.Learning.Python.Design.Patterns.2nd.Edition之单实例模式

class Singleton(object):
    def __new__(cls):
        if not hasattr(cls, 'instance'):
            cls.instance = super(Singleton, cls).__new__(cls)
        return cls.instance

s = Singleton()
print("Object created", s, id(s))

s1 = Singleton()
print("Object created", s1, id(s1))

class SingletonA:
    __instance = None
    def __init__(self):
        if not SingletonA.__instance:
            print("__init__ method called..")
        else:
            print("Instance already created:", self.getInstance())

    @classmethod
    def getInstance(cls):
        if not cls.__instance:
            cls.__instance = SingletonA()
        return cls.__instance

s = SingletonA()
print("Object created", SingletonA.getInstance())
s = SingletonA()

class Borg:
    __shared_state = {"1": "2"}
    def __init__(self):
        self.x = 1
        self.__dict__ = self.__shared_state
        pass

b = Borg()
b1 = Borg()
b.x = 4

print("Borg Object 'b':", b)
print("Borg Object 'b1':", b1)
print("Object State 'b':", b.__dict__)
print("Object State 'b1':", b1.__dict__)

class BorgA:
    _shared_state = {"1": "2"}
    def __new__(cls, *args, **kwargs):
        obj = super(BorgA, cls).__new__(cls, *args, **kwargs)
        obj.__dict__ = cls._shared_state
        return obj

b = BorgA()
b1 = BorgA()
b.x = 4

print("BorgA Object 'b':", b)
print("BorgA Object 'b1':", b1)
print("Object State 'b':", b.__dict__)
print("Object State 'b1':", b1.__dict__)