events.py

class EventDispatcher:
    def __init__(self):
        self._handlers = defaultdict(set)

    def handle(self, type, handler):
        self._handlers[type].add(handler)

    def send_event(self, event):
        for handler in self._handlers[type(event)]:
            handler(event)

main.py

from events import EventDispatcher
from users import UserCreated, insert_user

event_dispatcher = EventDispatcher()
event_dispatcher.handle(UserCreated, lambda id: print('created user {}'.format(id)))
event_dispatcher.handle(UserCreated, lambda id: send_password_to_user(id))

insert_user(event_dispatcher, db, User(...))

users.py

class UserCreated:
    def __init__(self, user_id):
        self.user_id = user_id


def insert_user(event_dispatcher, db, user):
    user_id = db.execute(...)
    event_dispatcher.send_event(UserCreated(user_id))
    return user_id