Question: what is proper way to define python function as event_trigger?

Hello. 

Problem: 

1. any attempt to define python function as an event_trigger, i.e.: 

CREATE FUNCTION public.verify()
RETURNS event_trigger
LANGUAGE 'plpython3u'
AS $$ print("hello-kitty") $$; 

2. fails with message: 

ERROR: trigger functions can only be called as triggers 

SQL state: 0A000 

3. here in the source: 

https://github.com/postgres/postgres/blob/master/src/pl/plpython/plpy_procedure.c#L226


Question: 

what is proper way to define python function as event_trigger? 

Thank you.