python-设计模式-单例模式

简述

单例模式用于创建对象的场景。很多时候我们要用到全局对象，也就是说这个对象在程序运行过程中只实例化一次。
我们还继续用"从小喝到大的椰奶"的例子，无论盒装还是罐装的椰奶，它的代言人都是徐冬冬老师，所以对于代言人这个对象，我们只需创建一次即可。

实践一下

class SpokesMenSingle(type):

    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(SpokesMenSingle, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


class CmSpokesMen(metaclass=SpokesMenSingle):

    def __init__(self):
        self.name = "xu dong dong"

    def get_spokes_men(self):
        print(f"椰奶的代言人是 {self.name}")
        return self.name


class CmSpokesMenOther():

    def __init__(self):
        self.name = "xu dong dong"

    def get_spokes_men(self):
        print(f"椰奶的代言人是 {self.name}")
        return self.name


if __name__ == "__main__":
    print("单例模式==>")
    single_cm = CmSpokesMen()
    single_cm.get_spokes_men()
    single_cm_01 = CmSpokesMen()
    print(f"{id(single_cm_01)}  {id(single_cm)}  {id(single_cm_01) == id(single_cm)}")
    print("非单例模式==>")
    single_other_cm = CmSpokesMenOther()
    single_other_cm_01 = CmSpokesMenOther()
    print(f"{id(single_other_cm)}  {id(single_other_cm_01)}  {id(single_other_cm) == id(single_other_cm_01)}")

从运行结果可以看出，单例模式是没有创建新实例的。