Python单例模式的4种实现方法

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

print '----------------------方法1--------------------------'

#方法1,实现__new__方法

#并在将一个类的实例绑定到类变量_instance上,

#如果cls._instance为None说明该类还没有实例化过,实例化该类,并返回

#如果cls._instance不为None,直接返回cls._instance

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)

#29097904

print id(two)

#29097904

print one == two

#True

print one is two

#True

print '----------------------方法2--------------------------'

#方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法)

#同一个类的所有实例天然拥有相同的行为(方法),

#只需要保证同一个类的所有实例具有相同的状态(属性)即可

#所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict)

#可参看:http://code.activestate.com/recipes/66531/

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()

#one和two是两个不同的对象,id, ==, is对比结果可看出

two.a = 3

print one.a

#3

print id(one)

#28873680

print id(two)

#28873712

print one == two

#False

print one is two

#False

#但是one和two具有相同的（同一个__dict__属性）,见:

print id(one.__dict__)

#30104000

print id(two.__dict__)

#30104000

print '----------------------方法3--------------------------'

#方法3:本质上是方法1的升级（或者说高级）版

#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

print '----------------------方法4--------------------------'

#方法4:也是方法1的升级（高级）版本,

#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