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
plugin.name,
            )
def initialize_extra_operators_links_plugins():
    """Creates modules for loaded extension from extra operators links plugins"""
    global global_operator_extra_links
    global operator_extra_links
    global registered_operator_link_classes
    if (
        global_operator_extra_links is not None
        and operator_extra_links is not None
        and registered_operator_link_classes is not None
    ):
        return
    ensure_plugins_loaded()
    if plugins is None:
        raise AirflowPluginException("Can't load plugins.")
    log.debug("Initialize extra operators links plugins")
    global_operator_extra_links = []
    operator_extra_links = []
    registered_operator_link_classes = {}
    for plugin in plugins:
        global_operator_extra_links.extend(plugin.global_operator_extra_links)
        operator_extra_links.extend(list(plugin.operator_extra_links))
        registered_operator_link_classes.update(
            {
                f"{link.__class__.__module__}.{link.__class__.__name__}": link.__class__
                for link in plugin.operator_extra_links
            }
        )
def integrate_executor_plugins() -> None: