If you’re already in a Jinja statement or expression, you should not use a second set of delimiters to set off expressions.

Wrong:

{{ foo[{{ bar }}] }}
{{ hostvars['{{ 'myhost-' + {{ foo }}']['bar'] }}
{{ '{{ foo }}' if bar else 'baz' }}

{% set foo == "{{ bar }}foo" %}

Right:

{{ foo[bar] }}
{{ hostvars['myhost-' ~ foo]['bar'] }}
{{ foo if bar else 'baz' }}

{% set foo = bar ~ 'foo' %}

Conditionals

An Ansible-specific twist is that conditional statements are implicitly a Jinja context, so you rarely want to include Jinja delimiters in them.

- hosts: localhost
  vars:
    foo: bar
    bar: false
    baz: example.com
  tasks:
    - debug:
        msg: Heyla
      when: foo is truthy

    - debug:
        msg: La hey
      when: "{{ foo }}"

    - debug:
        msg: Wa hey
      when: "{{ baz }}"

TASK [debug] *******************************************************************
ok: [localhost] => 
    msg: Heyla

TASK [debug] *******************************************************************
skipping: [localhost]

TASK [debug] *******************************************************************
[ERROR]: Task failed: Error while evaluating conditional: 'example' is undefined

Task failed.
Origin: test.yml:15:7

13       when: "{{ foo }}"
14
15     - debug:
         ^ column 7

<<< caused by >>>

Error while evaluating conditional.
Origin: test.yml:17:13

15     - debug:
16         msg: Wa hey
17       when: "{{ baz }}"
               ^ column 13

<<< caused by >>>

'example' is undefined
Origin: test.yml:5:10

3     foo: bar
4     bar: false
5     baz: example.com
           ^ column 10

fatal: [localhost]: FAILED! => 
    msg: 'Task failed: Error while evaluating conditional: ''example'' is undefined'