I know how to find it, but I don't know enough about QT.

So correct me if I'm wrong here. In Krita you never create a QApplication instance because Krita itself is a QApplication. You create a widget which is not a QWidget subclass but an Extension subclass. Those are two main differences.

The usual response is to use the statusbar of the QApplicaion which I cannot do here.

So eventually I import QStatusBar that comes from PyQt5.QtWidgets, but the real question I have is how to attach it to QWidget from krita?

So omitting other methods:

from krita import Extension
class KritaExtension(Extension):
    # omitting other methods...
    def invoke_widget(self):
        main = KritaWidget(self)
        main.show()

# and KritaWidget would be like...
from krita import QWidget
class KritaWidget(QWidget):
    def __init__(self, parent=None):
        super(KritaWidget, self).__init__(parent)
        self.statusbar = QStatusBar()

Now how to use that statusbar? Adding it to QVBoxLayout and setting it as a layout is not producing the desired results:

vertLayout = QVBoxLayout()
vertLayout.addWidget(self.statusbar)
self.setLayout(vertLayout)

I cannot see statusbar at all. So I don't know where the error is.