Python实现单例模式的5种方式

写在前面

学究嘛，就记录一下;
本质都是通过设置一个标志位来实现, 通俗的讲就是当第一次实例化时, 记录下"已经实例化了", 当再次实例化时, 将"记录"的地址返回;
题外话, 函数它不香么? 还面向对象, 要什么对象.

1. 模块(module)

  • 这种方式不用干啥, 正常写code就可以了, 一旦这个模块被导入就会生成.pyc文件, 第二次导入时加载的就是.pyc

    # girl.pyclass GirlFriend:    passLucy = GirlFriend()# me.pyfrom girl import Lucy

2. 装饰器实现

  • 定义一个装饰器如下

    def singleton(cls):    instances = {}    @functools.wraps(cls)    def _wrapper(*args, **kw):        if cls not in instances:            print('没有该实例，创建一个')            instances[cls] = cls(*args, **kw)        return instances[cls]    return _wrapper

  • 使用装饰器

    @singletonclass GirlFriend:    pass

3. 类装饰器使其

class Singleton:    def __init__(self, cls):        self._cls = cls        self._instance = {}    def __call__(self, *args):        if self._cls not in self._instance:            self._instance[self._cls] = self._cls(*args)        return self._instance[self._cls]# 用法@Singletonclass GirlFriend:    pass

4. __new__方法实现

class GirlFriend:    def __new__(cls, *args, **kw):        if not hasattr(cls, '_instance'):            cls._instance = super().__new__(cls)        return cls._instance# 用法Lucy = GirlFriend()

5. 元类实现

class GirlFriend(type):        def __new__(cls, name, bases, attrs):        cls._instance = None        return type.__new__(cls, name, bases, attrs)    def __call__(cls, *args, **kw):        if cls._instance is None:            cls._instance = type.__call__(cls, *args, **kw)        return cls._instance# 用法class Lucy(metaclass=GirlFriend):    pass

总结

  1. 装饰器实现比较简单, 也好理解
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