python singleton

方法一：用元类的方式实现一个singleton。

liuxiaoyan@development:~/mysite$ cat Singleton.py
class Singleton(type):
        """Singleton Metaclass"""
        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 MyClass(object):
        __metaclass__ = Singleton  

        def __init__(self,arg):
                self.arg = arg  

myClass1=MyClass("myClass1")
myClass2=MyClass("myClass2")

print myClass1 is myClass2

liuxiaoyan@development:~/mysite$ python Singleton.py
True

方法二：用修饰类的方式实现一个Singleton。

liuxiaoyan@development:~/mysite$ cat Singleton.py
class Singleton:
    def __init__( self, decorated ):
        self._decorated = decorated
    def Instance( self ):
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated
            return self._instance  

    def __call__( self ):
        raise TypeError( 'single instance allowed' )  

@Singleton
class MyClass:
    def __init__( self ):
        print( 'created' )
#myClass=MyClass()  #不能再用这种方式创建MyClass实例，会产生异常

#用以下方法创建实例
myClass1=MyClass.Instance()
myClass2=MyClass.Instance()

print myClass1 is myClass2
print id(myClass1)
print id(myClass2)

liuxiaoyan@development:~/mysite$ python Singleton.py
True
140716352778528
140716352778528