[Tkinter-discuss] tkinter "monitor" and root.mainloop()

[Cameron Laird]

On Mon, May 26, 2008 at 11:29:40AM +1000, Peter Milliken wrote:
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> True, sleep() pauses the main app - if you run it there. It wasn't obvious
> to me from the original question that this might be a problem :-)
> 
> In my application (a simple egg timer that allows on the fly creation of new
> timers and allows timing in relative or absolute terms i.e. in 5 minutes 23
> seconds alert me or at 10:06:55 alert me) I leave the sleep() call down in
> the thread that I created for each timer - so each "timer" thread pauses for
> X seconds (1 second in my case because that is the smallest increment of
> time that I am interested in).
> 
> I use Pmw for all my GUI work - after getting your mind around how flexible
> it can be, you can create some amazingly power applications that can
> reconfigure themselves on the fly for the user - one of the best programming
> interfaces I have ever used! The guy who did this has my complete
> admiration! :-)
> 
> Perhaps (just for fun :-)) think about recreating your application using
> threads and (message) queues to pass information between the main Tk loop
> and the various threads that you create.
> 
> Be careful - Tkinter in not re-entrant and you need to leave all of your
> graphics manipulations in the mainloop - but you can safely do all sorts of
> other stuff (checking files for changes etc etc) in threads - as long as
> they don't do any GUI work then you are safe.
> 
> Doing threaded applications in Tk is definitely not for the beginner - you
> can find cookbook examples of using threading with Tk.
> 
> Personally I find it kind of fun to do - I work in real-time embedded
> systems programming, so threading and tasking is very familiar territory.
> But if you ever want to expand your horizons give it some thought - but be
> mindful that it requires a different mindset to that of straight "linear"
> thinking programming - many programmers have some difficulty when they are
> first exposed to threads and multi-tasking applications - but once you get
> past that barrier you start to think in those terms with ease :-)
> > >                       .
> > >                       .
> > >                       .
> > > > and thanks for the answers: how, i avoided the sleep() approach,
> > > > because, as Cameron said i supposed that it freezed the application:
> > > > being in sleep() it stops the mainloop()...
> >                         .
> >                        .
> >                        .
> > I need to put this minimal example of after()-based polling in the Wiki ...
> >
> >  import Tkinter
> >  import time
> >
> >  root = Tkinter.Tk()
> >
> >  def my_update():
> >      display.set("The time now is '%s'." % time.asctime(time.localtime()))
> >          # Re-invoke myself in two seconds.
> >      root.after(2000, my_update)
> >
> >  display = Tkinter.StringVar()
> >  window = Tkinter.Label(root, textvariable = display)
> >  window.pack()
> >  my_update()
> >  root.mainloop()
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I think we're all together now.  To be certain, I'll 
say some of this in my own words.

sleep() is not a "problem", but it frequently is a
*surprise* to those working in this area for the first
time.  Concurrency is indeed distinct from sequential-
procedural patterns, and I agree that folklore tells
us many newcomers to the area take a bit of time to
adjust.  I'd add that, to the best of my knowledge,
that adjustment is present whether coding is in a 
thread-based or event-based idiom.  While I'm also a
firm believer that the latter is less error-prone for
many common requirement sets, I agree that Pmw is a
considerable value for which we all should be thankful.

You're absolutely right that Tkinter (and X-windows-
compatible toolkits, more generally) need to keep their
GUIs in a single thread.

Is it handy for you to share a threaded-Pmw-based equi-
valent of the two-second-grainy clock I've included 
above?  I think it'll make for an interesting comparison
from which we perhaps might all learn.  If you don't get
to it in the next week, I'll try, although I'm rusty 
with Pmw.