title: Check existence of YAML key
body: <p>I use PyYAML to work with YAML files.
  I wonder how I can properly check existence of some key? In the example below <code>title</code> key is present only for list1. I want to process title value properly if it exists, and ignore if it is not there.</p>
  
  <pre><code>list1:
      title: This is the title
      active: True
  list2:
      active: False
  </code></pre>