单例模式元类

元类实现单例模式

#coding=utf8
class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


# Python2
class MyClass(object):
    pass
    __metaclass__ = Singleton

# Python3
# class MyClass(metaclass=Singleton):
#    pass

m1=MyClass()

m2=MyClass()

print id(m1),id(m2)