code: def transform(self, arg, *args, **kwargs):
          
          return self._selected_obj.groupby(self.groupby).transform(
              arg, *args, **kwargs)
docs: Call function producing a like-indexed Series on each group and return
          a Series with the transformed values.
  
          Parameters
          ----------
          arg : function
              To apply to each group. Should return a Series with the same index.
  
          Returns
          -------
          transformed : Series
  
          Examples
          --------
          >>> resampled.transform(lambda x: (x - x.mean()) / x.std())