def get_knob_defaults(cls):
        """ Returns a string with defaults
        >>> Knob.get_knob_defaults()
        '# \n# HAVE_RUM=True\n\n# Yar\n# JOLLY_ROGER_PIRATES=124\n\n# Foo Bar\n# WUNDER=BAR\n'
        """

        return '\n'.join(
            [
                '# {description}\n# {knob}={default}\n'.format(
                    description=cls.get_registered_knob(name).description,
                    knob=name,
                    default=cls.get_registered_knob(name).default
                ) for name in sorted(cls._register.keys())
            ]
        )