How to use the buku.DELIM function in buku

@pytest.mark.parametrize(
    'keywords, exp_res',
    [
        (None, None),
        ([], None),
        (['tag1', 'tag2'], ',tag1 tag2,'),
        (['tag1,tag2', 'tag3'], ',tag1,tag2 tag3,'),
    ]
)
def test_parse_tags(keywords, exp_res):
    """test func."""
    import buku
    if keywords is None:
        pass
    elif not keywords:
        exp_res = buku.DELIM
    res = buku.parse_tags(keywords)
    assert res == exp_res

with mock.patch('buku.get_system_editor', return_value=editor), \
            mock.patch('buku.is_editor_valid', return_value=is_editor_valid_retval), \
            mock.patch('buku.edit_rec', return_value=edit_rec_retval) as m_edit_rec:
        import buku
        buku.edit_at_prompt(obj, nav)
        # test
        if nav == 'w' and not is_editor_valid_retval:
            return
        if nav == 'w':
            m_edit_rec.assert_called_once_with(editor, '', None, buku.DELIM, None)
        elif buku.is_int(nav[2:]):
            obj.edit_update_rec.assert_called_once_with(int(nav[2:]))
            return
        else:
            editor = nav[2:]
        m_edit_rec.assert_called_once_with(editor, '', None, buku.DELIM, None)
        if edit_rec_retval is not None:
            obj.add_rec(*edit_rec_retval)