title: AttributeError: 'list' object has no attribute 'copy'
body: <p>I have the following code snippet </p>
  
  <pre><code>classifier = NaiveBayesClassifier.train(train_data)
  #classifier.show_most_informative_features(n=20)
  results = classifier.classify(test_data)
  </code></pre>
  
  <p>and the error shows in the following line</p>
  
  <pre><code>results = classifier.classify(test_data)
  </code></pre>
  
  <p>error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "trial_trial.py", line 46, in &lt;module&gt;
      results = classifier.classify(test_data)
    File "c:\Users\Amr\Anaconda\lib\site-packages\nltk\classify\naivebayes.py", line 88, in classify
      return self.prob_classify(featureset).max()
    File "c:\Users\Amr\Anaconda\lib\site-packages\nltk\classify\naivebayes.py", line 94, in prob_classify
      featureset = featureset.copy()
  AttributeError: 'list' object has no attribute 'copy'
  </code></pre>
  
  <p>I think of extending base class list in python and add copy function
  but I'm not expert in python and I don't know how to solve this problem.</p>