undefined variable error in ansible when using jinja2 templates

Question

I am practising ansible. I have come all way through basic and now I am working on building a jinja2 template and using it. There is an exercise when I need to build a report for groups and upload upload them to their respective dns server. Reports for all the server in american group will be uploaded to dns_server_america, similarly for asia.

dns_server_america ansible_host=172.20.1.100 ansible_ssh_pass=Passw0rd ansible_user=root
dns_server_asia ansible_host=172.20.1.101 ansible_ssh_pass=Passw0rd ansible_user=root

[america]
web0001 ansible_hostname=web0001.company.com ansible_host=10.1.1.101 
web0002 ansible_hostname=web0002.company.com ansible_host=10.1.1.102 


[asia]
web2001 ansible_hostname=web2001.company.com ansible_host=10.1.1.201 
web2002 ansible_hostname=web2002.company.com ansible_host=10.1.1.202 

This is the YAML.

- name: Generate dns hosts files on americas servers
  hosts: dns_server_america
  tasks:
  - template: src=templates/hosts.j2 dest=/tmp/hosts.txt
    vars:
      GROUP_NAME: america

- name: Generate dns hosts files on asia servers
  hosts: dns_server_asia
  tasks:
  - template: src=templates/hosts.j2 dest=/tmp/hosts.txt
    vars:
      GROUP_NAME: asia

This is the jinja2 template.

{% for host in groups[GROUP_NAME] %}
{{ host }} {{ hostvars[host]['ansible_host'] }}
{% endfor %}

Why are we not quoting [host] and [GROUP_NAME] in the jinja2 template. Ansible says that when variables are put in square brackets, they are supposed to be wrapped within quotes. When I enclose then with quotes, I get an error message "undefined variable" and when I remove the quotes I am able to run the playbook successfully. Please advise, I may be missing something or my theory of understanding variables could be wrong.

Answer 1

Q: "Why are we not quoting [host] and [GROUP_NAME] in the jinja2 template?"

A: Both host and GROUP_NAME are variables. The value of the variables is needed in the index. If the names of the variables are quoted 'host' or 'GROUP_NAME' the names of the variables are used instead of the variable's values.

---

Direct and indirect

For example, the template

shell> cat test.txt.j2
{{ dict[index] }}
{{ dict['index'] }}

and the playbook

shell> cat playbook.yml
- hosts: localhost
  vars:
    dict:
      index: value of attribute index
      attr1: value of attribute attr1
  tasks:
    - template:
        src: test.txt.j2
        dest: test.txt
      vars:
        index: attr1

give

shell> cat test.txt
value of attribute attr1
value of attribute index

---

This is not limited to templates. It's generally valid. For example

    - debug:
        msg:
          - "{{ dict[index] }}"
          - "{{ dict['index'] }}"
      vars:
        index: attr1

gives

  msg:
  - value of attribute attr1
  - value of attribute index

---

Dotted

It's possible to use the "dotted" reference. For example

    - debug:
        var: dict.index

gives

  dict.index: value of attribute index

The "dotted" reference can be used in nested dictionaries. For example with the nested dictionary

    dict:
      index:
        var1: value of attribute index
      attr1:
        var1: value of attribute attr1

both versions work as expected

    - debug:
        msg:
          - "{{ dict.index.var1 }}"
          - "{{ dict['index'].var1 }}"
      vars:
        index: attr1

gives

  msg:
  - value of attribute index
  - value of attribute index

---

Dotted references in template

But there is a difference when a template is used. When a reference is put into the brackets [] all subsequent references must be put into the brackets as well. Otherwise, the template will fail. For example

shell> cat test.txt.j2
{{ dict.index.var1 }}        # OK
{{ dict.index['var1'] }}     # OK
{{ dict['index']['var1'] }}  # OK
{{ dict['index'].var1 }}     # WRONG: has no attribute var1

will fail

fatal: [localhost]: FAILED! => changed=false 
  msg: 'AnsibleUndefinedVariable:
        ''ansible.parsing.yaml.objects.AnsibleUnicode object''
        has no attribute ''var1'''