code: def _index2word(self):
          
          # TODO(kelvinguu): it would be nice to just use `dict.viewkeys`, but unfortunately those are not indexable
  
          compute_index2word = lambda: self.keys()  # this works because self is an OrderedDict
  
          # create if it doesn't exist
          try:
              self._index2word_cache
          except AttributeError:
              self._index2word_cache = compute_index2word()
  
          # update if it is out of date
          if len(self._index2word_cache) != len(self):
              self._index2word_cache = compute_index2word()
  
          return self._index2word_cache
docs: Mapping from indices to words.
  
          WARNING: this may go out-of-date, because it is a copy, not a view into the Vocab.
  
          :return: a list of strings