[Python知识库]Python线程—对守护线程的执行理解

在说守护线程之前，先回归下线程的执行 threading模块

1. 没有使用join时

import threading, time

def run(i):
    print('Thread %s is start' % i)
    time.sleep(2)
    print('thread %s is end.......' % i)

for i in range(5):
    t = threading.Thread(target=run, args=(i,))
    t.start()  # 启动线程

print('主线程，all threading done...........', threading.currentThread())

执行结果：

Thread 0 is start
Thread 1 is start
Thread 2 is start
Thread 3 is start
Thread 4 is start主线程，all threading done........... <_MainThread(MainThread, started 20736)>
thread 3 is end.......
thread 1 is end.......
thread 0 is end.......
thread 4 is end.......
thread 2 is end.......

结论：子线程没有执行完时，到主线程时，主线程就可以执行，然后直到子线程执行结束，到此程序结束。所以看到的主线程的内容在中间部分

2. 使用join时

import threading, time

def run(i):
    print('Thread %s is start' % i)
    time.sleep(2)
    print('thread %s is end.......' % i)


thread_obj = []

for i in range(5):
    t = threading.Thread(target=run, args=(i,))
    t.start()  # 启动线程
    thread_obj.append(t)

for thread in thread_obj: 
    thread.join()  # 加join

print('主线程，all threading done...........', threading.currentThread())

加join执行结果：

Thread 0 is start
Thread 1 is start
Thread 2 is start
Thread 3 is start
Thread 4 is start

thread 4 is end.......
thread 3 is end.......
thread 0 is end.......
thread 2 is end.......
thread 1 is end.......

主线程，all threading done........... <_MainThread(MainThread, started 14492)>

结论：加了join之后，其它线程需要等待该线程执行结束，上述代码中，主线程会等所有子线程执行完毕后才会执行,所以 all threading done最后才会看到.

不要这样写：否则就是串行的

import threading, time

def run(i):
    print('Thread %s is start' % i)
    time.sleep(2)
    print('thread %s is end.......' % i)

for i in range(5):
    t = threading.Thread(target=run, args=(i,))
    t.start()  # 启动线程
    thread.join()  # 每启动一个线程，其它线程就需要等待该线程，此时这5个线程就变成了串行，而不是并行。


print('主线程，all threading done...........', threading.currentThread())

# 执行顺序：线程1 -> 2 -> 3 -> 4 -> 5 ->主线程

3. 使用守护线程 setDaemon(True)

import threading, time

def run(i):
    print('Thread %s is start' % i)
    time.sleep(2)
    print('thread %s is end.......' % i)


for i in range(5):
    t = threading.Thread(target=run, args=(i,))
    t.setDaemon(True)  # 将5个子线程变为守护线程，守护主线程
    t.start()  # 启动线程
    thread_obj.append(t)

print('主线程，all threading done...........', threading.currentThread())

执行结果：

Thread 0 is start
Thread 1 is start
Thread 2 is start
Thread 3 is start
Thread 4 is start

主线程，all threading done........... <_MainThread(MainThread, started 11496)>

结论：守护线程就是主线程一旦执行结束，守护线程就要立刻退出，因为主线程不存在的话，就不存在守护。all threading done最后看到，即子线程只执行了一部分就退出了。

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