code: def __call__(self, groupby):
      indexed = groupby.op.build_mock_groupby()[self.selection]
  
      if indexed.ndim == 1:
          self.output_types = [OutputType.series_groupby]
          params = dict(
              shape=(groupby.shape[0],),
              name=self.selection,
              dtype=groupby.dtypes[self.selection],
              index_value=groupby.index_value,
              key_dtypes=groupby.key_dtypes,
          )
      else:
          self.output_types = [OutputType.dataframe_groupby]
  
          if isinstance(self.selection, Iterable) and not isinstance(
              self.selection, str
          ):
              item_list = list(self.selection)
          else:
              item_list = [self.selection]
  
          params = groupby.params.copy()
          params["dtypes"] = new_dtypes = groupby.dtypes[item_list]
          params["selection"] = self.selection
          params["shape"] = (groupby.shape[0], len(item_list))
          params["columns_value"] = parse_index(
              new_dtypes.index, store_data=True
          )
  
      return self.new_tileable([groupby], **params)
  
file_path: mars/mars/dataframe/groupby/getitem.py
imports: from collections.abc import Iterable
  from ...None import opcodes
  from ...core import OutputType
  from ...serialization.serializables import AnyField
  from ..operands import DataFrameOperandMixin, DataFrameOperand
  from ..utils import parse_index
label_desc: Use image node_id 5 for calling the GroupByIndex obj's underlying member method code with example usage: obj.__call__(groupby) and returns: self