def test_helpAuthLimitsOutput(self):
        """
        C{--help-auth} will only list checkers that purport to
        supply at least one of the credential interfaces our
        application can use.
        """
        options = OptionsForUsernamePassword()
        for factory in options._checkerFactoriesForOptHelpAuth():
            invalid = True
            for interface in factory.credentialInterfaces:
                if options.supportsInterface(interface):
                    invalid = False
            if invalid:
                raise strcred.UnsupportedInterfaces()

        C{--help-auth} will only list checkers that purport to
        supply at least one of the credential interfaces our
        application can use.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/test/test_strcred.py