Multi-platform Threading

[Peter Hansen]

Lane LiaBraaten wrote:
> 
> > Are you using the proper mechanisms provided by Tkinter to communicate
> > with the GUI thread?
> 
> I have no idea.  What proper mechanisms are you refering to?  All I know is
> that the program works on Linux and not on WinNT.  Are there and glaring
> differences that I should be aware of?

I don't use Tkinter, so I can't tell you about differences, but if I'm
right about the fundamental nature of the problem, the fact that it works
on one platform and not the other is incidental: it could just as well
have been the other way around, or it might even be intermittent on 
one or both platforms and you just haven't noticed it yet.

If this is, for example, due to a classic race condition, you could run
your Linux program for days without seeing the problem, then suddenly
see it "for no good reason at all", then not again for weeks.

I could be way off base.  That's why I asked.  The fact that you 
didn't say something like "I'm no rookie, sport: of course I didn't 
screw up the GUI thread" suggests I might be on to something. ;-)

In general, GUI frameworks don't deal well with multiple threads,
except through tightly controlled interfaces which generally (a)
defer handling certain external events until an appropriate time,
and (b) temporarily suspend regular event processing and GUI
work such as redrawing while handling the external event at the
later time.  

>From the wxPython demo, for example:

 '''The main issue with multi-threaded GUI programming is the thread 
  safety of the GUI itself.  On most platforms the GUI is not thread 
  safe and so any cross platform GUI Toolkit and applications written 
  with it need to take that into account.

  The solution is to only allow interaction with the GUI from a single
  thread, but this often severly limits what can be done in an
  application and makes it difficult to use additional threads at all.

  Since wxPython already makes extensive use of event handlers, it is a
  logical extension to allow events to be sent to GUI objects from
  alternate threads.  A function called wxPostEvent allows you to do
  this.  It accepts an event and an event handler (window) and instead
  of sending the event immediately in the current context like
  ProcessEvent does, it processes it later from the context of the GUI
  thread.'''

I'm sure Tkinter has similar restrictions, and solutions.

-Peter