关闭Tkinter应用时线程无法正常结束的问题排查

我用Python的Tkinter开发了一个仅含单个按钮的计时器，按钮同时显示剩余时间，计时器通过线程更新按钮文本。线程通过while循环运行，设置Event事件时循环终止。关闭窗口时用protocol方法调用函数设置Event并join线程，多数情况正常，但特定时机关闭程序时，线程会在窗口关闭后继续运行。

我不想使用thread.stop()方法，简化后的复现代码如下：

import tkinter as tk
from tkinter import TclError, ttk
from datetime import timedelta
import time
import threading
from threading import Event

def strfdelta(tdelta):
    # Includes microseconds
    hours, rem = divmod(tdelta.seconds, 3600)
    minutes, seconds = divmod(rem, 60)
    return str(hours).rjust(2, '0') + ":" + str(minutes).rjust(2, '0') + \
           ":" + str(seconds).rjust(2, '0') + ":" + str(tdelta.microseconds).rjust(6, '0')[0:2]

class App(tk.Tk):
    def __init__(self):
        super().__init__()
        self.is_running = False
        is_closing = Event()
        self.start_time = timedelta(seconds=4, microseconds=10, minutes=0, hours=0)
        self.current_start_time = self.start_time
        self.time_of_last_pause = time.time()
        self.time_of_last_unpause = None
        # region guisetup
        self.time_display = None
        self.geometry("320x110")
        self.title('Replace')
        self.resizable(False, False)
        box1 = self.create_top_box(self)
        box1.place(x=0, y=0)
        # endregion guisetup
        self.timer_thread = threading.Thread(target=self.timer_run_loop, args=(is_closing, ))
        self.timer_thread.start()

        def on_close():  # This occasionally fails when we try to close.
            is_closing.set()  # This used to be a boolean property self.is_closing. Making it an event didn't help.
            print("on_close()")
            try:
                self.timer_thread.join(timeout=2)
            finally:
                if self.timer_thread.is_alive():
                    self.timer_thread.join(timeout=2)
                    if self.timer_thread.is_alive():
                        print("timer thread is still alive again..")
                    else:
                        print("timer thread is finally finished")
                else:
                    print("timer thread finished2")
            self.destroy()
        self.protocol("WM_DELETE_WINDOW", on_close)

    def create_top_box(self, container):
        box = tk.Frame(container, height=110, width=320)
        box_m = tk.Frame(box, bg="blue", width=320, height=110)
        box_m.place(x=0, y=0)
        self.time_display = tk.Button(box_m, text=strfdelta(self.start_time), command=self.toggle_timer_state)
        self.time_display.place(x=25, y=20)
        return box

    def update_shown_time(self, time_to_show: timedelta = None):
        print("timer_run_loop must finish. flag 0015")  # If the window closes at this point, everything freezes
        self.time_display.configure(text=strfdelta(time_to_show))
        print("timer_run_loop must finish. flag 016")

    def toggle_timer_state(self):
        # update time_of_last_unpause if it has never been set
        if not self.is_running and self.time_of_last_unpause is None:
            self.time_of_last_unpause = time.time()
        if self.is_running:
            self.pause_timer()
        else:
            self.start_timer_running()

    def pause_timer(self):
        pass  # Uses self.time_of_last_unpause, Alters self.is_running, self.time_of_last_pause, self.current_start_time

    def timer_run_loop(self, event):
        while not event.is_set():
            if not self.is_running:
                print("timer_run_loop must finish. flag 008")
                self.update_shown_time(self.current_start_time)
            print("timer_run_loop must finish. flag 018")
        print("timer_run_loop() ending")

    def start_timer_running(self):
        pass  # Uses self.current_start_time; Alters self.is_running, self.time_of_last_unpause

if __name__ == "__main__":
    app = App()
    app.mainloop()

无需点击按钮，多次按Alt+F4即可触发问题。问题出现时最后打印内容为“timer_run_loop must finish. flag 0015”，说明self.time_display.configure(text=strfdelta(time_to_show))未执行完成，推测线程调用Tkinter按钮的configure方法时关闭窗口引发了问题。

想知道是否有更合适的资源共享模式，或者更优的线程终止方式？

1. 遵循Tkinter线程安全原则：线程不直接操作UI #

Tkinter的所有GUI操作必须在主线程执行，线程只负责时间计算逻辑，通过主线程的after()方法更新UI：

• 修改线程逻辑，仅更新时间变量，不调用UI操作方法
• 在主窗口中添加定时任务，由主线程检查时间变化并更新按钮文本

修改后的timer_run_loop：

def timer_run_loop(self, event):
    while not event.is_set():
        if not self.is_running:
            # 仅更新时间变量，交给主线程处理UI
            self.current_display_time = self.current_start_time
        time.sleep(0.01)  # 降低循环频率，减少资源占用
        print("timer_run_loop must finish. flag 018")
    print("timer_run_loop() ending")

在App类的__init__中添加主线程UI更新循环：

def __init__(self):
    # ... 原有代码 ...
    self.current_display_time = self.start_time
    self.update_ui()

def update_ui(self):
    try:
        self.time_display.configure(text=strfdelta(self.current_display_time))
    except TclError:
        # 窗口已销毁，终止定时更新
        return
    # 每10ms触发一次UI更新
    self.after(10, self.update_ui)

2. 在线程中操作UI时捕获TclError #

如果必须在线程中临时操作UI，必须捕获窗口销毁时触发的TclError，避免线程因异常卡住：
修改update_shown_time方法：

def update_shown_time(self, time_to_show: timedelta = None):
    print("timer_run_loop must finish. flag 0015")
    try:
        self.time_display.configure(text=strfdelta(time_to_show))
    except TclError:
        # 窗口已销毁，直接返回
        pass
    print("timer_run_loop must finish. flag 016")

3. 优化窗口关闭逻辑，避免线程阻塞 #

将on_close中的join和销毁逻辑拆分，用after()让主线程先处理完当前事件，再尝试join线程：

def __init__(self):
    # ... 原有代码 ...
    self.is_closing = Event()  # 把is_closing设为实例变量，方便其他方法访问
    self.timer_thread = threading.Thread(target=self.timer_run_loop, args=(self.is_closing, ))
    self.timer_thread.start()

    def on_close():
        self.is_closing.set()
        print("on_close()")
        # 延迟执行join和销毁，给线程处理剩余逻辑的时间
        self.after(100, self.join_and_destroy)

    self.protocol("WM_DELETE_WINDOW", on_close)

def join_and_destroy(self):
    self.timer_thread.join(timeout=1)
    if self.timer_thread.is_alive():
        print("timer thread is still alive, force exiting")
    else:
        print("timer thread finished")
    self.destroy()

内容的提问来源于stack exchange，提问作者wimworks