装饰器实现单例模式 java，python面试题之Python如何实现单例模式？

#使用__metaclass__（元类）的高级python用法  
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 MyClass3(object):  __metaclass__ = Singleton2  one = MyClass3()  
two = MyClass3()  two.a = 3  
print one.a  
#3  
print id(one)  
#31495472  
print id(two)  
#31495472  
print one == two  
#True  
print one is two#True#使用装饰器(decorator),  
#这是一种更pythonic,更elegant的方法,  
#单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的  
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 MyClass4(object):  a = 1  def __init__(self, x=0):  self.x = x  one = MyClass4()  
two = MyClass4()  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  
#1本文首发于Python黑洞网，csdn同步跟新