某班需要班长主持班会，应用单例模式（分别用懒汉式单例和饿汉式单例两种实现形式）产生班长，模拟班会举行

懒汉式单例实现：

class ClassPresident:
    __instance = None

    @staticmethod
    def get_instance():
        if ClassPresident.__instance is None:
            ClassPresident()
        return ClassPresident.__instance

    def __init__(self):
        if ClassPresident.__instance is not None:
            raise Exception("ClassPresident class is a singleton!")
        else:
            ClassPresident.__instance = self

    def lead_class_meeting(self):
        print("Welcome to the class meeting. Let's begin!")

# 测试代码
president1 = ClassPresident.get_instance()
president1.lead_class_meeting()  # Welcome to the class meeting. Let's begin!

president2 = ClassPresident.get_instance()
print(president1 == president2)  # True

president3 = ClassPresident()  # 会报错，因为 ClassPresident 类是单例模式，不能直接实例化

饿汉式单例实现：

class ClassPresident:
    __instance = ClassPresident()

    @staticmethod
    def get_instance():
        return ClassPresident.__instance

    def __init__(self):
        if ClassPresident.__instance is not None:
            raise Exception("ClassPresident class is a singleton!")

    def lead_class_meeting(self):
        print("Welcome to the class meeting. Let's begin!")

# 测试代码
president1 = ClassPresident.get_instance()
president1.lead_class_meeting()  # Welcome to the class meeting. Let's begin!

president2 = ClassPresident.get_instance()
print(president1 == president2)  # True

president3 = ClassPresident()  # 会报错，因为 ClassPresident 类是单例模式，不能直接实例化

模拟班会举行：

president = ClassPresident.get_instance()
president.lead_class_meeting()

# 输出：
# Welcome to the class meeting. Let's begin!