[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy  added the comment:

The information missing from this issue is that tcl and tk can be compiled 
without and with thread support, and that the latter became default in 8.6, 
which is included with recent 3.x versions.  Hence the launch example, as 
posted on #33257, works for me in 3.7 but not 2.7.

I opened tkinter doc issue 33479, with this additional info, as a followup to 
msg183774 above.

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[issue11077] Tkinter is not thread safe

[Mark Lawrence]

Change by Mark Lawrence :


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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy  added the comment:

This issue was effectively reopened by #33257, with altered versions of 
TkinterCrash2-2.  Serhiy's original thereof fails for me on 2.7.15.  I will put 
details on the new issue.

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[issue11077] Tkinter is not thread safe

[Serhiy Storchaka]

Changes by Serhiy Storchaka storch...@gmail.com:


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stage: test needed - resolved
status: pending - closed

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[issue11077] Tkinter is not thread safe

[Serhiy Storchaka]

Changes by Serhiy Storchaka storch...@gmail.com:


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status: open - pending

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[issue11077] Tkinter is not thread safe

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I can't reproduce the bug on 2.7.8+, but on 2.7.3 I sometimes got messages like 
Exception in thread Thread-26 (most likely raised during interpreter 
shutdown): or:

  File /usr/lib/python2.7/lib-tk/Tkinter.py, line 2204, in create_line
  File /usr/lib/python2.7/lib-tk/Tkinter.py, line 2184, in _create
type 'exceptions.TypeError': 'NoneType' object is not callable

This points to shutdown issue. When TkinterCrash2.py call quit(), other threads 
are still work. And when modules (including Tkinter and __main__) are cleaned, 
they try to call global functions which are set to None. After fixing 
TkinterCrash2.py to not invoke any code during shutdown, error messages on 
2.7.3 gone.

I don't see any bugs in Tkinter exposed by this script, only a bug in the 
script itself.

Here is a script with quick and dirty fix. This solution is not perfect, there 
is small chance of race conditions (happened once in about 500 runs). Correct 
solution should be more complicated. If there no other errors in 
TkinterCrash2-2.py, this issue can be closed.

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[issue11077] Tkinter is not thread safe

[Mark Lawrence]

Mark Lawrence added the comment:

Is this still an issue with 2.7.8 which has presumably had more fixes since 
2.7.6?

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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy added the comment:

Summarizing other messages, it seems that tkinter is intended to be thread-safe 
(as long tk.mainloop is started in the tk thread) and mostly is until it is not 
(probably from timing issue). With fresh 2.7.8 install, I ran TkinterCrash2.py. 
I got the following twice:

Exception in thread Thread-12:
Traceback (most recent call last):
  File C:\Programs\Python27\lib\threading.py, line 810, in __bootstrap_inner
self.run()
  File tkcrash2.py, line 49, in run
self.deliverToQueue((self.target, z, y))
  File tkcrash2.py, line 131, in arrival_122
new_yz[1])
  File C:\Programs\Python27\lib\lib-tk\Tkinter.py, line 2282, in create_line
return self._create('line', args, kw)
  File C:\Programs\Python27\lib\lib-tk\Tkinter.py, line 2270, in _create
*(args + self._options(cnf, kw
ValueError: invalid literal for int() with base 10: 'None'

and the progrm continued. After another 100-200 launches, I tried to close the 
tk window and got the python has stopped working message from Windows. After 
this, every time I restart, I soon got the message again. Something seems to 
have been altered, though I do not see a relevant process in task manager.

I closed and reopened the command prompt process and restarted with pythonw 
instead of python and the program ran until too sluggish to continue. I then 
closed and reran and got 'pythonw has stopped working' at about launch 5. Third 
time ran until I closed at 50 launches. Fourth time stopped after 10 launches.

Serhiy, have any fresh insight?

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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy added the comment:

With 3.4.1, (64 bit), I ran, saturated (200 launches?) and closed 4 windows 
without incident. This extends Alexander's claim, 'not 3.x problem', in 
msg127648, from 3.2 to 3.4.

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[issue11077] Tkinter is not thread safe

[Keith Bannister]

Keith Bannister added the comment:

Hi,

I'm trying to make an application with GNU readline support, and some plots. 
But it seems that tkInter and raw_input don't play nicely.

