title: Set nested dict value and create intermediate keys
body: <p>I feel like I saw a way to do this recently. Say I've got an empty dict and I want to set a value in a nested dict inside that empty dict, but obviously that nested dict hasn't been created yet. Is there a 1-line way to create the intermediate keys? This is what I want to do:</p>
  
  <pre><code>mydict = {}
  mydict['foo']['bar']['foobar'] = 25
  </code></pre>
  
  <p>If you execute this code you'll get a KeyError exception for 'foo'. Is there a function to create intermediate keys?</p>
  
  <p>Thanks.</p>