title: Attribute error in DictVectorizer in Scikit Learn
body: <p>I am trying the following code in Python scikit learn.</p>
  
  <pre><code>from sklearn.feature_extraction import DictVectorizer
  onehot_encoder = DictVectorizer()
  instances = [{'city': 'New York'}, {'city', 'Boston'}, {'city', 'London'}]
  onehot_encoder.fit_transform(instances)
  </code></pre>
  
  <p>However, I get the following error:</p>
  
  <blockquote>
    <p>'set' object has no attribute 'iteritems'</p>
  </blockquote>
  
  <p>Could you help me in understanding what this error is about and how to fix it?</p>