code: def _get_list_key(self, obj, key, vc, obj_nex_id=None):
          
          k = obj.get(key)
          if k is None:
              return None
          if isinstance(k, dict):
              k = [k]
          if not isinstance(k, list):
              self._error_event(vc.curr_element_type,
                                obj=obj,
                                err_type=gen_MissingExpectedListWarning,
                                anc=vc.anc_list,
                                obj_nex_id=obj_nex_id,
                                key_list=[key, ])
              return None
          return k
docs: Either:
              * Returns a list, or
              * Generates a MissingExpectedListWarning and returns None (if
                   the value is not a dict or list)