Hongyao Tang

Design pattern

Which SOLID principle states that a class should have only one reason to change?

Single Responsibility Principle

Which design pattern is demonstrated by Python’s new method?

The new method can be used to implement the Singleton pattern by ensuring that only one instance of a class is created.

数据库连接池,线程池
单例模式（Singleton Pattern）- 一个类只能有一个实例，并提供一个全局访问点。
在 new 中判断是否已经创建过实例，如果有，就直接返回已有的实例

class Singleton:
    _instance = None  # 用于保存唯一实例

    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super().__new__(cls)

        # __new__ 必须返回一个类的实例（通常是 super().__new__(cls)）
        # 如果你返回的不是该类的实例，__init__ 就不会被调用
        return cls._instance

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


a = Singleton(10)
b = Singleton(20)

print(a is b)         # True，说明是同一个对象

Design pattern#

Design pattern