The attached script (18 lines) crashes immediately on my Mac with SIGABRT and 
outputs the following:

$ python tk_v_raw.py
Yes?  Tcl_WaitForEvent: Notifier not initialized
Abort trap: 6

Is this related to this issue?

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Added file: http://bugs.python.org/file33637/tk_v_raw.py

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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy added the comment:

tk_v_raw.py is not obviously related to this issue. First, you only use tkinter 
in the main thread. Second, it works on Windows 7-64, 3.4 Idle.
 
Yes?  abc
Got cmd abc
Yes?  def
Got cmd def
Yes?  
It also runs in the console on 2.7, 3.3, 3.4. The main problem is that closing 
the Label window does not stop while True: in the thread, making a graceful 
stop difficult. The loop needs
if cmd == 'q': break

Keith, did you install the appropriate ActiveState tcl/tk, as described here: 
http://www.python.org/download/mac/tcltk/

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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy added the comment:

Is this still a problem with 2.7.6, which has a few hundred bug fixes since 
2.7.1.

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[issue11077] Tkinter is not thread safe

[Honest Abe]

Honest Abe added the comment:

I have been researching this due to the recurring recommendations, on 
stackoverflow.com, to use the queue.Queue + after() technique when using 
multiple threads.

———

From comments in _tkinter.c:

The Tcl interpreter is only valid in the thread that created it, 
and all Tk activity must happen in this thread, also. 
That means that the mainloop must be invoked in the thread that
created the interpreter. Invoking commands from other threads is possible; 
_tkinter will queue an event for the interpreter thread, which will then 
execute the command and pass back the result. If the main thread is not in the 
mainloop, and invoking commands causes an exception; if the main loop is 
running but not processing events, the command invocation will block.

http://hg.python.org/cpython/file/ef8ea052bcc4/Modules/_tkinter.c#l174

/* This is the main entry point for calling a Tcl command.
It supports three cases, with regard to threading:
1. Tcl is not threaded: Must have the Tcl lock, then can invoke command in
the context of the calling thread.
2. Tcl is threaded, caller of the command is in the interpreter thread:
Execute the command in the calling thread. Since the Tcl lock will
not be used, we can merge that with case 1.
3. Tcl is threaded, caller is in a different thread: Must queue an event to
the interpreter thread. Allocation of Tcl objects needs to occur in the
interpreter thread, so we ship the PyObject* args to the target thread,
and perform processing there. */

http://hg.python.org/cpython/file/ef8ea052bcc4/Modules/_tkinter.c#l1314

———

The following quote seems to imply that it is thread safe:

Invoking commands from other threads is possible;
_tkinter will queue an event for the interpreter thread, 
which will then execute the command and pass back the result.

But... Allen B. Taylor, author of mtTkinter, states:

The problems stem from the fact that the _tkinter module attempts to gain 
control of the main thread via a polling technique when processing calls from 
other threads. If it succeeds, all is well. If it fails (i.e., after a 
timeout), the application receives an exception with the message: 
RuntimeError: main thread is not in main loop.

http://tkinter.unpythonic.net/wiki/mtTkinter

This seems to be somewhat validated by the first quote from the source code.
However, the statement main thread is not in main loop
seems backwards to me. Wouldn't a loop be in a thread?
How can the thread that spawned the mainloop ever be inside of the mainloop? 
Note that all of my experience is with Python, not C.

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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy added the comment:

I am somewhat puzzled by the claim that tkinter *is* thread safe (as opposed to 
*intended to be*), in the sense of accessing the gui from multiple threads. 

First, 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.

This statement must be based on some sort of experience.

Second, there have been many reports of problems with tkinter and threads. 
From currently open tracker issues:
#11029 Same Scott M as here fixed problem by feeding data to single gui thread 
with queue.
#16823 I turned this into a doc issue to say (perhaps erroneously) that tkinter 
is not thread safe, quoting the above.
#6933 I closed this as invalid because of using multiple threads.
#10909 This is ambiguous because problem continued after stopping 2nd thread 
access.
I believe these have most of the details requested.

There have been posts on python-list where people were advised to and solved 
problems by only accessing tkinter from one thread, and the main thread at 
that. Ditto for Stackoverflow: searching [python tkinter threads] gets 368 
hits. Just one example:
http://stackoverflow.com/questions/13753545/tkinter-freeze-on-grid-remove/13753865#13753865
 from Adrian Larsen got this response:

Tkinter is not thread safe. If you do anything in a thread other than the main 
thread that touches a GUI object then you will get unpredictable results. ... 
(Brian Oakley, experienced responder)

Adrian responded ... The real problem is that it is not in the correct thread 
(i.e. the mainloop). I solved the problem by sending the ungrid order to a 
queue in the GUI, which then issues the command from inside the mainloop.

