Sequential programming, threads, causality, and the nature of time

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Dan wrote

I just started with PyQt and it seems pretty cool but tried to use a "progress bar" to track a long I/O process which introduced me to threads. I have things mostly working and understand how a long I/O would mess up the event queue for functions called AFTER the I/O process is called but I have experienced the following:

  1. FIRST I try & update some GUI widgets
  2. Immediately AFTERWARDS I call the long I/O function

Somehow step #2 interferes with step #1! It's like a time machine! ;-) I'm guessing that the call to step #1 just places GUI commands in a queue which immediately gets interfered with by step #2? So.... I guess all I'm asking for is a way to "flush the > queue" or somehow force step #1 to complete before step #2? I tried calling various "update" & "repaint" functions but that doesn't seem to help. I can (of course) deal with this via threads but it seems a bit inelegant... Any suggestions?


Qt always runs application logic before it runs UI code, even if Ui code was called first, that is what is going on.

To fix this call the I/O function in a separate thread.


Hello Dan, welcome to the forum! 🙂

Martin has written a wonderful article about threads and thread-blocking in GUI applications. You'll find all your answers there.


Thanks! It really is an excellent article.

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Sequential programming, threads, causality, and the nature of time was written by Martin Fitzpatrick .

Martin Fitzpatrick has been developing Python/Qt apps for 8 years. Building desktop applications to make data-analysis tools more user-friendly, Python was the obvious choice. Starting with Tk, later moving to wxWidgets and finally adopting PyQt.