time.sleep() crashes program when using while-loop in PyQt

Question

When I use the function time.sleep() in the terminal, it is no problem. When I use PyQt and want to use a label, it crashes and/or only shows the last number.

By the way: I want a function, that counts e.g. a year from 2020 to 2030 in various speed, that the user can change and the year should be shown in a label.

Thanks a lot for your help.

# timer that counts in the future with various speed // still crashing
def timer(self):
    x=datetime.datetime.now()
    z=x.ctime()
    self.ui.labelDateTime.setText(z)
    var=x.year
    while True:
        if var==2030:
            break
        else:
            var+=1
            y=x.replace(year=var)
            z=y.ctime()
            self.ui.labelDateTime.setText(z)
            time.sleep(0.5)
    print("You are dead")

Answer 1

You should call QtCore.QCoreApplication.processEvents() within your for loop to make the Qt's event loop proceed the incoming event (from keyboard or mouse or sleep).

Although calling QtCore.QCoreApplication.processEvents() works , I have read in many places on the web that it should be a last resort. Unfortunately none of the sources clearly explain why -- but see for example

• How to make Qt work when main thread is busy?
• Should I use QCoreApplication::processEvents() or QApplication::processEvents()?
• How to implement a QThread that runs forever{} with a QWaitCondition but still needs to catch another Slot while doing that
• http://qt-project.org/forums/viewthread/22967

So it does seem allowed, but in general, it seems to be better design to use QTimer or QThread.