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 =
print one.a
#
print id(one)
#
print id(two)
#
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 =
    def __init__(self, x=):
        self.x = x  

one = MyClass4()
two = MyClass4()  

two.a =
print one.a
#
print id(one)
#
print id(two)
#
print one == two
#True
print one is two
#True
one.x =
print one.x
#
print two.x
#

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