Python实现单例模式

使用函数装饰器

def singleton(cls):
    _instance = {}

    def inner():
        if cls not in _instance:
            _instance[cls] = cls()
        return _instance[cls]
    return inner
    
@singleton
class Demo(object):
    def __init__(self):
        pass

def test():
    b1 = Demo()
    b2 = Demo()
    print(b1, b2)

使用类装饰器

class single1:
    def __init__(self, cls):
        self.cls = cls
        self.cls.instance = None

    def __call__(self, *args, **kwargs):
        if not self.cls.instance:
            self.cls.instance = self.cls(*args, **kwargs)
        return self.cls.instance

@single1
class Demo:
    def test(self):
        print("hello world")

def test():
    b1 = Demo()
    b2 = Demo()
    print(b1, b2)

基于__new__ 方法实现

import threading

class Demo(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        pass


    def __new__(cls, *args, **kwargs):
        if not hasattr(Demo, "_instance"):
            with Demo._instance_lock:
                if not hasattr(Demo, "_instance"):
                    Demo._instance = object.__new__(cls)  
        return Demo._instance
    
def test():
    b1 = Demo()
    b2 = Demo()
    print(b1, b2)

打印结果显示b1和b2的地址一致：

<demo.Demo object at 0x1103ca980> <demo.Demo object at 0x1103ca980>