global variables

[HeinKurz]

I've read a view posts and articles about global variables so I don't understand why it doesn't work in my app.
In the code below pos1, pos2 and pos2 create an error at keyPressEvent: "local variable 'pos...' referenced before assignment".
Why? And how can I do it right?
pos is an index that I want to increase/decrease at keypress.

import sys

global pos1

class MainWindow(QtWidgets.QMainWindow):

    global pos2
    pos2 = 1
    pos1 = 1
    
    def __init__(self):
        super().__init__()

        global pos3
        pos3 = 1

        # main window
        self.setWindowTitle("Uhren II")
        self.setFixedSize(500, 500)
        self.setWindowIcon(QIcon("Uhren2.ico"))

        # ...

    def keyPressEvent(self, event):
        if type(event) == QtGui.QKeyEvent and (event.key() == QtCore.Qt.Key_Left or event.key() == QtCore.Qt.Key_Down) :
            pos1 -= 1
            pos2 -= 1
            pos3 -= 1
        if type(event) == QtGui.QKeyEvent and (event.key() == QtCore.Qt.Key_Right or event.key() == QtCore.Qt.Key_Up) :
            pos1 += 1
            pos2 += 1
            pos3 += 1
        print(pos1, pos2, pos3)

app = QtWidgets.QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec()