Python behavioral patterns - observer

observer.py

from weakref import WeakSet


class Subject:
    def __init__(self):
        self.observers = WeakSet()
        self.subject_state = None

    def attach(self, observer):
        observer.subject = self
        self.observers.add(observer)

    def detach(self, observer):
        observer.subject = None
        self.observers.discard(observer)

    def notify(self):
        for observer in self.observers:
            observer.update(self.subject_state)


class Observer:
    def __init__(self):
        self.observer_state = None
        self.subject = None

    def update(self, state):
        self.observer_state = state


subject = Subject()
observers = [Observer() for _ in range(3)]  # create 3 observers
[subject.attach(observer) for observer in observers]

print([observer.observer_state for observer in observers])  # [None, None, None]
subject.subject_state = "Game On"
subject.notify()
print([observer.observer_state for observer in observers])  # ['Game On', 'Game On', 'Game On']