[PyQt] label doesn't update on each iteration

[daa]

Hi all,
  I'm not new to programming, but I am new to Python.
  Can someone tell me why the label (lblTest) in my form (PyQt) doesn't update on each iteration? It only updates finally when the loop is complete.
  I've tried using ui.lblTest.update() and ui.lblTest.hide() and ui.lblTest.show() on each iteration (as I've seen suggested elsewhere), but it doesn't work.

def btnTest_clicked():
    x = 0
    while x < 4:
        print("Test: " + str(x))
        ui.lblTest.setText("Test: " + str(x))
        x = x + 1
        time.sleep(1.0)

Thanks,
David