Using Multiprocessing module for updating Tkinter GUI

Question

I have been trying to use Multiprocessing module for updating Tkinter GUI but when I run this code, it is giving Pickling error.

# Test Code for Tkinter with threads
import Tkinter
from multiprocessing import Queue
import multiprocessing
import time

# Data Generator which will generate Data
def GenerateData():
    global q
    for i in range(10):
        print "Generating Some Data, Iteration %s" %(i)
        time.sleep(2)
        q.put("Some Data from iteration %s \n" %(i))

def QueueHandler():
    global q, text_wid
    while True:
        if not q.empty():
            str = q.get()
            text_wid.insert("end", str)

# Main Tkinter Application
def GUI():
    global text_wid
    tk = Tkinter.Tk()
    text_wid = Tkinter.Text(tk)
    text_wid.pack()
    tk.mainloop()

if __name__ == '__main__':
# Queue which will be used for storing Data
    tk = Tkinter.Tk()
    text_wid = Tkinter.Text(tk)
    q = multiprocessing .Queue()
    t1 = multiprocessing.Process(target=GenerateData,args=(q,))
    t2 = multiprocessing.Process(target=QueueHandler,args=(q,text_wid))
    t1.start()
    t2.start()
    text_wid.pack()
    tk.mainloop()

Error:

PicklingError: Can't pickle <type 'thread.lock'>: it's not found as thread.lock

If I remove the argument text_wid, then no error is reported but text widget is not updated with the data from the queque.

UPDATE :

I modified code so as to call the function to update the GUI whenever there is value in queue, thus preventing Tkinter widgets from being passed to separate process. Now, I am not getting any error but the widget is not updated with the data. However if i use mix of Threading and Multiprocessing module i.e. create a separate thread for handling data from the queue, then it works fine. My question why didn't it worked when i run the handler code in separate process. Am I not passing the data correctly. Below is the modified code:

# Test Code for Tkinter with threads
import Tkinter
import multiprocessing
from multiprocessing import Queue
import time
import threading

# Data Generator which will generate Data
def GenerateData(q):
    for i in range(10):
        print "Generating Some Data, Iteration %s" %(i)
        time.sleep(2)
        q.put("Some Data from iteration %s \n" %(i))

def QueueHandler(q):
    while True:
        if not q.empty():
            str = q.get()
            update_gui(str)
            #text_wid.insert("end", str)

# Main Tkinter Application
def GUI():
    global text_wid
    tk = Tkinter.Tk()
    text_wid = Tkinter.Text(tk)
    text_wid.pack()
    tk.mainloop()

def update_gui(str):
    global text_wid
    text_wid.insert("end", str)

if __name__ == '__main__':
# Queue which will be used for storing Data
    tk = Tkinter.Tk()
    text_wid = Tkinter.Text(tk)
    q = multiprocessing.Queue()
    t1 = multiprocessing.Process(target=GenerateData,args=(q,))
    t2 = multiprocessing.Process(target=QueueHandler,args=(q,))
    t1.start()
    t2.start()
    text_wid.pack()
    tk.mainloop()

Answer 1

You missed out an important part, you should protect your calls with a __main__ trap:

if __name__ == '__main__': 
    q = Queue.Queue()
    # Create a thread and run GUI & QueueHadnler in it
    t1 = multiprocessing.Process(target=GenerateData,args=(q,))
    t2 = multiprocessing.Process(target=QueueHandler,args=(q,))

    ....

Note that the Queue is passed as a parameter rather than using a global.

Edit: just spotted another issue, you should be using Queue from the multiprocessing module, not from Queue:

from multiprocessing import Queue