def test_co_group_by_key_on_list(self):
    pipeline = TestPipeline()
    pcoll_1 = pipeline | 'Start 1' >> beam.Create(
        [('a', 1), ('a', 2), ('b', 3), ('c', 4)])
    pcoll_2 = pipeline | 'Start 2' >> beam.Create(
        [('a', 5), ('a', 6), ('c', 7), ('c', 8)])
    result = (pcoll_1, pcoll_2) | beam.CoGroupByKey()
    assert_that(result, equal_to([('a', ([1, 2], [5, 6])),
                                  ('b', ([3], [])),
                                  ('c', ([4], [7, 8]))]))
    pipeline.run()