面试-python单例模式实现

概念:
一个对象只能最多存在一个实例

优点:
节省资源空间

缺点:
多线程失效，需要加锁解决

两种实现方式:

需要用到的知识点
1.单例模式思想

2.装饰器

3.类对象
4.__new__方法

4.with语句
5.多线程资源竞争使单例模式失效，导致脏读，使用锁解决

# 方式1：装饰器实现 非线程安全
def singleton(cls):
    _instances = {}
    def wrapper(*args, **kwargs):
        if cls not in _instances:
            _instances[cls] = cls(*args, **kwargs)
        return _instances[cls]
    return wrapper
@singleton
class ConfigManager:
    def __init__(self):
        self.settings = {}

# 方式1：装饰器实现 线程安全
import threading
def singleton(cls):
    __instances = {}
    __lock = threading.Lock()
    def wrapper(*args, **kwargs):
			with __lock:
		       if cls not in _instances:
		           _instances[cls] = cls(*args, **kwargs)
		       return _instances[cls]
    return wrapper
    
@singleton
class ConfigManager:
    def __init__(self):
        self.settings = {}

# 方式2：__new__方法重写    重点++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
class CacheManager:
    _instance = None
    def __new__(cls):
        if not cls._instance:
            cls._instance = super().__new__(cls)
            cls._instance._cache = {}
        return cls._instance

# 方式2：线程安全版（带锁）
from threading import Lock

class ThreadSafeSingleton:
    _instance = None
    _lock = Lock()
    
    def __new__(cls):
        if not cls._instance:
            with cls._lock:
                if not cls._instance:
                    cls._instance = super().__new__(cls)
        return cls._instance

方式3:
所有模块导入的类也是默认单例