Python单例模式

实现单例模式的三种方法：类实现、装饰器实现和元类实现

#coding-utf-8

#单例模式

#类的方式实现单例模式
'''class Singleton(object):

    _instance = None
    def __init__(self):
        print ("__init__")
        pass

    @staticmethod
    def __new__(cls, *argv, **kwargv):
       print ("__new__")
       if not cls._instance:
           cls._instance = super(Singleton, cls).__new__(cls,*argv, **kwargv)

       return cls._instance    

class A(Singleton):pass'''

#使用装饰器的方式实现单例模式
'''def Singleton(cls):
    instances = {}
    
    def getinstance(*argv, **kwargv):
        if cls not in instances:
            instances[cls] = cls(*argv, **kwargv)   
        return instances[cls]

    return getinstance

@Singleton
class A(object):
    def __init__(self, parm):
        print ("class A __init__:%s" %(parm))'''

#使用元类的方法实现单例模式
class Singleton(type):
    instances = {}
    def __call__(self, *argv, **kwargv):
        if self not in self.instances:
            print ("!!!not have instances")
            self.instances[self] = super(Singleton, self).__call__(*argv, **kwargv)
        return super(Singleton, self).__call__(*argv, **kwargv)    

    def __new__(mcs, name, bases, namespace, **kwargs):
        print ("Singleton new")
        return super(Singleton, mcs).__new__(mcs,name, bases, namespace, **kwargs)

class A(metaclass = Singleton):
    def __init__(self):
        self.aa = 1

if __name__ == "__main__":
    a = A()
    b = A()
    print (id(a))
    print (id(b))