對於程序來說，如果主進程在子進程還未結束時就已經退出，那么Linux內核會將子進程的父進程ID改為1（也就是init進程），當子進程結束后會由init進程來回收該子進程。
主線程退出后子線程的狀態依賴於它所在的進程，如果進程沒有退出的話子線程依然正常運轉。如果進程退出了，那么它所有的線程都會退出，所以子線程也就退出了。
主線程退出，進程等待所有子線程執行完畢后才結束
進程啟動后會默認產生一個主線程，默認情況下主線程創建的子線程都不是守護線程（setDaemon(False)）。因此主線程結束后，子線程會繼續執行，進程會等待所有子線程執行完畢后才結束
所有線程共享一個終端輸出（線程所屬進程的終端）
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()
輸出為：
parent_thread_running...
parent_thread_exit...
child_thread1_running...
child_thread1_running...
child_thread1_running...
child_thread1_running...
...
可見父線程結束后，子線程仍在運行，此時結束進程，子線程才會被終止
主線程結束后進程不等待守護線程完成，立即結束
當設置一個線程為守護線程時，此線程所屬進程不會等待此線程運行結束，進程將立即結束
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()
輸出：
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是守護線程，thread2非守護線程，因此，進程會等待thread2完成后結束，而不會等待thread1完成
注意：子線程會繼承父線程中daemon的值，即守護線程開啟的子線程仍是守護線程
主線程等待子線程完成后結束
在線程A中使用B.join()表示線程A在調用join()處被阻塞，且要等待線程B的完成才能繼續執行
'''
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'''
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()
輸出：
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
主線程在執行到thread2.join()時被阻塞，等待thread2結束后才會執行下一句
1/0 會使主線程報錯退出，且thread1設置了daemon=True，因此主線程意外退出時thread1也會立即結束。thread1.join()沒有被主線程執行