单例模式

class Single(object):
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            cls._instance = super(Single, cls).__new__(cls, *args, **kwargs)
        return cls._instance


class A(Single):
    pass


def single2(cls):
    _instance = {}

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


@single2
class B(object):
    pass