def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def get_bind(self, mapper=None, *args, **kwargs):
                    return super(MySession, self).get_bind(
                        mapper, *args, **kwargs
                    )