code: def test_agg_one_grouper(df):
      res = GroupBy(["a"]).agg(df, {"y": "max"})
      assert_array_equal(res.index, [0, 1])
      assert_array_equal(res.columns, ["a", "y"])
      assert_array_equal(res["a"], ["a", "b"])
      assert_array_equal(res["y"], [0.8, 0.5])
  
file_path: seaborn/tests/_core/test_groupby.py
imports: import numpy
  import pandas
  import pytest
  from numpy.testing import assert_array_equal
  from seaborn._core.groupby import GroupBy
label_desc: Use image node_id 6 for calling a global function with example usage: test_agg_one_grouper(df) without return types