Label in PyQt4 GUI not updating with every loop of FOR loop

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The label gets updated all right, but the GUI isn’t redrawn before the end of your loop.

Here’s what you can do about it:

  • Move your long-running loop to a secondary thread, drawing the GUI is happening in the main thread.

  • Call app.processEvents() in your loop. This gives Qt the chance to process events and redraw the GUI.

  • Break up your loop and let it run using a QTimer with a timeout of 0.

Using a thread is the best option, but involves quite a bit more work than just calling processEvents. Doing it with a timer is the old fashioned way and is not recommanded anymore. (see the documentation)

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