python学习教程（十三）python实现单例模式

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.instance

class MyClass(object):
    __metaclass__ = Singleton

print 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。