Ansible playbook condition fails when variable has a default value

Question

Given the following playbook (deployment.yml):

---
- name: Debug
  hosts: applicationservers
  tasks:
  - debug: msg="{{add_host_entries | default('false')}}"
  - debug: msg="{{add_host_entries | default('false') == 'true'}}"
  - debug: msg="Add host entries = {{add_host_entries | default('false') == 'true'}}"

- include: add_host_entries.yml
  when: add_host_entries | default('false') == 'true'

The condition to include add_host_entries.yml always fails, even if all of the above debug messages print some sort of true (I know that in the first debug message it's a String, whereas the other two result in Booleans).

When I omit the part with the default value, add_host_entries.yml will be executed:

  when: add_host_entries

I need this default value behaviour though, because it's an optional value which is only set on certain stages.

Other Attempts (without success)

Brackets

  when: (add_host_entries | default('false')) == 'true'

Casting to boolean

  when: add_host_entries|default('false')|bool

Other Sources and Information

Here are all the resources needed to reproduce the problem.

add_host_entries.yml

---
- name: add_host_entries
  hosts: applicationservers
  gather_facts: false
  tasks:
    - debug: msg="Add Host Entries"

inventory

[applicationservers]
127.0.0.1

[all:vars]
add_host_entries=true

Call

markus@lubuntu:~/foobar$ ansible-playbook deployment.yml -i inventory

Versions

markus@lubuntu:~/foobar$ ansible --version
ansible 2.1.1.0
  config file = /etc/ansible/ansible.cfg
  configured module search path = Default w/o overrides

markus@lubuntu:~/foobar$ ansible-playbook --version
ansible-playbook 2.1.1.0
  config file = /etc/ansible/ansible.cfg
  configured module search path = Default w/o overrides

Answer 1

You try to conditionally include playbook. See my other answer about different include types.

The thing is, this only works when variable is defined before Ansible parses your playbook.
But you try to define add_host_entries as host-level fact (group variable) – these variables are not yet defined during parse time.

If you call your playbook with -e add_host_entries=true your condition will work as expected, because extra-vars are known during parse time.

Answer 2

Use bool to convert the string value of add_host_entries into a boolean and then the condition will work.

---
- name: Debug
  hosts: applicationservers
  tasks:
  - debug: msg="{{add_host_entries | default('false')}}"
  - debug: msg="{{add_host_entries | default('false') == 'true'}}"
  - debug: msg="Add host entries = {{add_host_entries | default('false') == 'true'}}"

- include: add_host_entries.yml
  when: add_host_entries | default('false') | bool