如何在Python中实现观察者模式？

class Subject:
    def __init__(self):
        self._observers = []

    def attach(self, observer):
        if observer not in self._observers:
            self._observers.append(observer)

    def detach(self, observer):
        try:
            self._observers.remove(observer)
        except ValueError:
            pass

    def notify(self):
        for observer in self._observers:
            observer.update(self)

class Observer:
    def update(self, subject):
        pass

class ConcreteObserver(Observer):
    def update(self, subject):
        print(f"Observer {id(self)} got notified by {subject.__class__.__name__}")

import weakref

class WeatherStation:
    def __init__(self):
        self._observers = weakref.WeakSet()
        self._temperature = None

    def attach(self, observer):
        self._observers.add(observer)

    def detach(self, observer):
        self._observers.discard(observer)

    def set_temperature(self, temperature):
        self._temperature = temperature
        self.notify()

    def notify(self):
        for observer in self._observers:
            observer.update(self)

class TemperatureObserver:
    def update(self, subject):
        if isinstance(subject, WeatherStation):
            print(f"Temperature updated to {subject._temperature}°C")

class AlertObserver:
    def __init__(self, threshold):
        self.threshold = threshold

    def update(self, subject):
        if isinstance(subject, WeatherStation) and subject._temperature > self.threshold:
            print(f"Alert: Temperature exceeded {self.threshold}°C!")

# 使用示例
weather_station = WeatherStation()
temp_observer = TemperatureObserver()
alert_observer = AlertObserver(threshold=30)

weather_station.attach(temp_observer)
weather_station.attach(alert_observer)

weather_station.set_temperature(25)  # 输出: Temperature updated to 25°C
weather_station.set_temperature(35)  # 输出: Temperature updated to 35°C 和 Alert: Temperature exceeded 30°C!