Python单例模式

#-*- encoding=utf-8 -*-

class Singleton(object):

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

        if not hasattr(cls, '_instance'):

            orig = super(Singleton, cls)

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

        return cls._instance



class MyClass(Singleton):

    a = 1



one = MyClass()

two = MyClass()



two.a = 3

print one.a

#3

#one和two完全相同,可以用id(), ==, is检测

print id(one)



print id(two)



print one == two

#True

print one is two

#True



class Borg(object):

    _state = {}

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

        ob = super(Borg, cls).__new__(cls, *args, **kw)

        ob.__dict__ = cls._state

        return ob



class MyClass2(Borg):

    a = 1



one = MyClass2()

two = MyClass2()



two.a = 3

print one.a

print id(one)

print id(two)

print one == two

print one is two



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





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

class3使用了元类。 

class4使用了美妙的decorator。