单例：即一个类只能生成唯一的一个实例，python中的类如果没有被实例化，则cls._instance为None

如下：

class Singleton(object):

def __new__(cls,*args,**kw):

if not hasattr(cls,"_instance"):

orgi=super(Singleton,cls)

cls._instance=orgi.__new__(cls,*args,**kw)

return cls._instance

class MyClass(Singleton):

pass

one=MyClass()

two=MyClass()

print id(one)

print id(two)

print one==two

print one is two