def METHOD_NAME():
    index = pd.to_datetime(["2021-01-01", "2021-01-02", "2021-01-03"])
    index.name = "__timestamp"
    columns = pd.MultiIndex.from_arrays(
        [
            ["level1,value1", "level1,value2", "level1,value3"],
            ["level2, value1", "level2, value2", "level2, value3"],
        ],
        names=["level1", "level2"],
    )
    df = pd.DataFrame(index=index, columns=columns, data=1)
    assert list(pp.flatten(df).columns.values) == [
        "__timestamp",
        "level1\\,value1" + FLAT_COLUMN_SEPARATOR + "level2\\, value1",
        "level1\\,value2" + FLAT_COLUMN_SEPARATOR + "level2\\, value2",
        "level1\\,value3" + FLAT_COLUMN_SEPARATOR + "level2\\, value3",
    ]