    all_os_stage:
      - TST
      - DEV
      - INT
      - PR

  tasks:

  - name: "Set from template"
    set_fact:
      my_list: "{{ lookup('ansible.builtin.template', 'templates/my_list.j2') }}"

  - debug:
      msg: "{{ my_list | from_yaml }}"

will result into an output of

TASK [debug] ***************
ok: [localhost] =>
  msg:
    my_list_from_template:
    - name: Template for TST
    - name: Template for DEV
    - name: Template for INT
    - name: Template for PR