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

Question

I have a small GUI test with a  Start  button and a Progress bar  The desired behavior is    Click Start Progressbar oscillates for 5 seconds Progressbar stops   The observed behavior is the  Start  button freezes for 5 seconds  then a Progressbar is displayed  no oscillation    Here is my code so far   class GUI      def   init   self  master           self master   master         self test button   Button self master  command self tb click          self test button configure              text  Start   background  Grey               padx 50                       self test button pack side TOP       def progress self           self prog bar   ttk Progressbar              self master  orient  horizontal               length 200  mode  indeterminate                        self prog bar pack side TOP       def tb click self           self progress           self prog bar start             Simulate long running process         t   threading Thread target time sleep  args  5            t start           t join           self prog bar stop    root   Tk   root title  Test Button   main ui   GUI root  root mainloop     Based on the information from Bryan Oakley here  I understand that I need to use threads  I tried creating a thread  but I m guessing that since the thread is started from within the main thread  it doesn t help   I had the idea to place the logic portion in a different class  and instantiate the GUI from within that class  similar to the example code by A  Rodas here   My question   I can t figure out how to code it so that this command   self test button   Button self master  command self tb click    calls a function that is located in the other class  Is this a Bad Thing to do or is it even possible  How would I create a 2nd class that can handle the self tb click  I tried following along to A  Rodas  example code which works beautifully  But I cannot figure out how to implement his solution in the case of a Button widget that triggers an action   If I should instead handle the thread from within the single GUI class  how would one create a thread that doesn t interfere with the main thread

User · Accepted Answer

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:

Create a Queue object in the main thread
Create a new thread with access to that queue
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 Queue

class 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(0)
            # 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):
        threading.Thread.__init__(self)
        self.queue = queue
    def run(self):
        time.sleep(5)  # Simulate long running process
        self.queue.put("Task finished")

User · Answer

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

User · Answer

I have used RxPY which has some nice threading functions to solve this in a fairly clean manner  No queues  and I have provided a function that runs on the main thread after completion of the background thread  Here is a working example  import rx from rx scheduler import ThreadPoolScheduler import time import tkinter as tk  class UI     def   init   self         self root   tk Tk         self pool scheduler   ThreadPoolScheduler 1    thread pool with 1 worker thread       self button   tk Button text  quot Do Task quot   command self do task  pack       def do task self         rx empty   subscribe           on completed self long running task            scheduler self pool scheduler             def long running task self           your long running task here    eg        time sleep 3          if you want a callback on the main thread        self root after 5  self on task complete      def on task complete self          pass   runs on main thread  if   name       quot   main   quot       ui   UI       ui root mainloop    Another way to use this construct which might be cleaner  depending on preference   tk Button text  quot Do Task quot   command self button clicked  pack         def button clicked self       def do task           time sleep 3    runs on background thread                  def on task done          pass   runs on main thread     rx just 1  subscribe        on next do task         on completed lambda  self root after 5  on task done          scheduler self pool scheduler

User · Answer

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 threading  class 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

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

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