How to fix "false" being evaluated to True in Ansible (Jinja2 templates)

Question

How to make sure Jinja2 templates used within Ansible plays / roles are prone to "false" being evaluated to True?

---

Background:

Boolean handling in Ansible is tricky and may lead to an unexpected results when used along with Jinja2 templating.

Turns out that Ansible handles differently "false" value:

• When used in when clause it evaluates to False
• When used in Jinja2 template it evaluates to True

Take a look at this simple example:

---
- name: Sample play
  hosts: localhost
  gather_facts: false
  vars:
    enabled: 'false'
  tasks:
    - name: Print enabled / disabled with Jinja2 inline condition
      debug:
        msg: "{{ 'enabled' if enabled else 'disabled' }}"
    - name: Print enabled
      debug:
        msg: Enabled
      when: enabled

First task prints enabled even though variable is 'false', the second task skips:

PLAY [Sample play] *********************************************************************************************************************************************************************

TASK [Print enabled / disabled with Jinja2 inline condition] ***************************************************************************************************************************
ok: [localhost] => {
    "msg": "enabled"
}

TASK [Print enabled] *******************************************************************************************************************************************************************
skipping: [localhost]

Answer 1

There are two approaches you can use to make sure "false" is evaluated to False in Jinja2 templates:

1. Use bool filter

Simply add bool filter to every Jinja2 condition when a variable of boolean type is being used:

---
- hosts: localhost
  gather_facts: false
  tasks:
    - debug:
        msg: "{{ 'enabled' if item | bool else 'disabled' }}"
      with_items:
        - "true"
        - "false"
        - true
        - false

This results with:

TASK [debug] ***************************************************************************************************************************************************************************
ok: [localhost] => (item=true) => {
    "msg": "enabled"
}
ok: [localhost] => (item=false) => {
    "msg": "disabled"
}
ok: [localhost] => (item=True) => {
    "msg": "enabled"
}
ok: [localhost] => (item=False) => {
    "msg": "disabled"
}

Or in a template file:

{% if enabled | bool %}
# ...
{% endif %}

2. Use assertion

You can also enforce that variable value is a boolean and not a string with an assertion:

- assert:
  that:
    - enabled == enabled | bool

    # or you may use `type_debug` filter
    - enable | type_debug == 'bool'

Simply add this at the beginning of your Ansible role / playbook. It will fail if the variable is not a true or false.

For this play:

---
- hosts: localhost
  gather_facts: false
  tasks:
    - assert:
        that:
          - item == item | bool
      with_items:
        - "true"
        - "false"
        - true
        - false

Only true and false pass, "true" and "false" is rejected:

TASK [assert] **************************************************************************************************************************************************************************
failed: [localhost] (item=true) => {
    "assertion": "item == item | bool", 
    "changed": false, 
    "evaluated_to": false, 
    "item": "true"
}
failed: [localhost] (item=false) => {
    "assertion": "item == item | bool", 
    "changed": false, 
    "evaluated_to": false, 
    "item": "false"
}
ok: [localhost] => (item=True) => {
    "changed": false, 
    "item": true, 
    "msg": "All assertions passed"
}
ok: [localhost] => (item=False) => {
    "changed": false, 
    "item": false, 
    "msg": "All assertions passed"
}