面向对象–设计模式：单列模式

单例模式

1. 单例是什么

举个常见的单例模式例子，我们日常使用的电脑上都有一个回收站，在整个操作系统中，回收站只能有一个实例，整个系统都使用这个唯一的实例，而且回收站自行提供自己的实例。因此回收站是单例模式的应用。

确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例，这个类称为单例类，单例模式是一种对象创建型模式。

2. 创建单例-保证只有1个对象

<code class="lang-python">
<span class="hljs-comment"># 实例化一个单例
<span class="hljs-class"><span class="hljs-keyword">class <span class="hljs-title">Singleton<span class="hljs-params">(object):
    __instance = <span class="hljs-keyword">None

    <span class="hljs-function"><span class="hljs-keyword">def <span class="hljs-title">__new__<span class="hljs-params">(cls, age, name):
        <span class="hljs-comment">#如果类属性__instance的值为None，
        <span class="hljs-comment">#那么就创建一个对象，并且赋值为这个对象的引用，保证下次调用这个方法时
        <span class="hljs-comment">#能够知道之前已经创建过对象了，这样就保证了只有1个对象
        <span class="hljs-keyword">if <span class="hljs-keyword">not cls.__instance:
            cls.__instance = object.__new__(cls)
        <span class="hljs-keyword">return cls.__instance

a = Singleton(<span class="hljs-number">18, <span class="hljs-string">"dongGe")
b = Singleton(<span class="hljs-number">8, <span class="hljs-string">"dongGe")

print(id(a))
print(id(b))

a.age = <span class="hljs-number">19 <span class="hljs-comment">#给a指向的对象添加一个属性
print(b.age)<span class="hljs-comment">#获取b指向的对象的age属性
</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code>

www#gaodaima.com来源gaodai#ma#com搞@代~码网搞代码

运行结果：

<code class="lang-python">In [<span class="hljs-number">12]: <span class="hljs-class"><span class="hljs-keyword">class <span class="hljs-title">Singleton<span class="hljs-params">(object):
    ...:     __instance = <span class="hljs-keyword">None
    ...: 
    ...:     <span class="hljs-function"><span class="hljs-keyword">def <span class="hljs-title">__new__<span class="hljs-params">(cls, age, name):
    ...:         <span class="hljs-keyword">if <span class="hljs-keyword">not cls.__instance:
    ...:             cls.__instance = object.__new__(cls)
    ...:         <span class="hljs-keyword">return cls.__instance
    ...: 
    ...: a = Singleton(<span class="hljs-number">18, <span class="hljs-string">"dongGe")
    ...: b = Singleton(<span class="hljs-number">8, <span class="hljs-string">"dongGe")
    ...: 
    ...: print(id(a))
    ...: print(id(b))
    ...: 
    ...: a.age = <span class="hljs-number">19
    ...: print(b.age)
    ...: 
<span class="hljs-number">4391023224
<span class="hljs-number">4391023224
<span class="hljs-number">19
</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code>

3. 创建单例时，只执行1次__init__方法

<code class="lang-python">
<span class="hljs-comment"># 实例化一个单例
<span class="hljs-class"><span class="hljs-keyword">class <span class="hljs-title">Singleton<span class="hljs-params">(object):
    __instance = <span class="hljs-keyword">None
    __is_first = <span class="hljs-keyword">True

    <span class="hljs-function"><span class="hljs-keyword">def <span class="hljs-title">__new__<span class="hljs-params">(cls, age, name):
        <span class="hljs-keyword">if <span class="hljs-keyword">not cls.__instance:
            cls.__instance = object.__new__(cls)
        <span class="hljs-keyword">return cls.__instance

    <span class="hljs-function"><span class="hljs-keyword">def <span class="hljs-title">__init__<span class="hljs-params">(self, age, name):
        <span class="hljs-keyword">if self. __is_first:
            self.age = age
            self.name = name
            Singleton. __is_first = <span class="hljs-keyword">False


a = Singleton(<span class="hljs-number">18, <span class="hljs-string">"习大大")
b = Singleton(<span class="hljs-number">28, <span class="hljs-string">"习大大")

print(id(a))
print(id(b))


print(a.age)
print(b.age)

a.age = <span class="hljs-number">19
print(b.age)
</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code>

运行结果:

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