Tkinter: How to use threads to preventing main event loop from "freezing"

When you join the new thread in the main thread, it will wait until the thread finishes, so the GUI will block even though you are using multithreading.

If you want to place the logic portion in a different class, you can subclass Thread directly, and then start a new object of this class when you press the button. The constructor of this subclass of Thread can receive a Queue object and then you will be able to communicate it with the GUI part. So my suggestion is:

1. Create a Queue object in the main thread
2. Create a new thread with access to that queue
3. Check periodically the queue in the main thread

Then you have to solve the problem of what happens if the user clicks two times the same button (it will spawn a new thread with each click), but you can fix it by disabling the start button and enabling it again after you call self.prog_bar.stop().

import queueclass GUI:    # ...    def tb_click(self):        self.progress()        self.prog_bar.start()        self.queue = queue.Queue()        ThreadedTask(self.queue).start()        self.master.after(100, self.process_queue)    def process_queue(self):        try:            msg = self.queue.get_nowait()            # Show result of the task if needed            self.prog_bar.stop()        except queue.Empty:            self.master.after(100, self.process_queue)class ThreadedTask(threading.Thread):    def __init__(self, queue):        super().__init__()        self.queue = queue    def run(self):        time.sleep(5)  # Simulate long running process        self.queue.put("Task finished")

I will submit the basis for an alternate solution. It is not specific to a Tk progress bar per se, but it can certainly be implemented very easily for that.

Here are some classes that allow you to run other tasks in the background of Tk, update the Tk controls when desired, and not lock up the gui!

Here's class TkRepeatingTask and BackgroundTask:

import threadingclass TkRepeatingTask():    def __init__( self, tkRoot, taskFuncPointer, freqencyMillis ):        self.__tk_   = tkRoot        self.__func_ = taskFuncPointer                self.__freq_ = freqencyMillis        self.__isRunning_ = False    def isRunning( self ) : return self.__isRunning_     def start( self ) :         self.__isRunning_ = True        self.__onTimer()    def stop( self ) : self.__isRunning_ = False    def __onTimer( self ):         if self.__isRunning_ :            self.__func_()             self.__tk_.after( self.__freq_, self.__onTimer )class BackgroundTask():    def __init__( self, taskFuncPointer ):        self.__taskFuncPointer_ = taskFuncPointer        self.__workerThread_ = None        self.__isRunning_ = False    def taskFuncPointer( self ) : return self.__taskFuncPointer_    def isRunning( self ) :         return self.__isRunning_ and self.__workerThread_.isAlive()    def start( self ):         if not self.__isRunning_ :            self.__isRunning_ = True            self.__workerThread_ = self.WorkerThread( self )            self.__workerThread_.start()    def stop( self ) : self.__isRunning_ = False    class WorkerThread( threading.Thread ):        def __init__( self, bgTask ):                  threading.Thread.__init__( self )            self.__bgTask_ = bgTask        def run( self ):            try :                self.__bgTask_.taskFuncPointer()( self.__bgTask_.isRunning )            except Exception as e: print repr(e)            self.__bgTask_.stop()

Here's a Tk test which demos the use of these. Just append this to the bottom of the module with those classes in it if you want to see the demo in action:

def tkThreadingTest():    from tkinter import Tk, Label, Button, StringVar    from time import sleep    class UnitTestGUI:        def __init__( self, master ):            self.master = master            master.title( "Threading Test" )            self.testButton = Button(                 self.master, text="Blocking", command=self.myLongProcess )            self.testButton.pack()            self.threadedButton = Button(                 self.master, text="Threaded", command=self.onThreadedClicked )            self.threadedButton.pack()            self.cancelButton = Button(                 self.master, text="Stop", command=self.onStopClicked )            self.cancelButton.pack()            self.statusLabelVar = StringVar()            self.statusLabel = Label( master, textvariable=self.statusLabelVar )            self.statusLabel.pack()            self.clickMeButton = Button(                 self.master, text="Click Me", command=self.onClickMeClicked )            self.clickMeButton.pack()            self.clickCountLabelVar = StringVar()                        self.clickCountLabel = Label( master,  textvariable=self.clickCountLabelVar )            self.clickCountLabel.pack()            self.threadedButton = Button(                 self.master, text="Timer", command=self.onTimerClicked )            self.threadedButton.pack()            self.timerCountLabelVar = StringVar()                        self.timerCountLabel = Label( master,  textvariable=self.timerCountLabelVar )            self.timerCountLabel.pack()            self.timerCounter_=0            self.clickCounter_=0            self.bgTask = BackgroundTask( self.myLongProcess )            self.timer = TkRepeatingTask( self.master, self.onTimer, 1 )        def close( self ) :            print "close"            try: self.bgTask.stop()            except: pass            try: self.timer.stop()            except: pass                        self.master.quit()        def onThreadedClicked( self ):            print "onThreadedClicked"            try: self.bgTask.start()            except: pass        def onTimerClicked( self ) :            print "onTimerClicked"            self.timer.start()        def onStopClicked( self ) :            print "onStopClicked"            try: self.bgTask.stop()            except: pass            try: self.timer.stop()            except: pass                                def onClickMeClicked( self ):            print "onClickMeClicked"            self.clickCounter_+=1            self.clickCountLabelVar.set( str(self.clickCounter_) )        def onTimer( self ) :            print "onTimer"            self.timerCounter_+=1            self.timerCountLabelVar.set( str(self.timerCounter_) )        def myLongProcess( self, isRunningFunc=None ) :            print "starting myLongProcess"            for i in range( 1, 10 ):                try:                    if not isRunningFunc() :                        self.onMyLongProcessUpdate( "Stopped!" )                        return                except : pass                   self.onMyLongProcessUpdate( i )                sleep( 1.5 ) # simulate doing work            self.onMyLongProcessUpdate( "Done!" )                        def onMyLongProcessUpdate( self, status ) :            print "Process Update: %s" % (status,)            self.statusLabelVar.set( str(status) )    root = Tk()        gui = UnitTestGUI( root )    root.protocol( "WM_DELETE_WINDOW", gui.close )    root.mainloop()if __name__ == "__main__":     tkThreadingTest()

Two import points I'll stress about BackgroundTask:

1) The function you run in the background task needs to take a function pointer it will both invoke and respect, which allows the task to be cancelled mid way through - if possible.

2) You need to make sure the background task is stopped when you exit your application. That thread will still run even if your gui is closed if you don't address that!

The problem is that t.join() blocks the click event, the main thread does not get back to the event loop to process repaints.See Why ttk Progressbar appears after process in Tkinter or TTK progress bar blocked when sending email