def __call__(self):
        try:
            x = eval(self.arg, globals(), self._dict)

            if isinstance(x, _GetColumns):
                return x.cls
            else:
                return x
        except __HOLE__ as n:
            raise exc.InvalidRequestError(
                "When initializing mapper %s, expression %r failed to "
                "locate a name (%r). If this is a class name, consider "
                "adding this relationship() to the %r class after "
                "both dependent classes have been defined." %
                (self.prop.parent, self.arg, n.args[0], self.cls)
            )
dataset/ETHPy150Open RoseOu/flasky/venv/lib/python2.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py/_class_resolver.__call__