Test what vars evalute to true or false in Ansible 2.5

ansible_when_truthy_test.yml

#!/usr/bin/env ansible-playbook
---
- name: Test truthiness
  hosts: localhost
  gather_facts: False
  vars:
    truthy_vars:
    # TRUE
    - True
    - 1
    - "true"
    # FALSE
    - "false"
    - null
    - False
    - 0
    # ERROR (invalid sytax error stops the loop of the first of these)
    - "null"
    - "defined string"
    # ERROR
    # truthy_var_undefined
    # FALSE
    truthy_var_defined:
  tasks:
  - name: Test truthy
    debug:
      msg: "is truthy"
    ignore_errors: True # beware, the loo
    when: item
    loop: "{{ truthy_vars }}"
    loop_control:
      label: Test {{ item }}
  - name: truthy_var_undefined
    debug:
    when: truthy_var_undefined
    ignore_errors: true
  - name: truthy_var_defined
    debug:
    when: truthy_var_defined

- name: Test | default(False)
  hosts: localhost
  gather_facts: False
  vars:
    default_pipe_vars:
    # TRUE
    - True
    - 1
    - "true"
    # beware these:
    - "false"
    - "null"
    - "defined string" 
    # FALSE
    - null
    - False
    - 0
    # FALSE
    # default_pipe_undefined
    # FALSE
    default_pipe_defined:
  tasks:
  - name: Test | default(False)
    debug:
      msg: "is not | default(False)"
    when: item | default(False)
    loop: "{{ default_pipe_vars }}"
    loop_control:
      label: Test {{ item }}
  - name: default_pipe_undefined | default(False)
    debug:
    when: default_pipe_undefined | default(False)
  - name: default_pipe_defined | default(False)
    debug:
    when: default_pipe_defined | default(False)

- name: Test | bool
  hosts: localhost
  gather_facts: False
  vars:
    bool_vars:
    # TRUE
    - True
    - 1
    - "true"
    # FALSE
    - "defined string"
    - "null"
    - "false"
    - null
    - False
    - 0
    # ERROR
    # bool_undefined
    # FALSE
    bool_defined:
  tasks:
  - name: Test bool parsing
    debug:
      msg: "parsed as true booleans"
    when: item | bool
    loop: "{{ bool_vars }}"
    loop_control:
      label: Test {{ item }}
  - name: bool_undefined | bool
    debug:
    when: bool_undefined | bool
    ignore_errors: true
  - name: bool_defined var | bool
    debug:
    when: bool_defined | bool


- name: Test is defined and | bool
  hosts: localhost
  gather_facts: False
  vars:
    defined_bool_vars:
    # TRUE
    - True
    - 1
    - "true"
    # FALSE
    - "defined string"
    - "null"
    - "false"
    - null
    - False
    - 0
    # FALSE
    # defined_bool_undefined
    # FALSE
    defined_bool_defined:
  tasks:
  - name: Test bool parsing
    debug:
      msg: "parsed as true booleans"
    when:
    - item is defined
    - item | bool
    loop: "{{ defined_bool_vars }}"
    loop_control:
      label: Test {{ item }}
  - name: defined_bool_undefined | bool
    debug:
    when:
    - defined_bool_undefined is defined
    - defined_bool_undefined | bool
    ignore_errors: true
  - name: defined_bool_defined var | bool
    debug:
    when:
    - defined_bool_defined is defined
    - defined_bool_defined | bool