python singleton

方法一：

class Singleton(object):
    __instance = None

    def __init__():
        '''
        disable __init__
        '''
        pass

    def initInstance(self):
        pass

    @staticmethod
    def getInstance():
        '''
        this is the only access to get the instance of this class
        '''
        if(Singleton.__instance == None):
           Singleton.__instance = object.__new__(Singleton)
           Singleton.__instance.initInstance()
        return Singleton.__instance

    @staticmethod
    def destroyInstance():
        if(Singleton.__instance):
            Singleton.__instance = None


if(__name__ == '__main__'):
    _singleton = Singleton.getInstance()
    _singleton.value = 'abc'
    print _singleton.value
    Singleton.destroyInstance()

方法二：
还可以在创建对象时利用__new__方法判断本类是否有对象存在，如果存在就返回已经存在的对象，如果不存在就创建新的对象并返回。可以直接在一个基类中记录类和生成的对象：

class Singleton(object):
    __instances = {}
    def __new__(cls, *args, **kwargs):
        name = '.'.join((cls.__module__, cls.__name__))
        if name not in Singleton.__instances:
            Singleton.__instances[name] = object.__new__(cls, *args, **kwargs)
        return Singleton.__instances[name]

if __name__ == "__main__":
    class T(Singleton):
        pass
    print T() is T()

方法三：
和方法二相似，但不记录类的对象，利用gc垃圾回收机制的引用来查找对象

import gc

class Singleton(object):
    def __new__(cls, *args, **kwargs):
        for obj in gc.get_referrers(cls):
            if obj.__class__ == cls:
                return obj
        return object.__new__(cls, *args, **kwargs)

if __name__ == "__main__":
    class T(Singleton):
        pass
    print T() is T()