使用元类的单例

单例是一种模式，它将类的实例化限制为一个实例/对象。有关 python singleton 设计模式的更多信息，请参见此处 。

class SingletonType(type):
    def __call__(cls, *args, **kwargs):
        try:
            return cls.__instance
        except AttributeError:
            cls.__instance = super(SingletonType, cls).__call__(*args, **kwargs)
            return cls.__instance

Python 2.x <= 2.7

class MySingleton(object):
    __metaclass__ = SingletonType

Python 3.x >= 3.0

class MySingleton(metaclass=SingletonType):
    pass

MySingleton() is MySingleton()  # True, only one instantiation occurs