code: def sum(df: DataFrame) -> "pd.Series":
      data_manager = df.data_manager
      operable_blocks = data_manager.infer_operable_blocks()
  
      def _mapper(blocks, op_bids):
          ret = []
          for bid in op_bids:
              ret.append(blocks[bid].sum(axis=0))
  
          return ret
  
      def _reducer(blocks1, blocks2):
          return [
              block1 + block2
              for block1, block2 in zip(blocks1, blocks2)
          ]
  
      mapper_func = functools.partial(_mapper, op_bids=operable_blocks)
  
      reduce_ret = df.block_table.mapValues(mapper_func).reduce(
          _reducer
      )
      return _post_process(reduce_ret, operable_blocks, data_manager)
  
file_path: FATE/python/fate/arch/dataframe/ops/_stat.py
imports: import functools
  import numpy
  import pandas
  import torch
  from .._dataframe import DataFrame
  from ..manager import DataManager
label_desc: Use image node_id 3 for calling a global function with example usage: sum(df) and returns: _post_process, ret, unknown