title: Python transpose explanation
body: <p>Playing around with Python dictionary comprehensions. Can you help me understand why this flips the key, value pairs around?  I'm not grokking why the value is also flipped.  (also it's 2am so.)  It's an interesting syntax.  </p>
  
  <pre><code>capitals = {'USA': 'Washington, DC','France':'Paris','Italy':'Rome' } ​
  
  capitals_flip = {capitals[key]: key for key in capitals}
  
  
  capitals_flip
  
  ​ {'Paris': 'France', 'Rome': 'Italy', 'Washington, DC': 'USA'}
  </code></pre>