Different ways to compose variables from other variables in Ansible

compose_variables.yml

---
- name: Compose variables and merge lists
  hosts: localhost
  connection: local
  gather_facts: no

  vars:
    api_versions:
      - 1
      - 2
      - 3
      - 5
    api_url: "https://api.example.com"
    api_resource: "resource_num_"
    api_resources: []
    api_urls: []

  pre_tasks:
    - name: Compose the list of api_urls
      set_fact:
        # [A] = [A] + [item] 
        api_urls: "{{ api_urls }} + [ '{{ api_url }}/v{{ item }}' ]"
        api_resources: "{{ api_resources }} + [ '{{ api_resource }}{{ item }}' ]"
      loop: "{{ api_versions }}"

  tasks:
    - name: Show the list of api_urls
      debug:
        msg: "{{ item }}"
      loop: "{{ api_urls }}"

    - name: Show the list of api_resources
      debug:
        msg: "{{ item }}"
      loop: "{{ api_resources }}"

    - name: Show the WHAT resources get from WHERE
      debug:
        msg: "GET: {{ item.1 }} FROM: {{ item.0 }}"
      # Merge api_urls and api_resources lists using the zip() filter
      loop: "{{ api_urls | zip(api_resources) | list }}"

output.sh

$ ansible-playbook compose_variables.yml

PLAY [Compose variables and merge lists] *****************************************************************************************************************************************************************************************************

TASK [Compose the list of api_urls] **********************************************************************************************************************************************************************************************************
ok: [localhost] => (item=1)
ok: [localhost] => (item=2)
ok: [localhost] => (item=3)
ok: [localhost] => (item=5)

TASK [Show the list of api_urls] *************************************************************************************************************************************************************************************************************
ok: [localhost] => (item=https://api.example.com/v1) => 
  msg: https://api.example.com/v1
ok: [localhost] => (item=https://api.example.com/v2) => 
  msg: https://api.example.com/v2
ok: [localhost] => (item=https://api.example.com/v3) => 
  msg: https://api.example.com/v3
ok: [localhost] => (item=https://api.example.com/v5) => 
  msg: https://api.example.com/v5

TASK [Show the list of api_resources] ************************************************************************************************************************************************************************************************************
ok: [localhost] => (item=resource_num_1) => 
  msg: resource_num_1
ok: [localhost] => (item=resource_num_2) => 
  msg: resource_num_2
ok: [localhost] => (item=resource_num_3) => 
  msg: resource_num_3
ok: [localhost] => (item=resource_num_5) => 
  msg: resource_num_5

TASK [Show the WHAT resources get from WHERE] ************************************************************************************************************************************************************************************************
ok: [localhost] => (item=['https://api.example.com/v1', 'resource_num_1']) => 
  msg: 'GET: resource_num_1 FROM: https://api.example.com/v1'
ok: [localhost] => (item=['https://api.example.com/v2', 'resource_num_2']) => 
  msg: 'GET: resource_num_2 FROM: https://api.example.com/v2'
ok: [localhost] => (item=['https://api.example.com/v3', 'resource_num_3']) => 
  msg: 'GET: resource_num_3 FROM: https://api.example.com/v3'
ok: [localhost] => (item=['https://api.example.com/v5', 'resource_num_5']) => 
  msg: 'GET: resource_num_5 FROM: https://api.example.com/v5'

PLAY RECAP ***********************************************************************************************************************************************************************************************************************************
localhost                  : ok=4    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0