初步实现代码如下:
class Singleton(object): instance = None def __init__(self): pass @classmethod def get_instance(cls): if Singleton.instance is None: Singleton.instance = Singleton() return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() print('a id=', id(a)) print('b id=', id(b))
输出:
>>> a id= 39314488 >>> b id= 39314488
class Singleton(object): instance = None def __init__(self): pass @classmethod def get_instance(cls): if Singleton.instance is None: Singleton.instance = Singleton() return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() c = Singleton() print('a id=', id(a)) print('b id=', id(b)) print('c id=', id(c))输出:
>>> a id= 43371992 >>> b id= 43371992 >>> c id= 43384904c是通过类实例化的id和a、b的不一样。
有没有办法禁止通过类来实例化呢?我们可以在__init__中做文章,在__init__中抛出异常即可禁止实例化了,但是还是有问题,因为get_instance里面就是通过类来实例化的:
@classmethod def get_instance(cls): if Singleton.instance is None: Singleton.instance = Singleton() return Singleton.instance
除了通过类来实例化就没有其他方法实例化了吗?有,我们可以通过父类object的__new__函数来实例化, get_instance函数修改为:
@classmethod def get_instance(cls): if Singleton.instance is None: Singleton.instance = object.__new__(Singleton) return Singleton.instance
完整代码如下:
class Singleton(object): instance = None def __init__(self): raise SyntaxError('can not instance, please use get_instance') @classmethod def get_instance(cls): if Singleton.instance is None: Singleton.instance = object.__new__(Singleton) return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() print('a id=', id(a)) print('b id=', id(b)) c = Singleton() print('c id=', id(c))
输出:
>>> Traceback (most recent call last): File "E:/Code/python3/otherTest/classmethodSingleton.py", line 18, in <module> c = Singleton() File "E:/Code/python3/otherTest/classmethodSingleton.py", line 5, in __init__ raise SyntaxError('can not instance, please use get_instance') SyntaxError: can not instance, please use get_instance >>> a id= 39125064 >>> b id= 39125064从完整代码和输出可以看出只能通过get_instance获取实例,不能通过类来实例化,通过类来实例化会报异常。
PS:同样的通过staticmethod也能实现单例模式,见python一种用staticmethod实现单例模式方法的探讨。