Update the Opacity of QGraphicsItem

I want to update the opacity of some QGraphicsItem after the mouse clicking. As suggested from other solution, the QGraphicScene manually update the GraphicsItem after the mouser press event. I have tried different setOpacity() and update() in QGraphicsScene and QGraphicsItem. But none works and do not know what is wrong.

import sys
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

CUBE_POS = {
    "a":(   8.281,  18.890),
    "b":(   8.668,  23.692),
    "c":(   21.493, 23.423),
    "d":(   21.24,  15.955),
    }        

class CubeItem(QGraphicsItem):

    def __init__(self, x, y, parent=None):
        super(CubeItem,self).__init__(parent)
        self.x = x
        self.y = y
        self.polygon = QPolygonF([
            QPointF(self.x-10, self.y-10), QPointF(self.x-10, self.y+10),
            QPointF(self.x+10, self.y+10), QPointF(self.x+10, self.y-10),
            ])
        self._painter = QPainter()

    ##Estimate the drawing area
    def boundingRect(self):
        return QRectF(self.x-10, self.y-10, 20, 20)

    ##Real Shape of drawing area
    def shape(self):
        path = QPainterPath()
        path.addRect(self.x-10, self.y-10, 20, 20)
        return path

    ##paint function called by graphicview
    def paint(self, painter, option, widget):
        painter.setBrush(Qt.red)
        painter.setOpacity(0.2)
        painter.drawRect(self.x-10, self.y-10, 20, 20)
        self._painter = painter

    def activate(self):
        try:
            #self._painter.setOpacity(1.0)
            self.setOpacity(1.0)
            self.update()
        except ValueError as e:
            print(e)

class TagScene(QGraphicsScene):

    def __init__(self, parent=None):
        super(TagScene, self).__init__(parent)

        self.cubes_items_ref = {}
        self.addCubes()

    def addCubes(self):
        for cube in CUBE_POS:
            newCube = CubeItem(CUBE_POS[cube][0]*15, 
                                   CUBE_POS[cube][1]*15)
            self.addItem(newCube)
            self.cubes_items_ref[cube] = newCube

    def mousePressEvent(self, event):
        print("mouse pressed")

        #for cube in self.cubes_items_ref:
        #    self.cubes_items_ref[cube].setOpacity(1.0)
        #    #self.cubes_items_ref[cube].activate()
        #self.update(QRectF(0,0,500,500))

        for cube in self.items():
            cube.setOpacity(1.0)
        self.update(QRectF(0,0,500,500))

class MainWindow(QMainWindow):

     def __init__(self):
        super(MainWindow, self).__init__()
        layout = QHBoxLayout()
        self.scene = TagScene()
        self.view = QGraphicsView(self.scene)
        self.scene.setSceneRect(QRectF(0,0,500,500))
        layout.addWidget(self.view)
        self.widget = QWidget()
        self.widget.setLayout(layout)
        self.setCentralWidget(self.widget)

if __name__ == "__main__":

    app = QApplication(sys.argv)
    test = MainWindow()
    test.show()
    sys.exit(app.exec_())