title: How can i run ansible command if certain file changed
body: <p>I am using <code>ansible</code> to deploy my <code>django App</code></p>
  
  <p>using</p>
  
  <pre><code>- name: Upgrade the virtualenv.
    pip: requirements={{project_root}}/www/requirements.txt virtualenv={{project_root}}/www/virtualenv
  </code></pre>
  
  <p>But i only want to run that if requirements.txt changed since last run</p>