关于python的单实例模式

单实例模式一直是常用的设计模式，对于python的单实例模式，其实其本身就有实现

http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python/31887#31887

里面说到module，module只会初始化一次，天然的singleton。这是最为python的解决方案。将你所需要的属性和方法，直接暴露在模块中变成模块的全局变量和方法即可。

 

另外，如果真的要使用代码进行singleton，下面给出几个代码

# Singleton/ClassVariableSingleton.py
class SingleTone(object):
    __instance = None
    def __new__(cls, val):
        if SingleTone.__instance is None:
            SingleTone.__instance = object.__new__(cls)
        SingleTone.__instance.val = val
        return SingleTone.__instance

# Singleton/SingletonDecorator.py
class SingletonDecorator:
    def __init__(self,klass):
        self.klass = klass
        self.instance = None
    def __call__(self,*args,**kwds):
        if self.instance == None:
            self.instance = self.klass(*args,**kwds)
        return self.instance

class foo: pass
foo = SingletonDecorator(foo)

x=foo()
y=foo()
z=foo()
x.val = 'sausage'
y.val = 'eggs'
z.val = 'spam'
print(x.val)
print(y.val)
print(z.val)
print(x is y is z)

# Singleton/SingletonMetaClass.py
class SingletonMetaClass(type):
    def __init__(cls,name,bases,dict):
        super(SingletonMetaClass,cls)\
          .__init__(name,bases,dict)
        original_new = cls.__new__
        def my_new(cls,*args,**kwds):
            if cls.instance == None:
                cls.instance = \
                  original_new(cls,*args,**kwds)
            return cls.instance
        cls.instance = None
        cls.__new__ = staticmethod(my_new)

class bar(object):
    __metaclass__ = SingletonMetaClass
    def __init__(self,val):
        self.val = val
    def __str__(self):
        return `self` + self.val

x=bar('sausage')
y=bar('eggs')
z=bar('spam')
print(x)
print(y)
print(z)
print(x is y is z)