def test_decorateNestedTestSuite(self):
        """
        Calling L{decorate} on a test suite with nested suites will return a
        test suite that maintains the same structure, but with all tests
        decorated.
        """
        test = self.TestCase()
        suite = unittest.TestSuite([unittest.TestSuite([test])])
        decoratedTest = unittest.decorate(suite, unittest.TestDecorator)
        expected = unittest.TestSuite(
            [unittest.TestSuite([unittest.TestDecorator(test)])])
        self.assertSuitesEqual(decoratedTest, expected)

        Calling L{decorate} on a test suite with nested suites will return a
        test suite that maintains the same structure, but with all tests
        decorated.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_tests.py