def METHOD_NAME(self):
    fs = StringIO('\n'.join(map('\t|\t'.join, [
        ('1', '2', 'family'),
        ('3', '4', 'genus'),
        ('x', '6', 'species'),  # 'x' is not a number
    ])))
    with self.assertRaises(ValueError) as ctx:
        _taxdump_to_data_frame(fs, scheme='nodes_slim')
    self.assertEqual(str(ctx.exception),
                     'Invalid taxdump file format.')