python学习教程（十三）python实现单例模式

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首先，要知道什么是python的单例模式，所谓单例模式就是一个类只能创建一个实例化。

然后，就是python单例模式的方法，总共可以分为两大种，四小种，一会就回说的。

首先，方法一：

class Singleton(type):    def __init__(cls, name, bases, dict):super(Singleton, cls).__init__(name, bases, dict)cls.instance = None    def __call__(cls, *args, **kw):if cls.instance is None:    cls.instance = super(Singleton, cls).__call__(*args, **kw)    return cls.instanceclass MyClass(object):    __metaclass__ = Singletonprint MyClass()print MyClass()

方法二：使用装饰器（decorator）

def singleton(cls, *args, **kw):      instances = {}      def _singleton():          if cls not in instances:              instances[cls] = cls(*args, **kw)          return instances[cls]      return _singleton   @singleton  class MyClass(object):      a = 1      def __init__(self, x=0):          self.x = x    one = MyClass()  two = MyClass()    two.a = 3  print one.a  #3  print id(one)  #29660784  print id(two)  #29660784  print one == two  #True  print one is two  #True  one.x = 1  print one.x  #1  print two.x

方法三：使用__metaclass__元类来实现

class Singleton2(type):      def __init__(cls, name, bases, dict):          super(Singleton2, cls).__init__(name, bases, dict)          cls._instance = None      def __call__(cls, *args, **kw):          if cls._instance is None:              cls._instance = super(Singleton2, cls).__call__(*args, **kw)          return cls._instance    class MyClass(object):      __metaclass__ = Singleton2    one = MyClass()  two = MyClass()    two.a = 3  print one.a  #3  print id(one)  #31495472  print id(two)  #31495472  print one == two  #True  print one is two  #True

方法四：通过共享属性来实现，所谓共享属性，最简单直观的方法就是通过__dict__属性指向同一个字典dict

class Borg(object):      _state = {}      def __new__(cls, *args, **kw):          ob = super(Borg, cls).__new__(cls, *args, **kw)          ob.__dict__ = cls._state          return ob    class MyClass(Borg):      a = 1    one = MyClass()  two = MyClass()    #one和two是两个不同的对象,id, ==, is对比结果可看出  two.a = 3  print one.a  #3  print id(one)  #28873680  print id(two)  #28873712  print one == two  #False  print one is two  #False  #但是one和two具有相同的（同一个__dict__属性）,见:  print id(one.__dict__)  #30104000  print id(two.__dict__)

其实吧，从本质上来讲，方法一二三都属于一种单例化模式的方法，与第四种不同，所以认为python中有两种或四种方法实现单例模式都可以。

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