【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

 1 class Singleton(object):
 2     _instance = None
 3 
 4     def __new__(cls, *args, **kwargs):
 5         if not cls._instance:
 6             cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
 7         return cls._instance
 8 
 9 if __name__ == '__main__':
10     s1 = Singleton()
11     s2 = Singleton()
12     assert id(s1) == id(s2)

2，利用元类实现：

 1 class Singleton(type):
 2     def __init__(cls, name, bases, dic):
 3         super(Singleton, cls).__init__(name, bases, dic)
 4         cls.instance = None
 5 
 6     def __call__(cls, *args, **kwargs):
 7         if cls.instance is None:
 8             cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
 9         return cls.instance
10 
11 
12 class MySingleton(object):
13     __metaclass__ = Singleton

3，利用装饰器实现

 1 def Singleton1(aclass):
 2     instance = None
 3     def oncall(*args):
 4         nonlocal instance
 5         if instance == None
 6             instance = aclass(*args)
 7         return instance
 8     return oncall
 9 
10 
11 class singleton2:
12     def __init__(self, aclass):
13         self.aclass = aclass
14         self.instance = None
15         
16     def __call__(self, *args):
17         if self.instance == None:
18             self.instance = self.aclass(*args)
19         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

 1 # mysingleton.py
 2 class My_Singleton(object):
 3     def foo(self):
 4         pass
 5 
 6 my_singleton = My_Singleton()
 7 
 8 # to use
 9 from mysingleton import my_singleton
10 
11 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