def test_isChecker(self):
        """
        Verifies that strcred.makeChecker('unix') returns an object
        that implements the L{ICredentialsChecker} interface.
        """
        self.assertTrue(checkers.ICredentialsChecker.providedBy(self.checker))
        self.assertIn(credentials.IUsernamePassword,
                      self.checker.credentialInterfaces)
        self.assertTrue(checkers.ICredentialsChecker.providedBy(
                        self.checkerBytes))
        self.assertIn(credentials.IUsernamePassword,
                      self.checkerBytes.credentialInterfaces)

        Verifies that strcred.makeChecker('unix') returns an object
        that implements the L{ICredentialsChecker} interface.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/test/test_strcred.py