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