title: Automatically add key to Python dict
body: <p>I want to automatically add keys to a Python dictionary if they don't exist already.
  For example,</p>
  
  <pre><code>a = "a"
  b = "b"
  c = "c"
  
  dict = {}
  dict[a][b] = c # doesn't work because dict[a] doesn't exist
  </code></pre>
  
  <p>How do I automatically create keys if they don't exist?</p>