def METHOD_NAME(mocker, bq_client_mock):
    bq_client_mock.__class__ = Client 
    mocker.patch.object(bq_client_mock, "get_dataset", side_effect=Dataset)
    query = Query(bq_client_mock)
    result = query.dataset_exist(DatasetReference("test_unittest","test_unittest"))
    assert result[0] == True and isinstance(result[1], Dataset)