【Python】单例模式Singleton

前两天一个面试被问到python中单例模式有几种实现方式，只答出了可以用元类实现。。。然后就想不起来了。

之后翻书，原来这些之前都见过的啊。。。。

1.手动实现真正创建实例的方法__new__()来实现

class Singleton(object):
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
        return cls._instance

if __name__ == '__main__':
    s1 = Singleton()
    s2 = Singleton()
    assert id(s1) == id(s2)

2，利用元类实现：

class Singleton(type):
    def __init__(cls, name, bases, dic):
        super(Singleton, cls).__init__(name, bases, dic)
        cls.instance = None

    def __call__(cls, *args, **kwargs):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
        return cls.instance


class MySingleton(object):
    __metaclass__ = Singleton

3，利用装饰器实现

def Singleton1(aclass):
    instance = None
    def oncall(*args):
        nonlocal instance
        if instance == None
            instance = aclass(*args)
        return instance
    return oncall


class singleton2:
    def __init__(self, aclass):
        self.aclass = aclass
        self.instance = None
        
    def __call__(self, *args):
        if self.instance == None:
            self.instance = self.aclass(*args)
        return self.instance

4.共享属性

class Borg(object):
    _state = {}
    def __new__(cls, *args, **kw):
        ob = super(Borg, cls).__new__(cls, *args, **kw)
        ob.__dict__ = cls._state
        return ob

class MyClass2(Borg):
    a = 1

5.import实现

# mysingleton.py
class My_Singleton(object):
    def foo(self):
        pass

my_singleton = My_Singleton()

# to use
from mysingleton import my_singleton

my_singleton.foo()

参考资料：1.改善python程序的91个建议

　　　　　 2.Python学习手册

                3.http://python.jobbole.com/85231/

　　　　　4.Python Cookbook