创建型模式-单例模式（使用元类实现）

# 创建型模式-单例模式（使用元类实现）
# 线程锁：防止创建多个实例
# 双重检查：实例创建后，防止重复加锁占用资源
# __call__作用：__call__方法使一个类的实例像方法一样调用，当我们调用一个实例时，实际就是调用了它的__call__方法
import time
from threading import Lock
from concurrent.futures import ThreadPoolExecutor


class Singletion(type):
_lock = Lock()
_instance = None

def __call__(cls, *args, **kwargs):
if not cls._instance:
with cls._lock:
if not cls._instance:
time.sleep(1)
cls._instance = super(Singletion, cls).__call__(*args, **kwargs)
return cls._instance


class Test(metaclass=Singletion):
pass


def get_instance():
b = Test()
print(id(b))


if __name__ == "__main__":
poll = ThreadPoolExecutor(max_workers=5)
for i in range(10):
poll.submit(get_instance)