def test_iterateIsLeftToRightDepthFirst(self):
        """
        L{_iterateTests} returns tests in left-to-right, depth-first order.
        """
        test = self.TestCase()
        suite = runner.TestSuite([runner.TestSuite([test]), self])
        self.assertEqual([test, self], list(_iterateTests(suite)))

        L{_iterateTests} returns tests in left-to-right, depth-first order.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_tests.py