Приведи пример использования паттернов программирования

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

obj1 = Singleton()
obj2 = Singleton()
print(obj1 is obj2)  # True

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

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

def get_pet(pet="dog"):
    pets = {"dog": Dog(), "cat": Cat()}
    return pets[pet]

animal = get_pet("cat")
print(animal.speak())  # Meow!

class Subject:
    def __init__(self):
        self._observers = []
    
    def attach(self, observer):
        self._observers.append(observer)
    
    def notify(self):
        for observer in self._observers:
            observer.update(self)

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