单例模式的python实现

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# 本实例主要介绍单例模式 # 1.什么是单例模式   # 1. 确保有且只有一个对象被创建   # 2. 为对象提供一个访问点,以使程序可以全局访问该对象   # 3. 控制共享资源的并行访问   # 2.单例模式实例 class Singleton(object):     def __new__(cls):         if not hasattr(cls, 'instance'):             cls.instance = super(Singleton, cls).__new__(cls)         return cls.instance   s = Singleton() print("Object created",s) s1 = Singleton() print("Object created",s1)                       # 3.单例设计模式中的懒汉式实例化 class Singleton1:     __instance = None     def __init__(self):         if not Singleton1.__instance:             print("__init__ method called")         else:             print("Instance already created", self.getInstance())           @classmethod     def getInstance(cls):         if not cls.__instance:             cls.__instance = Singleton1()         return cls.__instance   s = Singleton1() print("Object created", Singleton1.getInstance()) s1 = Singleton1()         # 4.Monostate(Borg) 单态模式 # 所有对象共享相同状态,改变一个实例的状态,另一个实例也会改变 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("Borg object b", b) print("Borg object b1", b1) print("Object state b", b.__dict__) print("Object state b1", b1.__dict__)   # 基于__new__方法本身实现Borg class BorgNew:     __shared_state = {}     def __new__(cls, *args, **kwargs):         obj = super(BorgNew , cls).__new__(cls, *args, **kwargs)         obj.__dict__ = cls.__shared_state         return obj   b = BorgNew() b1 = BorgNew() b.x = 4   print("Borg object b", b) print("Borg object b1", b1) print("Object state b", b.__dict__) print("Object state b1", b1.__dict__)                 # 5.单例和元类 # 元类: 元类是一个类的类,这意味着该类是它元类的一个实例 class MyInt(type):     def __call__(cls, *args, **kwargs):         print("******Here is MyInt******",args)         print("How do whatever you want with these objects...")         return type.__call__(cls, *args, **kwargs)   class int(metaclass=MyInt):     def __init__(self, x, y):         self.x = x         self.y = y   i = int(4, 5)   # 基于元类的单例实现 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 Logger(metaclass=MetaSingleton):     pass   logger1 = Logger() logger2 = Logger() print(logger1, logger2)         # 6.单例模式的应用     # A. 数据库的操作 import sqlite3 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 Database(metaclass=MetaSingleton):     connection = None     def connect(self):         if self.connection is None:             self.connection = sqlite3.connect('db.sqlite3')             self.cursorobj = self.connection.cursor()         return self.cursorobj   db1 = Database().connect() db2 = Database().connect()   print("Database objcursor db1", db1) print("Database objcursor db2", db2)   # 单一的应用可以使用该方法,节约CPU,内存等资源 # 集群化应用, 就需要使用数据库连接池       # B.监控服务 class HealthCheck:     _instances = None     def __new__(cls, *args, **kwargs):         if not HealthCheck._instances:             HealthCheck._instances = super(HealthCheck, cls).__new__(cls, *args, **kwargs)         return HealthCheck._instances           def __init__(self):         self._servers = []           def addServer(self):         self._servers.append("server1")         self._servers.append("server2")         self._servers.append("server3")         self._servers.append("server4")           def changeServer(self):         self._servers.pop()         self._servers.append("server5")   hc1 = HealthCheck() hc2 = HealthCheck() print(hc1,hc2)   hc1.addServer() for i in range(4):     print("checking ", hc1._servers[i])   hc2.changeServer() for i in range(4):     print("checking ", hc2._servers[i])                                     # 7.单例模式的优缺点 # 优点: 1. 创建有且只有一个对象, 2.节省CPU,内存资源. # 缺点: 1. 全局变量被误改, 但是别的对象还在引用 2.同一个对象,创建多个引用 3.可能影响其他类