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 = Nonedef __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 = 1def __init__(self, x=0):self.x = xone = 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 = Nonedef __call__(cls, *args, **kw):if cls._instance is None:cls._instance = super(Singleton2, cls).__call__(*args, **kw)return cls._instanceclass MyClass(object):__metaclass__ = Singleton2one = 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dictclass Borg(object):_state = {}def __new__(cls, *args, **kw):ob = super(Borg, cls).__new__(cls, *args, **kw)ob.__dict__ = cls._statereturn obclass MyClass(Borg):a = 1one = 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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