title: Create vocabulary dictionary for text mining
body: <p>I have the following code:</p>
  
  <pre><code>train_set = ("The sky is blue.", "The sun is bright.")
  test_set = ("The sun in the sky is bright.",
      "We can see the shining sun, the bright sun.")
  </code></pre>
  
  <p>Now Im trying to calculate the word frequency like this:</p>
  
  <pre><code>    from sklearn.feature_extraction.text import CountVectorizer
      vectorizer = CountVectorizer()
  </code></pre>
  
  <p>Next I would like to print the voculabary. Therefore I do:</p>
  
  <pre><code>vectorizer.fit_transform(train_set)
  print vectorizer.vocabulary
  </code></pre>
  
  <p>Right now I get the ouput none. While I expect something like:</p>
  
  <pre><code>{'blue': 0, 'sun': 1, 'bright': 2, 'sky': 3}
  </code></pre>
  
  <p>Any thoughts where this goes wrong?</p>