python面試題之Python如何實現單例模式？

#使用__metaclass__（元類）的高階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
#使用裝飾器(decorator),  
#這是一種更pythonic,更elegant的方法,  
#單例類本身根本不知道自己是單例的,因為他本身(自己的**)並不是單例的  
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本文首發於python黑洞網，csdn同步跟新

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