def test_decorateInPlaceMutatesOriginal(self):
        """
        Calling L{decorate} on a test suite will mutate the original suite.
        """
        test = self.TestCase()
        suite = unittest.TestSuite([test])
        decoratedTest = unittest.decorate(
            suite, unittest.TestDecorator)
        self.assertSuitesEqual(
            decoratedTest, unittest.TestSuite([unittest.TestDecorator(test)]))
        self.assertSuitesEqual(
            suite, unittest.TestSuite([unittest.TestDecorator(test)]))

        Calling L{decorate} on a test suite will mutate the original suite.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_tests.py