[Tkinter-discuss] loop delay

[Paul Simon]

Thanks very much for your response.  What I wrote is just a very simple 
script to familiarize myself with tkinter.  I'm making a lot of mistakes and 
so learning a lot :-)

The "sleep" command is in there so I can just see if the write loop is 
running, using the "sleep" for the delay.  I'm still working on figuring out 
widget placement.

The entire program is a replacement of a MS Access app written in VBA, and 
requires three parts.  A gui (tkinter), data access using the serial port 
with RS-485, and a simple database using sqlite.  The latter two pieces 
work, sort of, and am now working on the gui.  The RS-485 interface needs 
the sleep command or a do-while loop to wait for hardware turn around due to 
the absence of hand shake in the serial interface.

I would be happy to keep you guys informed of my slow process ("life 
interferes") if you wish.  You can see the current implementation on my web 
site, www.gophergulch.net under "temperature."
Paul
"Bryan Oakley" <bryan.oakley at gmail.com> wrote in message 
news:CAKypQny33xJ=w_Ti7CZyA4axopx+wxAh5ZO+qjFvLuGr_Ytsmw at mail.gmail.com...
> On Sun, Oct 21, 2012 at 5:31 PM, Paul Simon <psimon at sonic.net> wrote:
>> I' m a novice python programmer and now starting to learn tkinter for a
>> front and db back end to replace an application written MS Access/VBA. 
>> The
>> following program doesn't work as I expected as data is not written to 
>> the
>> text box until the def is exited.  If I include a print statement with 
>> the
>> increment it is printed.
>>
>> Any suggestions are welcome.
>
> The crux of the matter is this: the windows are (re)drawn in response
> to events which instruct them to redraw themselves. The only way that
> can happen is if the event loop is running.
>
> When you have a loop in your code, while that loop is running the
> event loop is _not_ running. This is especially bad if you put a sleep
> in your code, because that causes the entire GUI to freeze. As a rule
> of thumb you should never, ever use sleep in a GUI.
>
> The quick fix for your problem is to add a call to `update_idletasks`
> in your loop. Why `update_idletasks` instead of `update`? Because the
> redrawing of windows is considered an "idle" task -- it is a class of
> event that Tk will service when it is otherwise idle. That, and
> calling `update` is potentially dangerous since it effectively creates
> a new event loop nested inside the current event loop.
>
> However, that's just a hack. The better solution is to architect your
> code differently. If you have a long running process you should
> consider running it in a separate thread or separate process. Even
> better, if you can divide your work up into small chunks you can set
> it up so that each chunk can be run during one iteration of the event
> loop. You can request that Tkinter execute something during a future
> iteration of the event loop with the `after` method.
>
> Typically you do this with a function that does one chunk of work. It
> then checks to see if there are any more chunks (by checking a flag,
> or a list of data to be processed, etc) . If it finds there is more
> work to do it uses itself as an argument to `after`, with a delay of a
> few ms. That way, each time the event loop loops, it does one chunk of
> your processing. No threads, no processes, no sleeps.
>
> If you want to see an example of a countdown timer using this
> technique see http://stackoverflow.com/a/10599462/7432