python主线程与子线程的结束顺序

引用自 主线程退出对子线程的影响--YuanLi 的一段话:

对于程序来说,如果主进程在子进程还未结束时就已经退出,那么Linux内核会将子进程的父进程ID改为1(也就是init进程),当子进程结束后会由init进程来回收该子进程。

主线程退出后子线程的状态依赖于它所在的进程,如果进程没有退出的话子线程依然正常运转。如果进程退出了,那么它所有的线程都会退出,所以子线程也就退出了。

主线程退出,进程等待所有子线程执行完毕后才结束

进程启动后会默认产生一个主线程,默认情况下主线程创建的子线程都不是守护线程(setDaemon(False))。因此主线程结束后,子线程会继续执行,进程会等待所有子线程执行完毕后才结束

所有线程共享一个终端输出(线程所属进程的终端)

import threading
import time


def child_thread1():
    for i in range(100):
        time.sleep(1)
        print('child_thread1_running...')


def parent_thread():
    print('parent_thread_running...')
    thread1 = threading.Thread(target=child_thread1)
    thread1.start()
    print('parent_thread_exit...')


if __name__ == "__main__":
    parent_thread()

输出为:

parent_thread_running...
parent_thread_exit...
child_thread1_running...
child_thread1_running...
child_thread1_running...
child_thread1_running...
...

可见父线程结束后,子线程仍在运行,此时结束进程,子线程才会被终止

主线程结束后进程不等待守护线程完成,立即结束

当设置一个线程为守护线程时,此线程所属进程不会等待此线程运行结束,进程将立即结束

import threading
import time


def child_thread1():
    for i in range(100):
        time.sleep(1)
        print('child_thread1_running...')


def child_thread2():
    for i in range(5):
        time.sleep(1)
        print('child_thread2_running...')


def parent_thread():
    print('parent_thread_running...')
    thread1 = threading.Thread(target=child_thread1)
    thread2 = threading.Thread(target=child_thread2)
    thread1.setDaemon(True)
    thread1.start()
    thread2.start()
    print('parent_thread_exit...')


if __name__ == "__main__":
    parent_thread()

输出:

parent_thread_running...
parent_thread_exit...
child_thread1_running...child_thread2_running...

child_thread1_running...child_thread2_running...

child_thread1_running...child_thread2_running...

child_thread1_running...child_thread2_running...

child_thread2_running...child_thread1_running...

Process finished with exit code 0

thread1是守护线程,thread2非守护线程,因此,进程会等待thread2完成后结束,而不会等待thread1完成

注意:子线程会继承父线程中daemon的值,即守护线程开启的子线程仍是守护线程

主线程等待子线程完成后结束

在线程A中使用B.join()表示线程A在调用join()处被阻塞,且要等待线程B的完成才能继续执行

import threading
import time


def child_thread1():
    for i in range(10):
        time.sleep(1)
        print('child_thread1_running...')


def child_thread2():
    for i in range(5):
        time.sleep(1)
        print('child_thread2_running...')


def parent_thread():
    print('parent_thread_running...')
    thread1 = threading.Thread(target=child_thread1)
    thread2 = threading.Thread(target=child_thread2)
    thread1.setDaemon(True)
    thread2.setDaemon(True)
    thread1.start()
    thread2.start()
    thread2.join()
    1/0
    thread1.join()
    print('parent_thread_exit...')


if __name__ == "__main__":
    parent_thread()

输出:

parent_thread_running...
child_thread1_running...
child_thread2_running...
child_thread1_running...
child_thread2_running...
child_thread1_running...
child_thread2_running...
child_thread1_running...
child_thread2_running...
child_thread1_running...
child_thread2_running...
Traceback (most recent call last):
  File "E:/test_thread.py", line 31, in <module>
    parent_thread()
  File "E:/test_thread.py", line 25, in parent_thread
    1/0
ZeroDivisionError: integer division or modulo by zero

主线程在执行到thread2.join()时被阻塞,等待thread2结束后才会执行下一句

1/0 会使主线程报错退出,且thread1设置了daemon=True,因此主线程意外退出时thread1也会立即结束。thread1.join()没有被主线程执行

posted @ 2019-11-27 19:17  luozx207  阅读(9265)  评论(3编辑  收藏  举报