code: def test_flat_should_not_droplevel():
      assert pp.flatten(timeseries_df, drop_levels=(0,)).equals(
          pd.DataFrame(
              {
                  "index": pd.to_datetime(
                      [
                          "2019-01-01",
                          "2019-01-02",
                          "2019-01-05",
                          "2019-01-07",
                      ]
                  ),
                  "label": ["x", "y", "z", "q"],
                  "y": [1.0, 2.0, 3.0, 4.0],
              }
          )
      )
  
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 6 for calling a global function with example usage: test_flat_should_not_droplevel() without return types