title: How to update a python dict with default key:value pairs given in another dict
body: <p>Suppose a dictionary is expected to have certain keys present. Is there a simple way of adding these specific keys with default values if they are missing?</p>
  
  <p>For example:</p>
  
  <pre><code>default_dict = {'name': '', 'height': 100, 'age': 20} 
  
  d = {'name': 'James', 'age': 65}
  d.set_defaults(default_dict)
  </code></pre>
  
  <p>would update the dictionary <code>d</code> to</p>
  
  <pre><code>{'name': 'James', 'age': 65, 'height': 100}
  </code></pre>
  
  <p>where the original values of <code>d</code> are kept and only the missing keys are added.</p>
  
  <p>The <code>default_dict</code> should not be destroyed in the process.</p>