python 单例模式

#!/usr/bin/env python3
# _*_ coding:utf-8 _*_
# ========================================================
# Module         :  singleton
# Author         :  luting
# Create Date    :  2018/6/3
# Amended by     :  luting
# Amend History  :  2018/6/3
# ========================================================


# 单例    => 确保类只有一个对象被创建, 为对象提供一个访问点，以使程序可以全局访问该对象。控制共享资源的并行访问
# 使构造函数私有化，并创建一个静态方法来完成对象的初始化，这样，对象在第一次被创建时，此后，这个类将返回同一个对象

# 经典单例模式实现
class Singleton(object):

    def __new__(cls):
        if not hasattr(cls, 'instance'):
            cls.instance = super(Singleton, cls).__new__(cls)
        return cls.instance

s = Singleton()
print(s)
s1 = Singleton()
print(s1)


# 懒汉式实例化
class Singleton:

    __instance = None

    def __init__(self):
        if not Singleton.__instance:
            print('__init__ method called')
        else:
            print('instance already created', self.get_instance())

    @classmethod
    def get_instance(cls):
        if not cls.__instance:
            cls.__instance = Singleton()
        return cls.__instance

s = Singleton()
print(s)
s1 = Singleton()
print(s1)


# Monostate单例模式
class Borg:

    __shared_state = {'1': '2'}

    def __init__(self):
        self.x = 1
        self.__dict__ = self.__shared_state
        pass

b = Borg()
b1 = Borg()
b.x = 4
print(b, b1, b.__dict__, b1.__dict__)


class Borg(object):

    __shared_state = {}

    def __new__(cls, *args, **kwargs):
        obj = super(Borg, cls).__new__(cls, *args, **kwargs)
        obj.__dict = cls.__shared_state
        return obj


# 单例元类
class MetaSingleton(type):

    __instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls.__instances:
            cls.__instances[cls] = super(MetaSingleton, cls).__call__(*args, **kwargs)
            return cls.__instances[cls]
        

class Login(metaclass=MetaSingleton):
    pass

l = Login()
l1 = Login()
print(l, l1)