Python实现单例模式

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Abstract:

Singleton是创建模式中很典型的一类，在Java中是通过将构造函数private来实现的。在python中，没有private，我们通过share一个类或子类的所有成员变量来实现单例。注意，虽然所有对象都共享相同的成员，但和Java中的单例还是有差别。java中是真的只建了一个对象，但python中实际有多个对象(id不同), 但成员相同。

Content:

话不多说，直接上代码，注释写得很清楚。只要将你自己的类继承下面的Singleton类，即实现了将你的类转换成了单例模式。

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class Singleton:
    '''parent singleton class, make your class to inherit it to transfer yours to singleton''' 
    __shared_state = {} #create share dictionary, use to store member of class later
    self_variable = 0

    def __init__(self):
        self.__dict__ = self.__shared_state  #use shared dic to implement singleton
        self.state = 'Init'
        self.array = [1,2]

    def __str__(self): #you can delete it, just for transfer obj to str 
        return self.state

class SubSingleton(Singleton):
    pass

def main():
    rm1 = Singleton()
    rm2 = Singleton()
    rm1.self_variable = 1 #reassign shared variable will be visible for all singleton object
    rm2.state = 'Running'
    rm3 = Singleton() #even new object, it will get latest value if not reassign
    rm4 = SubSingleton() #for sub class, singleton function well. Share with parent
    rm4.array = [2,4]
    print('rm1: {0}'.format(rm1))
    print('rm1: {0}'.format(rm1.self_variable))
    print('rm1: {0}'.format(rm1.array))
    print('rm2: {0}'.format(rm2))
    print('rm2: {0}'.format(rm2.self_variable))
    print('rm2: {0}'.format(rm2.array))
    print('rm3: {0}'.format(rm3))
    print('rm3: {0}'.format(rm3.self_variable))
    print('rm3: {0}'.format(rm3.array))
    print('rm4: {0}'.format(rm4))
    print('rm4: {0}'.format(rm4.self_variable))
    print('rm4: {0}'.format(rm4.array))
    print('rm1 id: {0}'.format(id(rm1))) #id is different, means there are many obj sharing same members
    print('rm2 id: {0}'.format(id(rm2)))
    print('rm3 id: {0}'.format(id(rm3)))
    print('rm4 id: {0}'.format(id(rm4)))

if __name__ == '__main__':
    main()

运行上面代码，得到如下output，实现了共享方式的”单例”。

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rm1: Init
rm1: 1
rm1: [2, 4]
rm2: Init
rm2: 1
rm2: [2, 4]
rm3: Init
rm3: 1
rm3: [2, 4]
rm4: Init
rm4: 1
rm4: [2, 4]
rm1 id: 3078847052
rm2 id: 3078848332
rm3 id: 3078848364
rm4 id: 3078848396

最新实践发现，这种多个对象模拟单例对象的另一个问题————不能跨模块。如果在其它文件中导入该单例模块，再新建单例类的对象，会发现各模块中的单例对象不能实现共享了。

History:

  • 2016-09-11:将实现单例的代码和注释记录下来
  • 2016-09-21:更新跨模块单例失败的问题