Python线程执行顺序异常，如何让指定线程执行完再执行后续逻辑？

最近学习Python线程时遇到如下问题：需要让线程中的one函数执行完毕后，再执行two函数的收尾逻辑（打印'func two finished'），但join方法无法生效，显然是three函数中的while True循环导致的，尝试使用queue也没有效果。

原代码 #

from threading import Thread,Event
import time

def one():
    event.set()
    thr.join()
    for i in range(3):
        print('some print')
        time.sleep(1)

def two():
    t = Thread(target=one)
    t.start()
    #with t.join() here the program does not work at all, same thing with event.set()
    print('func two finished')

def three(callback, event):
    c = 0
    while True:
        c += 1
        time.sleep(1)
        print('func 3 is working')
        if c == 5:
            two()
        if event.is_set():
            callback(c)
            print('func 3 is stopped')
            break

def callback(t):
    print('callback ',t)

def thread(callback):
    global event, thr
    event = Event()
    thr = Thread(target=three, args=(callback, event,))
    thr.start()
    thr.join()

thread(callback)

当前输出 #

func 3 is working
func 3 is working
func 3 is working
func 3 is working
func 3 is working
func two finished
callback  5
func 3 is stopped
some print
some print
some print

期望输出 #

func 3 is working
func 3 is working
func 3 is working
func 3 is working
func 3 is working
callback  5
func 3 is stopped
some print
some print
some print
func two finished

问题修复 #

核心问题分析 #

1. one函数中调用thr.join()会触发死锁：thr是three所在的线程，此时three仍在运行，one等待它结束，three又在等待event触发，形成互相阻塞。
2. two函数启动one线程后直接打印收尾信息，没有等待one执行完毕，导致收尾信息提前输出。
3. 全局变量thr的冗余使用增加了线程间的耦合和出错概率。

修复后的代码 #

import time
from threading import Thread, Event

def one(event):
    event.set()  # 通知three线程退出
    for i in range(3):
        print('some print')
        time.sleep(1)

def two():
    t = Thread(target=one, args=(event,))
    t.start()
    t.join()  # 等待one线程执行完毕后再打印收尾
    print('func two finished')

def three(callback, event):
    c = 0
    while True:
        c += 1
        time.sleep(1)
        print('func 3 is working')
        if c == 5:
            two()
        if event.is_set():
            callback(c)
            print('func 3 is stopped')
            break

def callback(t):
    print('callback ', t)

def thread(callback):
    global event
    event = Event()
    thr = Thread(target=three, args=(callback, event,))
    thr.start()
    thr.join()

thread(callback)

关键改动说明 #

• 移除one中错误的thr.join()调用，消除死锁隐患。
• 在two中对one对应的线程调用t.join()，确保one执行完毕后再输出收尾信息。
• 删除全局变量thr的冗余定义，简化线程间的引用关系。

运行修复后的代码，输出顺序将与期望完全一致。

内容的提问来源于stack exchange，提问作者Аким