Call the backend method with args.

        If there is currently no backend, return default.

def _call_backend(self, method, default, *args, **kwargs):
        """Call the backend method with args.

        If there is currently no backend, return default."""
        meth = getattr(self.backend, method, None)
        if meth is None:
            return default
        else:
            return meth(*args, **kwargs)