code: def test_flat_should_flat_multiple_index():
      index = pd.to_datetime(["2021-01-01", "2021-01-02", "2021-01-03"])
      index.name = "__timestamp"
      iterables = [["foo", "bar"], [1, "two"]]
      columns = pd.MultiIndex.from_product(
          iterables, names=["level1", "level2"]
      )
      df = pd.DataFrame(index=index, columns=columns, data=1)
  
      assert pp.flatten(df).equals(
          pd.DataFrame(
              {
                  "__timestamp": index,
                  FLAT_COLUMN_SEPARATOR.join(["foo", "1"]): [1, 1, 1],
                  FLAT_COLUMN_SEPARATOR.join(["foo", "two"]): [1, 1, 1],
                  FLAT_COLUMN_SEPARATOR.join(["bar", "1"]): [1, 1, 1],
                  FLAT_COLUMN_SEPARATOR.join(["bar", "two"]): [1, 1, 1],
              }
          )
      )
  
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 4 for calling a global function with example usage: test_flat_should_flat_multiple_index() without return types