单例设计模式

单例模式

# 意图
- 确保类有且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
- 控制和共享资源的并行访问。

# 使用场景
日志记录，数据库操作等

# 实现方法
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，对象将在第一次调用时创建，此后，这个类将返回同一个对象

# 示例
class Singleton(object):
    def __new__(cls):
        if not hasattr(cls, 'instance'):
            cls.instance = super().__new__(cls)
        return cls.instance
    
s = Singleton()
print("Object created", s)
s1 = Singleton()
print("Object created", s1)
"""
object created <__main__.Singleton object at 0x000001DC119DB320>
object created <__main__.Singleton object at 0x000001DC119DB320>
"""

数据库单例模式示例

import sqlite3
class MetaSingleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]
    
class Database(metaclass=MetaSingleton):
    connection = None
    def connect(self):
        if self.connection is None:
            self.connection = sqlites.connect("db.sqlite3")
            self.cursorobj = self.connection.cursor()
        return self.cursorobj
    
db1 = Database().connect() #Database()实例化时将调用元类MetaSingleton的__call__方法
db2 = Database().connect()
print("Database Objects DB1", db1)
print("Database Objects DB2", db2)
"""
Database Objects DB1 <sqlite3.Cursor object at 0x0000024C23774DC0>
Database Objects DB2 <sqlite3.Cursor object at 0x0000024C23774DC0>
"""

监控服务单例模式示例

# 说明
有一个被监控的服务器列表。当一个服务器从这个列表中删除时，监控软件应该察觉到这一情况，并从被监控的服务器列表中将其删除，以上情况变动由同一个对象来完成

#
class HealthCheck:
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not HealthCheck._instance:
            HealthCheck._instance = super().__new__(cls, *args, **kwargs)
        return HealthCheck._instance
    
    def __init__(self):
        self._servers = []
    def addServer(self):
        self._servers.append("Server 1")
        self._servers.append("Server 2")
        self._servers.append("Server 3")
        self._servers.append("Server 4")
    
    def changeServer(self):
        self._servers.pop()
        self._servers.append("Server 5")
        
hc1 = HealthCheck()
hc2 = HealthCheck()
hc1.addServer(0)
print("Schedule health check for servers (1) ...")
for i in range(4):
    print("Checking", hc1._servers[i])

hc2.changeServer()
print("Schedule health check for server (2)...")
for i in range(4):
    print("Checking", hc2._servers[i])