怎样在Python中实现设计模式？

# singleton.py
class Singleton:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super(Singleton, cls).__new__(cls)
        return cls._instance

# 使用
from singleton import Singleton
s1 = Singleton()
s2 = Singleton()
print(s1 is s2)  # 输出: True

class Dog:
    def speak(self):
        return "Woof!"

class Cat:
    def speak(self):
        return "Meow!"

def animal_factory(animal_type):
    if animal_type == "dog":
        return Dog()
    elif animal_type == "cat":
        return Cat()
    else:
        raise ValueError("Unknown animal type")

# 使用
dog = animal_factory("dog")
print(dog.speak())  # 输出: Woof!

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()

class Observer:
    def __init__(self, name):
        self.name = name

    def __call__(self):
        print(f"{self.name} has been notified!")

# 使用
subject = Subject()
observer1 = Observer("Observer 1")
observer2 = Observer("Observer 2")

subject.attach(observer1)
subject.attach(observer2)

subject.notify()  # 输出: Observer 1 has been notified! 和 Observer 2 has been notified!