14.python单例模式

## _instance一定不要两个下划线，否则这种方式取不到属性
class Singleton(object):
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            orig = super(Singleton, cls)
            cls._instance = orig.__new__(cls, *args, **kwargs)
        return cls._instance


class Myclass(Singleton):
    a = 1


class Borg(object):
    _state = {}

    def __new__(cls, *args, **kwargs):
        ob = super(Borg, cls).__new__(cls, *args, **kwargs)
        ob.__dict__ = cls._state
        return ob


class Myclass2(Borg):
    a = 1


def singleton(cls, *args, **kwargs):
    instances = {}

    def getinstance():
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return getinstance