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[issue11077] Tkinter is not thread safe

[Andrew Svetlov]

Changes by Andrew Svetlov andrew.svet...@gmail.com:


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[issue11077] Tkinter is not thread safe

[Martin v . Löwis]

Martin v. Löwis mar...@v.loewis.de added the comment:

My claim is that Tkinter is thread-safe as it stands. A lot of thought has been 
put into making Tkinter thread-safe, so if there is any claim to the contrary, 
we would need more details: what exact Python version is being used, what exact 
operating system is being used, what exact code is run, and what exact output 
is produced.

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[issue11077] Tkinter is not thread safe

[Scott M]

Scott M scott.m...@comcast.net added the comment:

I'll look into making the crash easier to reproduce this coming week.

Is Tkinter's thread safety new? Because after I started getting crashes, I did 
my due diligence in Google and found a number of people writing about how it 
was necessary to use a Queue and Tkinter's after() timer, to draw from multiple 
threads. And in experiments in 2.7.1 on Windows, it turned out that it wasn't 
just the drawing calls from a thread other than the mainloop() thread, that 
invited a crash; even just trying to set an .after() timer from a different 
thread caused tracebacks, randomly.

At any rate this is less of an issue now, for me at least. I'm putting together 
a small graphic library for my coworkers. It will have a tiny fraction of 
Tkinter's capabilities, it will be Windows-only and won't even have native 
Windows look and feel; but it will be completely, rock-solid, idiot-proof 
thread safe - any operation (including widget deletes), any thread, any time, 
with no polling or timers needed. My coworkers are not technical and will get 
instantly lost if I have to describe queues and timers just to draw a line on a 
screen.

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[issue11077] Tkinter is not thread safe

[Martin v . Löwis]

Martin v. Löwis mar...@v.loewis.de added the comment:

 Is Tkinter's thread safety new?

It's supported on Unix since 1.5.1, and on Windows since 2.3.

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[issue11077] Tkinter is not thread safe

[Scott M]

Scott M scott.m...@comcast.net added the comment:

The new version runs 40 parabolas, then quits. I usually have to run this 
version 20 times or so to get the crash, so be patient. In general if it's 
going to crash it does so in the first 6 or so parabolas. Caveat: creates up to 
40 threads, so a bit of a CPU pig. You may want to change the 40 in 
(self.tracks  40) to 8.

Here's one crash. I got this by double clicking the .py file from Windows 
explorer, but I can get them with F5 in IDLE too.
---
UpdateStringProc should not be invoked for type cmdName

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
Exception in thread Thread-6:
Traceback (most recent call last):
  File C:\Python27\lib\threading.py, line 530, in __bootstrap_inner
self.run()
  File H:\PMT2\MyProjects\TkinterCrash2.py, line 47, in run
self.deliverToQueue((self.target, z, y))
  File H:\PMT2\MyProjects\TkinterCrash2.py, line 129, in arrival_122
new_yz[1])
  File C:\Python27\lib\lib-tk\Tkinter.py, line 2201, in create_line
return self._create('line', args, kw)
  File C:\Python27\lib\lib-tk\Tkinter.py, line 2189, in _create
*(args + self._options(cnf, kw
TclError: bad screen distance create

--
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Added file: http://bugs.python.org/file20684/TkinterCrash2.py

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[issue11077] Tkinter is not thread safe

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

I can't reproduce either, but the latest traceback posted in msg127985 seems to 
hint at a parameter marshalling problem. create is one of the parameters to 
the called Tk function, but it seemed to be mistaken for another.

Since it's arguably threading-related, saying I can't reproduce doesn't seem 
like a sufficient reason to close the issue. There can be all kinds of 
influencing factors (OS, CPU, background tasks) and the marshalling code in 
Modules/_tkinter.c is far from trivial.

