信号量（Semaphore）
信号量用来控制线程并发数的，BoundeSemaphore或Semaphore管理一个内置的计数器，每当调用acquire()时-1，调用release()时+1。

计数器不能小于0，当计数器为0时，acquire()将阻塞线程至同步锁定装填，直到其他线程调用release()。

BoundeSemaphore与Semaphore时唯一区别在于前者将在调用release()时检查计数器的值是否超过了计数器的初始值，如果超过了就跑出一个异常。

#!/usr/bin/env python3.8
# -*- coding: UTF-8 -*-
# __author: smoke
# file: Semaphore
# time: 2021/04/05

import threading,time

class myThread(threading.Thread):
    def run(self):
        if semaphore.acquire():
            print(self.name)
            time.sleep(3)
            semaphore.release()

if __name__ == "__main__":
    semaphore = threading.BoundedSemaphore(5)  #创建信号量锁,5代表多少个锁数量
    thrs=[]
    for i in range(100):
        thrs.append(myThread())
    for t in thrs:
        t.start()

/home/smoke/文档/DocumentFile/PycharmProjects/pythonProject/venv/bin/python /home/smoke/文档/DocumentFile/PycharmProjects/pythonProject/join/Semaphore.py
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Thread-9
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Process finished with exit code 0