def test_clearSuite(self):
        """
        Calling L{_clearSuite} on a populated L{TestSuite} removes
        all tests.
        """
        suite = unittest.TestSuite()
        suite.addTest(self.TestCase())
        # Double check that the test suite actually has something in it.
        self.assertEqual(1, suite.countTestCases())
        _clearSuite(suite)
        self.assertEqual(0, suite.countTestCases())

        Calling L{_clearSuite} on a populated L{TestSuite} removes
        all tests.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_tests.py