code: def test_flat_should_not_change():
      df = pd.DataFrame(
          data={
              "foo": [1, 2, 3],
              "bar": [4, 5, 6],
          }
      )
  
      assert pp.flatten(df).equals(df)
  
file_path: superset/tests/unit_tests/pandas_postprocessing/test_flatten.py
imports: import pandas
  from superset.utils import pandas_postprocessing
  from superset.utils.pandas_postprocessing.utils import FLAT_COLUMN_SEPARATOR
  from tests.unit_tests.fixtures.dataframes import timeseries_df
label_desc: Use image node_id 1 for calling a global function with example usage: test_flat_should_not_change() without return types