title: sklearn feature.Extraction 'DictVectorizer' object has no attribute 'feature_names_'
body: <p>Whenever I call <code>transform</code> it keeps giving me the following error :</p>
  
  <pre><code>AttributeError:'DictVectorizer' object has no attribute 'feature_names_'
  </code></pre>
  
  <p>This is the function call:</p>
  
  <pre><code>vec = DictVectorizer()
  x_test = vec.transform(X_features)
  </code></pre>
  
  <p>My python version is 2.7, <code>Scipy 0.16.0</code>, <code>numpy 1.9.2+mkl</code>, <code>scikit-learn 0.16.1</code>.</p>