paintEvent() is an event handler, which is called by Qt when a widget needs to paint it self (QPaintEvent).
You do not call event handlers your self, they get called by the event loop.
If you want a widget to have a custom painting, you have to re implement its paintEvent(), but leave the calling to the event loop.

Therefore, your question is "wrong", since paintEvent() never comes by its self, its a member method, which you do not call, just re implement.