code: def fit(self, X, y):
          
          self._word_vocab.add_documents(X)
          self._label_vocab.add_documents(y)
          if self._use_char:
              for doc in X:
                  self._char_vocab.add_documents(doc)
  
          self._word_vocab.build()
          self._char_vocab.build()
          self._label_vocab.build()
  
          return self
docs: Learn vocabulary from training set.
  
          Args:
              X : iterable. An iterable which yields either str, unicode or file objects.
  
          Returns:
              self : IndexTransformer.