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type: feature request - crash

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[issue11077] Tkinter is not thread safe

[Martin v . Löwis]

Martin v. Löwis mar...@v.loewis.de added the comment:

Tk produces bad screen distance in Tk_GetScreenMM (convert string to screen 
millimeters) and TkGetDoublePixels (convert string to number of pixels) when 
strtod fails on the string being passed. It also produces the error in 
SetPixelAny (convert object to pixel) if the string doesn't start with a 
double, and SetMMFromAny (convert object to millimeters) if the string either 
is no double, or not followed by [cimp]. Finally, TkPixelParseProc returns 
the error if TkGetDoublePixels returns a negative number.

In relationship to the canvas line command, TkPixelParseProc is used for 
converting the -width, -activewidth, and -disabledwidth arguments. None of 
these are used in the test case, though, so it's not clear which pixel parsing 
fails specifically.

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[issue11077] Tkinter is not thread safe

[Scott M]

Scott M scott.m...@comcast.net added the comment:

If it helps, over the many iterations of this test code, there have been two 
kinds of issues:

1. pythonw.exe crashes with the Windows variant of a SEGV. No traceback, just a 
crash. These are rare.

2. Evidence of confusion over which string the code should be looking at, and 
it's always down in the create function of Tkinter. Variations of this 
confusion have been Int() complaining that it can't translate None, strings 
like .667748474.7464 not being recognized as parameter names...  all of it 
sounds like Tkinter has somehow managed to be looking at the wrong string. Even 
when the only Tkinter call I do outside the mainloop thread is .after(), the 
crashes would happen when I went to draw a line.

My (trivial) experience in extending Python makes me wonder if some reference 
count, somewhere, didn’t get mismanaged. The only times I ever got a hard crash 
out pythonw.exe in my own projects is when I mishandled a count.

As a side note, if Tkinter is intended to be thread safe, the documentation 
should say so. Clearly, and in the first paragraph. Once I started having 
problems, I started Googling, and everything I read lead me to conclude that 
neither Tkinter nor wx were even intended to be thread safe, so I've started to 
write my own GUI code. This is a project I might have skipped if it has been 
clear that Tkinter is at least intended to be thread safe.

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[issue11077] Tkinter is not thread safe

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

The UpdateStringProc should not be invoked for type cmdName message (as 
quoted above in the traceback) apparently can mean that there's a mismanagement 
of Tcl reference counts.

From 
http://sourceforge.net/tracker/?func=detailatid=110894aid=1326087group_id=10894:

“This crash (actually, a panic) hints at defective Tcl_Obj
handling - possibly in the core, more likely in an extension
if you're using one. It indicates that Tcl_GetString() or
Tcl_GetStringFromObj() has been called on a cmdNameType
Tcl_Obj that has no string representation, a state that
should never occur.”

Intuitively, cmdNameType seems to refer to createcommand() / deletecommand(). 
Also, the following code in Tkinter.py looks a bit suspicious:

def after(self, ms, func=None, *args):
Call function once after given time.

MS specifies the time in milliseconds. FUNC gives the
function which shall be called. Additional parameters
are given as parameters to the function call.  Return
identifier to cancel scheduling with after_cancel.
if not func:
# I'd rather use time.sleep(ms*0.001)
self.tk.call('after', ms)
else:
def callit():
try:
func(*args)
finally:
try:
self.deletecommand(name)
except TclError:
pass
name = self._register(callit)
return self.tk.call('after', ms, name)

That is, we call deletecommand() while the command is still being executed.
Perhaps that, together with concurrent memory allocation from another thread 
(Tcl_Alloc() doesn't use Python's Tcl lock), might explain why the cmdName or 
other things sometimes become corrupted (Tcl_DeleteCommand deletes a command 
from a command interpreter. Once the call completes, attempts to invoke cmdName 
in interp will result in errors).

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[issue11077] Tkinter is not thread safe

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

 As a side note, if Tkinter is intended to be thread safe, the 
 documentation should say so. Clearly, and in the first paragraph.

I'm no Tkinter specialist but, judging by its source code, Tkinter (the Python 
module) *is* intended to be thread-safe using locking and/or marshalling.

As for wxPython, you are right that it is not thread safe.

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[issue11077] Tkinter is not thread safe

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

'Thread' is not found in the somewhat skimpy tkinter doc. Given the general 
state of Python with threads, I think it reasonable to take 'not thread-safe' 
as the default, making this a feature request. In any case, the point is moot 
until there is a tested fix. The use case presented, though, does make this 
look like a desireable new feature.

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[issue11077] Tkinter is not thread safe

[Christoph Gohlke]

Changes by Christoph Gohlke cgoh...@uci.edu:


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[issue11077] Tkinter is not thread safe. This is a bug.

