In-App Trigger Event

I am looking for a way to trigger an event on the application engine when creating a User. I have something like

def create_user(data):
    user = new User(data)
    user.put()
    trigger('user_created', user)
    return user

Thus, external modules can modify the object when creating a new user. I suppose I can add something like

add_hook('user_created', some_function)

for external modules (during application initialization). This function will add fields to an entity of type

def some_function(user):
    user.data = 'some value'

What would be the best way to do this in an application?