def _subclass_manager(self, cls):
        manager = manager_of_class(cls)
        if manager is None:
            manager = _instrumentation_factory.create_manager_for_cls(cls)
        return manager
Create a new ClassManager for a subclass of this ClassManager's
dataset/ETHPy150Open RoseOu/flasky/venv/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py/ClassManager._subclass_manager