title: Set Ansible variable to undefined through extra-vars or inventory variable
body: <p>So I have an Ansible playbook that looks like</p>
  
  <pre><code>---
  - hosts: mygroup
    tasks:
      - debug:
          msg: "{{ foo | default(inventory_hostname) }}"
  </code></pre>
  
  <p>My inventory file looks like</p>
  
  <pre><code>[mygroup]
  127.0.0.1
  </code></pre>
  
  <p>Since <code>foo</code> is not defined anywhere, the debug prints <code>127.0.0.1</code> as expected.</p>
  
  <p>But suppose my inventory file looks like</p>
  
  <pre><code>[mygroup]
  127.0.0.1 foo=null
  </code></pre>
  
  <p>When I run the playbook, it prints out the string <code>null</code>. I also tried with <code>foo=None</code> and it prints an empty string. How can set the variable to null through inventory or extra-vars?</p>
  
  <p>This may be useful when I want to unset a variable already defined in a playbook.</p>
  
  <p>I am using Ansible version 2.1.1.0.</p>