Observer pattern

Reference: エキスパートPythonプログラミング 改訂3版; 第17章 Pythonのためのデザインパターン

class Event:
    _observers = []

    def __init__(self, subject):
        self.subject = subject

    @classmethod
    def register(cls, observer):
        if observer not in cls._observers:
            cls._observers.append(observer)

    @classmethod
    def unregister(cls, observer):
        if observer in cls._observers:
            cls._observers.remove(observer)

    @classmethod
    def notify(cls, subject):
        event = cls(subject)
        for observer in cls._observers:
            observer(event)

class WriteEvent(Event):
    def __repr__(self):
        return 'WriteEvent'
    
def log(event):
    print(f'{event} was fired by the subject "{event.subject}"')
    
class AnotherObserver:
    def __call__(self, event):
        print(f'{event} called the action "{self.__class__.__name__}"')

WriteEvent.register(log)
WriteEvent.register(AnotherObserver())

WriteEvent.notify('Something happened')

WriteEvent was fired by the subject "Something happened"
WriteEvent called the action "AnotherObserver"