def test_canAddSupportedChecker(self):
        """
        When addChecker is called with a checker that implements at least one
        of the interfaces our application supports, it is successful.
        """
        options = OptionsForUsernamePassword()
        options.addChecker(self.goodChecker)
        iface = options.supportedInterfaces[0]
        # Test that we did get IUsernamePassword
        self.assertIdentical(
            options['credInterfaces'][iface][0], self.goodChecker)
        self.assertIdentical(options['credCheckers'][0], self.goodChecker)
        # Test that we didn't get IUsernameHashedPassword
        self.assertEqual(len(options['credInterfaces'][iface]), 1)
        self.assertEqual(len(options['credCheckers']), 1)

        When addChecker is called with a checker that implements at least one
        of the interfaces our application supports, it is successful.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/test/test_strcred.py