1 使用__new__方法

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()

>>> one == two

True

>>> one is two

True

>>> id(one), id(two)

(4303862608, 4303862608)

2 共享属性

创建实例时把所有实例的__dict__指向同一个字典,这样它们具有相同的属性和方法.

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

3 装饰器版本

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

    instances = {}

    def getinstance():

        if cls not in instances:

            instances[cls] = cls(*args, **kw)

        return instances[cls]

    return getinstance

@singleton

class MyClass:

装饰器

def deco(func):

    def _deco():

        print 'before func'

        func()

        print 'after func'

    return _deco

@deco

def myfunc():

    print 'myfunc() called'

myfunc()

执行结果

[图片]