[Tkinter-discuss] Proposal for thread-safe Tkinter

C. Allen callen at gemini.edu
Tue Oct 28 21:33:29 CET 2008

wait a second... I have a class system, and to exercise this class
system, and possibly as a general command line tool in the future, I
access this class from a script meant to run in a shell. To watch
progress of the system as it runs (it's processing data, and can take
significant amounts of time) so I thought I'd make a nice little
interface to watch the progress, and let the user pause and do other
control commands.  This is not the purpose of this command line tool,
however, so I only bring that up based on a command line flag.  Also, I
do not call mainloop from the main thread, because it is doing the
processing, and is the "real" program.  I start another thread, use it
(and only it) as the Tk thread.  

This all works fine, and I never get the exception mentioned below.


On Mon, 2008-10-27 at 15:10 -0200, Guilherme Polo wrote:

> On Mon, Oct 27, 2008 at 12:59 PM, Allen Taylor
> <Allen.Taylor at mdacorporation.com> wrote:
> > All,
> > After a very helpful discussion with "Guilherme Polo <ggpolo at gmail.com>", we
> > have come to the following conclusion regarding multithreaded applications
> > and Tkinter. Although Tk is technically thread-safe (if built with
> > --enable-threads), practically speaking it still has problems when used in
> > multithreaded applications.
> Now that I reread it, I see you said "Tk is technically thread-safe
> (if built with --enable-threads)". It is not true, what happens is
> that tkinter makes it thread safe by using some mutexes and scheduling
> actions to go through the thread that created the tcl interpreter,
> which otherwise would happen elsewhere. But tk isn't thread safe per
> se  (only tcl, since version 8.1).  At the same time, if tk is
> compiled without thread support but python has thread support, then
> tkinter fails miserably to protect against multiple threads from
> python from accessing tk.
> And again, your module does help in this case since all the
> interaction to tk will occur through main thread, independent of tk
> having thread support or not.
> > The problem stems from the fact that _tkinter
> > attempts to gain control of the main thread via a polling technique. If it
> > succeeds, all is well. If it fails, however, the application receives an
> > exception with the message "RuntimeError: main thread is not in main loop".
> > There is no way to tell when this might happen, so calling Tk routines from
> > multiple threads seems to be problematic at best.
> > The mtTkinter module I published last week
> > (http://tkinter.unpythonic.net/wiki/mtTkinter) solves this problem by
> > marshaling calls coming from multiple threads through a queue which is read
> > by an 'after' event running periodically in the main loop. This is similar
> > to the technique used in many other platforms (e.g., .NET's
> > InvokeRequired/Invoke mechanism). The technique used in mtTkinter is
> > exception-safe as well, marshaling exceptions through a response queue back
> > to the caller's thread.
> > If a similar technique were employed in _tkinter instead of the polling
> > technique, that would be a preferable solution. In the meantime, mtTkinter
> > will work as a good stop-gap measure.
> > Allen B. Taylor
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