code: def test_compare_multi_index_column():
      index = pd.to_datetime(["2021-01-01", "2021-01-02", "2021-01-03"])
      index.name = "__timestamp"
      iterables = [["m1", "m2"], ["a", "b"], ["x", "y"]]
      columns = pd.MultiIndex.from_product(
          iterables, names=[None, "level1", "level2"]
      )
      df = pd.DataFrame(index=index, columns=columns, data=1)
      """
                  m1          m2
      level1       a     b     a     b
      level2       x  y  x  y  x  y  x  y
      __timestamp
      2021-01-01   1  1  1  1  1  1  1  1
      2021-01-02   1  1  1  1  1  1  1  1
      2021-01-03   1  1  1  1  1  1  1  1
      """
      post_df = pp.compare(
          df,
          source_columns=["m1"],
          compare_columns=["m2"],
          compare_type=PPC.DIFF,
          drop_original_columns=True,
      )
      flat_df = pp.flatten(post_df)
      """
        __timestamp  difference__m1__m2, a, x  difference__m1__m2, a, y  difference__m1__m2, b, x  difference__m1__m2, b, y
      0  2021-01-01                         0                         0                         0                         0
      1  2021-01-02                         0                         0                         0                         0
      2  2021-01-03                         0                         0                         0                         0
      """
      assert flat_df.equals(
          pd.DataFrame(
              data={
                  "__timestamp": pd.to_datetime(
                      ["2021-01-01", "2021-01-02", "2021-01-03"]
                  ),
                  "difference__m1__m2, a, x": [0, 0, 0],
                  "difference__m1__m2, a, y": [0, 0, 0],
                  "difference__m1__m2, b, x": [0, 0, 0],
                  "difference__m1__m2, b, y": [0, 0, 0],
              }
          )
      )
  
file_path: superset/tests/unit_tests/pandas_postprocessing/test_compare.py
imports: import pandas
  from superset.constants import PandasPostprocessingCompare
  from superset.utils import pandas_postprocessing
  from superset.utils.pandas_postprocessing.utils import FLAT_COLUMN_SEPARATOR
  from tests.unit_tests.fixtures.dataframes import multiple_metrics_df, timeseries_df2
label_desc: Use image node_id 5 for calling a global function with example usage: test_compare_multi_index_column() without return types