单例模式

import threading

class Singleton(object): 
    def __init__(self):
        pass
    @classmethod 
    def instance(cls, *args, **kwargs): 
        if not hasattr(Singleton, "_instance"): 
        Singleton._instance = Singleton(*args, **kwargs) 
    return Singleton._instance
    
obj = Singleton.instance()

import time
import threading
class Singleton(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        time.sleep(1)

    @classmethod
    def instance(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    Singleton._instance = Singleton(*args, **kwargs)
        return Singleton._instance


# 第一次调用
def task(arg):
    obj = Singleton.instance()
    print(obj)
for i in range(10):
    t = threading.Thread(target=task,args=[i,])
    t.start()


# 第二次调用    
time.sleep(20)
obj = Singleton.instance()