Tkinter GUI freezing, used Thread then encountered RuntimeError: threads can only be started once

[MRAB]

On 2023-01-11 00:13, Cameron Simpson wrote:
> On 10Jan2023 18:32, MRAB <python at mrabarnett.plus.com> wrote:
>>I don't like how you're passing Thread...start as an argument. IMHO, it 
>>would be better/cleaner to pass a plain function, even if the only 
>>thing that function does is to start the thread.
> 
> Yes, and this is likely the thing causing the cited exception "threads
> can only be started once". Your setup of the button with the action
> defined as:
> 
>       Thread(....).start
> 
> creates a _single_ new Thread _when you define the button_, and makes
> hte button callback try to start it. On the second and following
> callback, you're trying to start the _same_ single Thread again.
> 
You're right! I missed that detail. :-(

> Do as MRAB suggests and have the callback create-and-start a Thread
> instead of just starting an _existing_ Thread.
> 
> Also, for simple quick things there's no need to use a Thread at all. If
> the final version of the programme is going to do something long running
> at that point, then sure.
> 
>>I can't tell what 'change_flag' is doing because of the formatting 
>>issue. Is it doing GUI stuff? In a thread? If yes, don't do that. The 
>>GUI doesn't like that. Only the main thread should do GUI stuff.
> 
> Aye. This is very important in almost all GUI toolkits.
> 
> Bit me very badly with Qt once, badly in that the segfaults (yes!
> segfaults! in a Python app!) were erratic and very timing dependent,
> making them hard to reproduce and understand. It wasn't until I
> _realised_ it was thread/concurrency related that I could fix it.
> 
> Note that in Tk you can have a callback do GUI work, just not in a
> separate thread.
>