i.e. has an impl.
        """
        return key in self and self[key].impl is not None
    def _subclass_manager(self, cls: Type[_T]) -> ClassManager[_T]:
        """Create a new ClassManager for a subclass of this ClassManager's
        class.
        This is called automatically when attributes are instrumented so that
        the attributes can be propagated to subclasses against their own
        class-local manager, without the need for mappers etc. to have already
        pre-configured managers for the full class hierarchy.   Mappers
        can post-configure the auto-generated ClassManager when needed.
        """
        return register_class(cls, finalize=False)
    def _instrument_init(self):
        self.new_init = _generate_init(self.class_, self, self.original_init)
        self.install_member("__init__", self.new_init)
    @util.memoized_property
    def _state_constructor(self) -> Type[state.InstanceState[_O]]:
        self.dispatch.first_init(self, self.class_)
        return state.InstanceState
    def manage(self):
        """Mark this instance as the manager for its class."""
        setattr(self.class_, self.MANAGER_ATTR, self)
    @util.hybridmethod
    def manager_getter(self):
        return _default_manager_getter
    @util.hybridmethod
    def state_getter(self):
        """Return a (instance) -> InstanceState callable.
        "state getter" callables should raise either KeyError or
        AttributeError if no InstanceState could be found for the
        instance.
        """
        return _default_state_getter
    @util.hybridmethod
    def dict_getter(self):
        return _default_dict_getter
    def instrument_attribute(
        self,
        key: str,