Python下简易的单例模式详解
Python 下的单例模式
要点:
- 1.某个类只能有一个实例;
- 2.它必须自行创建这个实例;
- 3.它必须自行向整个系统提供这个实例
方法:重写new函数
应该考虑的情况:
- 1.这个单例的类可能继承了别的类
- 2.这个单例的类还有可能要接收参数来实例化
要点:
实例化的过程其实不是直接调用init的,首先是new分配一块空间来创建实例,再由init对这个实例进行初始化.我们无法阻止new和init的调用,我们只能是限制他们的内容,以此使他们能达到单例的目的
代码:
class people(object): def __new__(cls,*args,**kargs): return super(people,cls).__new__(cls) def __init__(self,name): self.name = name def talk(self): print("hello,I am %s" %self.name) class student(people): def __new__(cls,*args,**kargs): if not hasattr(cls,"instance"): cls.instance = super(student,cls).__new__(cls,*args,**kargs) return cls.instance a = student("Timo") print(a) b = student("kysa") c = student("Luyi") a.talk() b.talk() print(c)
这里的输出结果是:
<__main__.student object at 0x0000025AC48BF2E8>
hello,I am Luyi
hello,I am Luyi
<__main__.student object at 0x0000025AC48BF2E8>
可以确定的是: 确实是单例了,因为a的id和b,c的id是一致的
但是为什么:a先创建明明是Timo,可是为什么a的name变成了Luyi呢?
原因:
虽然确实是a这个实例,但是在最后c重新调用了new,返回了a的实例,再经过init,改变了a的属性,执行时name ->Luyi.
解决:
这种情况下,我们只需要设置类变量,让init在类变量的限制下,只对类进行一次有效的初始化.
代码:
class people(object): def __new__(cls,*args,**kargs): return super(people,cls).__new__(cls) def __init__(self,name): self.name = name def talk(self): print("hello,I am %s" %self.name) class student(people): def __new__(cls,*args,**kargs): if not hasattr(cls,"instance"): cls.instance = super(student,cls).__new__(cls,*args,**kargs) return cls.instance def __init__(self,name): if not hasattr(self,"init_fir"): self.init_fir = True super(student,self).__init__(name) a = student("Timo") print(a) b = student("kysa") c = student("Luyi") a.talk() b.talk() print(c)
好了,到这里就用Python实现了一个简易的单例模式.
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