title: Trying to use text classification and receiving error code
body: <p>Code inputted:</p>
  
  <pre><code>from sklearn.feature_extraction.text import CountVectorizer
  count_vect = CountVectorizer()
  X_train_counts = count_vect.fit_transform(twenty_train.data)
  X_train_counts.shape
  
  from sklearn.feature_extraction.text import TfidfTransformer
  tfidf_transformer = TfidfTransformer()
  X_train_tfidf = tfidf_transformer.fit_transform(X_train_counts)
  X_train_tfidf.shape
  </code></pre>
  
  <p>Error message:</p>
  
  <pre class="lang-none prettyprint-override"><code>NameError: name 'tfidf_transformer' is not defined
  </code></pre>
  
  <p>I am very new to this so please bear with me.  </p>
  
  <p>Why am I getting this error message? I have defined <code>tfidf_transformer</code> haven't I?</p>