code: def _preprocess(self, data):
          
          transformed_data = _copy(data)
          for name, step in self._transformers[:-1]:
              transformed_data = step.fit_transform(transformed_data)
              if type(transformed_data) != _tc.SFrame:
                  raise RuntimeError("The transform function in step '%s' did not"
                      " return an SFrame (got %s instead)." % (name,
                                              type(transformed_data).__name__))
          return transformed_data
docs: Internal function to perform fit_transform() on all but last step.