Meant to be used as a context manager.

def global_instance(cls, cmdline_args, allow_override=False):
        """
        Meant to be used as a context manager.
        """
        orig_value = cls._instance
        assert (orig_value is None) or allow_override
        new_value = None
        try:
            new_value = CmdlineParser(cmdline_args)
            cls._instance = new_value
            yield new_value
        finally:
            assert cls._instance is new_value
            cls._instance = orig_value