python 线程thread详解

join详解

看到代码示例时，都会出现一个join，这个作用如何？先看结论

1. 阻塞主程序，专注于执行多线程中的程序

2. 多线程多join的情况下，依次执行各线程的join方法，子线程全部结束了才能执行主线程。

3. join无入参，则等待到该执行绪结束，才开始执行下一个线程的join。

4. 入参timeout为线程的阻塞时间，如timeout=2 就是按照着这个线程2s 以后，就不管他了，继续执行下面的程序。

观察下面的代码，

import threading, time

def worker(interval):
    print("worker start at "+time.strftime('%H:%M:%S'))
    time.sleep(interval)
    print('finish worker, ',time.strftime('%H:%M:%S'))


def master(interval):
    print("master start at "+time.strftime('%H:%M:%S'))
    time.sleep(interval)
    print('finish master, ', time.strftime('%H:%M:%S'))

if __name__ == '__main__':
    ts = []
    t1 = threading.Thread(target=worker, args=(3,))
    t2 = threading.Thread(target=master, args=(5,))
    ts.append(t1)
    ts.append(t2)
    t2.start()
    t1.start()
  
    print('End main thread, '+time.strftime('%H:%M:%S'))

代码里没有join时，上述有起了三个线程，分别是main，t1，t2，他们会各自同时并发执行各自的代码

master start at 16:29:08
worker start at 16:29:08
End main thread, 16:29:08
finish worker,  16:29:11
finish master,  16:29:13

如果我们加上join，原本各自为政的三个线程，就有了秩序

import threading, time

def worker(interval):
    print("worker start at "+time.strftime('%H:%M:%S'))
    time.sleep(interval)
    print('finish worker, ',time.strftime('%H:%M:%S'))
def master(interval):
    print("master start at "+time.strftime('%H:%M:%S'))
    time.sleep(interval)
    print('finish master, ', time.strftime('%H:%M:%S'))

if __name__ == '__main__':
    ts = []
    t1 = threading.Thread(target=worker, args=(3,))
    t2 = threading.Thread(target=master, args=(5,))
    ts.append(t1)
    ts.append(t2)
    t2.start()
    t1.start()
    for thread in ts:
        thread.join()
    print('End main thread, '+time.strftime('%H:%M:%S'))

master start at 16:32:43
worker start at 16:32:43
finish worker,  16:32:46
finish master,  16:32:48
End main thread, 16:32:48

main线程只有当所有子线程完成后才能执行，StackOverflow里有段很好的总结：

If, for example, you want to concurrently download a bunch of pages to concatenate them into a single large page, you may start concurrent downloads using threads, but need to wait until the last page/thread is finished before you start assembling a single page out of many. That’s when you use join().

那如果join()里带了参数呢？此时的子线程表示主线程你只要最多等我多长时间，你就可以自便了，这里参数改为join(2)：

master start at 16:35:42
worker start at 16:35:42
finish worker,  16:35:45
End main thread, 16:35:46
finish master,  16:35:47

可以看到main线程等了4秒，就管自己执行后续的代码了，而main最后才完成自己的程序，这里为何是四秒？因为两个线程分别都告诉了主线程等他2秒，总共4秒，你也可以拆开，让每个线程join不同的时间
Python 多執行緒 thread join() 的作用