code: def column_names(self):
          
          if self._is_vertex_frame():
              return self.__graph__.__proxy__.get_vertex_fields()
          elif self._is_edge_frame():
              return self.__graph__.__proxy__.get_edge_fields()
docs: Returns the column names.
  
          Returns
          -------
          out : list[string]
              Column names of the SFrame.