title: Nltk Sklearn Unigram + Bigram
body: <p>I'm building classificator using NLTK and nltk.sklearn wrapper.</p>
  
  <pre><code>classifier = SklearnClassifier(LinearSVC(), int,True)
  classifier.train(train_set)
  </code></pre>
  
  <p>When I was using only unigrams and build featureset for example:</p>
  
  <pre><code>{"Cristiano" : True, "Ronaldo : True}
  </code></pre>
  
  <p>evertyhing was fine. But when I want to use collocations there is a problem. Featureset looks different:</p>
  
  <pre><code>{ {"Cristiano" : True, "Ronaldo : True, ("Cristiano", "Ronaldo") : True }
  </code></pre>
  
  <p>Then I receive error:</p>
  
  <pre><code>feature_names.sort()TypeError: unorderable types: tuple() &lt; str()
  </code></pre>
  
  <p>How to create feature set properly for nltk sklearn wrapper using both unigrams and bigrams ? </p>