def METHOD_NAME():
    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],
            }
        )
    )