PyQt: Scrollbars are not shown with QGraphicsView class being nested under QMainWindo

I am supposed to add scrollbars to the screen. The class design shown below is the part of a software that I am supposed to modify and add scrollbars to. The scroll bar code in Window class has been added by me.

It doesn't get shown when I run the program. What am I supposed to correct?

Code:

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import sys
 
from PyQt4 import QtGui
from PyQt4 import QtCore
 
class Window(QtGui.QGraphicsView):
 
  def __init__(self, parent=None):
 
        QtGui.QGraphicsView.__init__(self, parent)
        self.scene = QtGui.QGraphicsScene(self)
        self.scene.setBackgroundBrush(QtGui.QBrush(QtCore.Qt.darkGray, QtCore.Qt.SolidPattern))
        self.setScene(self.scene)
 
        self.setDragMode(QtGui.QGraphicsView.ScrollHandDrag)
        self.setTransformationAnchor(QtGui.QGraphicsView.AnchorUnderMouse)
        self.viewport().setCursor(QtCore.Qt.CrossCursor)
        self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOn)
        self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOn)
 
        print "sdsads"
 
 
class CityscapesLabelTool(QtGui.QMainWindow):
    def __init__(self, parent=None):
 
        QtGui.QMainWindow.__init__(self, parent)
        Window()
 
app = QtGui.QApplication(sys.argv)
GUI = CityscapesLabelTool()
GUI.show()
sys.exit(app.exec_())

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