code: def _set_key_fn(self, on: str):
      self._key_fn = on
  
imports: import math
  from typing import TYPE_CHECKING, Callable, List, Optional, Union
  from ray.data._internal.null_aggregate import _null_wrap_accumulate_block, _null_wrap_accumulate_row, _null_wrap_finalize, _null_wrap_init, _null_wrap_merge
  from ray.data._internal.sort import SortKey
  from ray.data.block import AggType, Block, BlockAccessor, KeyType, T, U
  from ray.util.annotations import PublicAPI
label_desc: Use image node_id 1 for calling the _AggregateOnKeyBase obj's underlying member method code with example usage: obj._set_key_fn(on) without return types