[Scott M]

New submission from Scott M scott.m...@comcast.net:

The more I look at GUI support in Python, the more I realize that the lack of 
basic thread safety in GUI support is simply a bug. I know Java's Swing has the 
same thread limitation, but that doesn't make it right. Xlib is thread safe. 
The Windows SDK is thread safe. Python is supposed to be the language that's 
easy to use, and there is nothing easy about teaching new programmers that they 
have to mess with queues and timers just to get a basic set of displays 
running, just because when threads are in use.

I'm in the position of teaching folk with little-to-no programming experience, 
how to script simple applications in Python. The modules they have to use are 
inherently threaded, and delivery hunks of data from multiple sources to them. 
The most natural instinct is to put up some graphs and other widgits to display 
the data, and all of it is completely trivial right up until I have to explain 
that drawing a line isn't canvas.line(from, to), but becomes an exercise in 
Queue.Queue and theRoot.after(n, myself), before you even get to learn about 
widgits. Threading is supposed to simplify problems, not add to them. Having to 
hack around with special timers and polling, just to get some simple graphs up, 
is plain unpythonic.

Please consider this a bug, a glaring misfeature, in a language that is 
otherwise a very reasonable choice to get technical but non-programmerish 
people into toolmaking self-sufficiency.

--
components: Tkinter
messages: 127604
nosy: PythonInTheGrass
priority: normal
severity: normal
status: open
title: Tkinter is not thread safe. This is a bug.
type: behavior
versions: Python 2.7

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[issue11077] Tkinter is not thread safe

[Brian Curtin]

Changes by Brian Curtin cur...@acm.org:


--
title: Tkinter is not thread safe. This is a bug. - Tkinter is not thread safe

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[issue11077] Tkinter is not thread safe

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

One possible solution was presented in issue1252236: move tkinter event loop 
into Python main loop.  However, to consider this report a bug, we need an 
example code that shows the behavior that you consider incorrect.  Even then, 
it is likely that fixing the behavior won't be possible without introducing new 
features.  I am reclassifying this as a feature request.

--
nosy: +belopolsky
stage:  - unit test needed
type: behavior - feature request
versions: +Python 3.3 -Python 2.7

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[issue11077] Tkinter is not thread safe (and that's... bad)

[Scott M]

Scott M scott.m...@comcast.net added the comment:

I don't have an opinion on 1252236. I'm not certain it would help.

I have an extension that runs a bunch of (alien) threads into Python code. The 
threads deliver information for all sorts of real world events, asynchronously. 
Multiple threads are used, because people occasionally want to do blocking 
operations in these alien threads, but still want to be able to handle other 
incoming information at full speed, with the other threads.

If any of these threads attempts to update a Tkinter widgit - and this is the 
first thing I tried to do - then tkinter will except or crash in some horrible 
way, sooner or later. 

The attached .py (in Python 2.7.1) does it without any extensions. Click on 
Launch quickly 10-15 times; if it doesn't crash, kill and restart. It rarely 
takes more than 4-5 runs to get a traceback, or occasionally python itself just 
crashes. Bottom line, one thread in .mainloop() and another thread calling 
virtually any Tkinter function, even something as simple as .after(), is an 
invitation a crash.

--
title: Tkinter is not thread safe - Tkinter is not thread safe (and that's... 
bad)
Added file: http://bugs.python.org/file20630/TkinterCrash2.py

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[issue11077] Tkinter is not thread safe

[Brian Curtin]

Changes by Brian Curtin cur...@acm.org:


--
title: Tkinter is not thread safe (and that's... bad) - Tkinter is not thread 
safe

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[issue11077] Tkinter is not thread safe

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

Hmm, either my hand is too slow or my laptop is too fast, but I cannot 
reproduce the crash. Can you create a non-interactive script?  Maybe start a 
separate thread generating launch events?

What do you mean by crash?  Do you get a python backtrace or OS diagnostic 
saying that the process was killed by a signal of some sort?  If it is the 
former, please post the backtrace.  If the latter, can you produce C backtrace?

--

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[issue11077] Tkinter is not thread safe

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

I converted TkinterCrash2.py to 3.x using 2to3 (result attached as 
TkinterCrash3.py) and it works with 3.2rc2 just fine.

--
Added file: http://bugs.python.org/file20632/TkinterCrash3.py

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