Currently (as implemented in #17414), timetables are serialised by their classes’ full import path. This works most of the time, but not in some cases, including:

* Nested in class or function
* Declared directly in a DAG file without a valid import name (e.g. `12345.py`)

It’s fundamentally impossible to fix some of the cases (e.g. function-local class declaration) due to how Python works, but by requiring the user to explicitly register the timetable class, we can at least expose that problem so users don’t attempt to do that.

However, since the timetable actually would work a lot of times without any additional mechanism, I’m also wondering if we should _require_ registration.

1. Always require registration. A DAG using an unregistered timetable class fails to serialise.
2. Only require registration when the timetable class has wonky import path. “Normal” classes work out of the box without registering, and user sees a serialisation error asking for registration otherwise.
3. Don’t require registration. If a class cannot be correctly serialised, tell the user we can’t do it and the timetable must be declared another way.
airflow/plugins_manager.py
"""Validates that plugin has a name."""
        if not cls.name:
            raise AirflowPluginException("Your plugin needs a name.")
    @classmethod
    def on_load(cls, *args, **kwargs):
        """
        Executed when the plugin is loaded.
        This method is only called once during runtime.
        :param args: If future arguments are passed in on call.
        :param kwargs: If future arguments are passed in on call.
        """
def is_valid_plugin(plugin_obj):
    """
    Check whether a potential object is a subclass of
    the AirflowPlugin class.
    :param plugin_obj: potential subclass of AirflowPlugin
    :return: Whether or not the obj is a valid subclass of
        AirflowPlugin
    """
    global plugins
    if (
        inspect.isclass(plugin_obj)
        and issubclass(plugin_obj, AirflowPlugin)
        and (plugin_obj is not AirflowPlugin)
    ):
        plugin_obj.validate()
        return plugin_obj not in plugins
    return False
def register_plugin(plugin_instance):