@classmethod
def ignore_defaults(cls, *args, **kwargs):
    """Create a config where unspecified params are automatically None.

        Use this to ignore environment variables and disable alt functions.

        You can wrap other configs:

            cfg = Config.ignore_defaults(Config.parse_file(...))
        """
    cfg = cls(*args, **kwargs)
    for k in (cls._props - cfg._params.keys()):
        setattr(cfg, k, None)
    return cfg
