code: def test_apply_return_series(self):
      # SPARK-36907: Fix DataFrameGroupBy.apply without shortcut.
      pdf = pd.DataFrame(
          {
              "a": [1, 2, 3, 4, 5, 6],
              "b": [1, 1, 2, 3, 5, 8],
              "c": [1, 4, 9, 16, 25, 36],
          },
          columns=["a", "b", "c"],
      )
      psdf = ps.from_pandas(pdf)
  
      self.assert_eq(
          psdf.groupby("b").apply(lambda x: x.iloc[0]).sort_index(),
          pdf.groupby("b").apply(lambda x: x.iloc[0]).sort_index(),
      )
      self.assert_eq(
          psdf.groupby("b").apply(lambda x: x["a"]).sort_index(),
          pdf.groupby("b").apply(lambda x: x["a"]).sort_index(),
      )
      self.assert_eq(
          psdf.groupby(["b", "c"])
          .apply(lambda x: x.iloc[0])
          .sort_index(),
          pdf.groupby(["b", "c"])
          .apply(lambda x: x.iloc[0])
          .sort_index(),
      )
      self.assert_eq(
          psdf.groupby(["b", "c"]).apply(lambda x: x["a"]).sort_index(),
          pdf.groupby(["b", "c"]).apply(lambda x: x["a"]).sort_index(),
      )
  
      # multi-index columns
      columns = pd.MultiIndex.from_tuples(
          [("x", "a"), ("x", "b"), ("y", "c")]
      )
      pdf.columns = columns
      psdf.columns = columns
  
      self.assert_eq(
          psdf.groupby(("x", "b"))
          .apply(lambda x: x.iloc[0])
          .sort_index(),
          pdf.groupby(("x", "b"))
          .apply(lambda x: x.iloc[0])
          .sort_index(),
      )
      self.assert_eq(
          psdf.groupby(("x", "b"))
          .apply(lambda x: x[("x", "a")])
          .sort_index(),
          pdf.groupby(("x", "b"))
          .apply(lambda x: x[("x", "a")])
          .sort_index(),
      )
      self.assert_eq(
          psdf.groupby([("x", "b"), ("y", "c")])
          .apply(lambda x: x.iloc[0])
          .sort_index(),
          pdf.groupby([("x", "b"), ("y", "c")])
          .apply(lambda x: x.iloc[0])
          .sort_index(),
      )
      self.assert_eq(
          psdf.groupby([("x", "b"), ("y", "c")])
          .apply(lambda x: x[("x", "a")])
          .sort_index(),
          pdf.groupby([("x", "b"), ("y", "c")])
          .apply(lambda x: x[("x", "a")])
          .sort_index(),
      )
  
imports: import unittest
  import numpy
  import pandas
  from pyspark import pandas
  from pyspark.pandas.config import option_context
  from pyspark.testing.pandasutils import ComparisonTestBase
  from pyspark.testing.sqlutils import SQLTestUtils
label_desc: Use image node_id 10 for calling the GroupbyApplyFuncMixin obj's underlying member method code with example usage: obj.test_apply_return_series() without return types