PythonCard - My app stuck when button clicked

Dave Angel davea at
Wed May 13 19:42:43 CEST 2009

daved170 wrote:
> Hi there,
> I'm newbie in pythonCard.
> I have an application with 2 buttons : START , STOP
> Start execute a while(1) loop that execute my calculations.
> Stop suppose to raise a flag that will end that loop.
> Whenever I pish the START button my GUI is stuck. the calculation
> executes but I can't push the STOP button.
> I added thread that START start a thread that execute my calculations.
> I also added a Global variable that will hold the indication if the
> loop should continue.
> The problem now is that the thread ignore that variable and loop
> forever.
> Is there a simple way to make sure that the GUI won't stuck (without
> threads)?
> And if there isn't such way I would appriciet it very much if anyone
> could post an example of how to make my thread read that variable
> Thanks
> Dave
I don't know PythonCard, but most GUI's are similar enough that the 
concepts will work, even though the details differ.  I'll assume that 
PythonCard has a traditional event loop, from which all events are 

If your loop is fairly small, then you should keep it to one thread.  
Debugging it will usually be much easier.  The trick is to break the 
task into pieces (each piece might be once around what is now a loop), 
and invoke one piece each time the event loop empties.  I can't tell you 
how to do that without seeing your loop, but it's not usually very hard.

Now, there is some way of POSTing an event to the event loop.  That puts 
the event *after* all the events that are already there, but returns 
control immediately.  So create a custom event, and POST it from the 
START button's button-pressed event.  That will fire off one "loop" of 
the special task, in other words, make one function call to your new 
function.  Then at the end of the function, POST it again, unless the 
STOP button has been pressed in the meantime.

An optimization for this is to use coroutines, which are usually done 
with a generator.  It's much trickier to describe, but much easier to 
accomplish.  Roughly, you'd take your existing loop, and put a yield 
statement in it at appropriate place(s).  Then the custom event is 
simply a call to the .next() function of that generator.

Now, threading isn't that tough either, depending on how much data is 
being shared between the thread and the main program.  You say that 
sharing a global flag isn't working, but it should.  So how about if you 
show us some code, and somebody'll spot the trouble.  For example, is 
the thread defined in the same module as the App?  Global only shares 
between a single module.  Another reason globals might seem to fail is 
if you tried to do mutual imports between two or more modules.  (A 
imports B, which imports A).  Sometimes that fails in mysterious ways.

Make a simple (stripped) example of what you're trying, and we'll try to 
find the problem.  Without concrete code, we end up with ambiguities 
like the above usage of two different meanings for "the loop."

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