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 
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