vars_prompt defaults not evaluating correctly with bool filter

[Aaron Mills]

Greetings,

In one of my playbooks, I prompt for a value that, if specified as a string, will later be used in another play. However, I can't seem to find a consistent way to evaluate the truthiness/falsiness of the var. Here's a sample playbook:

---

- name: Test playbook.

  hosts: localhost

  gather_facts: False

  vars_prompt:

    - name: "test_var"

      prompt: "set a test var string"

      private: no

      default: ""

  tasks:
    # prints when test_var == ""
    # doesn't print when test_var == "a string"

    - debug: msg="I work as expected"

      when: not test_var

    # Doesn't run as expected per http://docs.ansible.com/playbooks_variables.html

    - debug: msg="I will never run"

      when: test_var | bool

    - debug: msg="I will never run either"

      when: "{{ test_var | bool }}"

    # I will fail if you don't enter a string

    - debug: msg="test_var is Truthy (no bool)"

      when: test_var

I think I understand the last task is failing because I'm not explicitly testing "test_var". However, I don't understand why the two bool tests consistently get skipped. My understanding is that the Jinja "when" syntax should pretty much follow Python logic, yes? e.g.:

>>> bool("")

False

>>> bool("a string")

True

Is there something I'm missing with regard to testing the truthiness/falsiness of a string? This is Ansible 1.8.2.

Many thanks,

-Aaron