Python 四种单例实现方法

1.py

#-*- coding: utf-8 -*-

"""
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()

print one.a
print two.a

print one._instance
print two._instance

print one is two

print one.__dict__
print two.__dict__

print Singleton.__dict__

2.py

#-*- coding: utf-8 -*-

"""
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()

two.a = 3
print one.a
print id(one)
print id(two)
print one == two
print one is two
print id(one.__dict__)
print id(two.__dict__)

3.py

#-*- coding: utf-8 -*-

"""
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
print id(one)
print id(two)
print one == two
print one is two

4.py

#-*- coding: utf-8 -*-

"""
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.

本人更喜欢这种方法.

"""

def singleton(cls):
    instances = {}
    def _singleton(*args, **kw):
        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
print id(one)
print id(two)
print one == two
print one is two
one.x = 1
print one.x
print two.x