Python quits on running tkinter code with threads

[Sarbjitsingh]

BPO	16823
Nosy	@terryjreedy, @asvetlov, @serhiy-storchaka
Superseder	bpo-33479: Document tkinter and threads
Files	debugstack.txt: debugstack trace tem.py: text code adapted to 3.x

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-05-13.04:16:15.071>
created_at = <Date 2012-12-31.03:23:09.608>
labels = ['type-bug', 'expert-tkinter', 'docs']
title = 'Python quits on running tkinter code with threads'
updated_at = <Date 2018-05-13.04:58:03.624>
user = 'https://bugs.python.org/Sarbjitsingh'

bugs.python.org fields:

activity = <Date 2018-05-13.04:58:03.624>
actor = 'terry.reedy'
assignee = 'docs@python'
closed = True
closed_date = <Date 2018-05-13.04:16:15.071>
closer = 'terry.reedy'
components = ['Documentation', 'Tkinter']
creation = <Date 2012-12-31.03:23:09.608>
creator = 'Sarbjit.singh'
dependencies = []
files = ['28503', '41966']
hgrepos = []
issue_num = 16823
keywords = []
message_count = 7.0
messages = ['178642', '179101', '180000', '183770', '260502', '260769', '316448']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'asvetlov', 'docs@python', 'serhiy.storchaka', 'Sarbjit.singh']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '33479'
type = 'behavior'
url = 'https://bugs.python.org/issue16823'
versions = ['Python 2.7', 'Python 3.5']

[Sarbjitsingh]

I have written a python tkinter code using threads so as the tkinter wizard updates automatically by tkinter mainloop running in the main thread and background process running in separate thread. But I noticed, that python crashes after some time when running the code. Moreover its random in nature but python crashes most of the time. I have written a small test code which can show this problem (My original code is similar to this but having some real processes and many other features, so I am sharing the test code).

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

# Data Generator which will generate Data
def GenerateData(q):
    for i in range(1000000):
        #print "Generating Some Data, Iteration %s" %(i)
        time.sleep(0.01)
        q.put("Some Data from iteration %s. Putting this data in the queue for testing" %(i))

# Queue which will be used for storing Data
q = Queue.Queue()

def QueueHandler(widinst, q):
    linecount = 0
    while True:
        print "Running"
        if not q.empty():
            str = q.get()
            linecount = linecount + 1
            widinst.configure(state="normal")
            str = str + "\n"
            widinst.insert("end", str)
            if linecount > 100:
                widinst.delete('1.0', '2.0')
                linecount = linecount - 1
            widinst.see('end')
            widinst.configure(state="disabled")

# Create a thread and run GUI & QueueHadnler in it
tk = Tkinter.Tk()
scrollbar = Tkinter.Scrollbar(tk)
scrollbar.pack(side='right', fill='y' )
text_wid = Tkinter.Text(tk,yscrollcommand=scrollbar.set)
text_wid.pack()
t1 = threading.Thread(target=GenerateData, args=(q,))
t2 = threading.Thread(target=QueueHandler, args=(text_wid,q))
t2.start()
t1.start()

tk.mainloop()
######################################################################

TO REPRODUCE:

If you open this code in IDLE and run it, it will sometimes appeared to be in hang state. So to reproduce, modify the sleep time to 0.1 from 0.01 and run it. After this stop the application, and modify it back to 0.01, do save and run it. This time it will run and after some time, python will stop working. I am using windows 7 (64 bit).

NOTE:

I am getting different stack trace's when it crashes on console:
Please refer to the attached file for the stack traces.

[terryjreedy]

What you are doing appears to be unsupported (invalid). From
http://www.astro.washington.edu/users/rowen/TkinterSummary.html
"all Tkinter access must be from the main thread (or more precisely,
from the thread that calls the mainloop). Violating this is likely to
cause nasty and mysterious symptoms such as freezes and core dumps."

There have been at multiple posts on python-list and stack overflow from other users that have run into this limitation. I cannot find it in our tkinter docs, but I think perhaps it should be (even though not-thread-safe is the default for any python structure).

[terryjreedy]

We use 'crash' to mean a segfault (and core dump, on *nix) or the Windows equivalent. We avoid those if at all possible. A Python traceback is not a crash but a semi-graceful shutdown that has be planned for, given the circumstances. What is annoying here is getting a different exception (or at least message type) each time. I presume it depends on what data gets corrupted, and that is somewhat haphazard. The quote seems to be accurate.

I know little about threading, but if it were possible to detect tkinter access from other than the main thread (without excessive slowdown), then a consistent exception might be raised. But I am doubtful that this can be done sensibly. Hence a doc change seems like the best first step.

[terryjreedy]

From bpo-11029: Doc about tkinter and treads should also give the alternative of using queue.queue to feed data from multiple threads.
See msg127316