title: Iterating over dictionaries using 'for' loops
body: <pre><code>d = {'x': 1, 'y': 2, 'z': 3}
  
  for key in d:
      print(key, 'corresponds to', d[key])
  </code></pre>
  <p>How does Python recognize that it needs only to read the <code>key</code> from the dictionary? Is <code>key</code> a special keyword, or is it simply a variable?</p>