问QPainter删除以前绘制的形状

EN

class Canvas(QWidget):
    def __init__(self):
        super().__init__()

        self.initUI()
        self.initLogic()

    def initUI(self):
        self.image = QImage(self.size(), QImage.Format_RGB32)
        self.image.fill(Qt.white)

    def initLogic(self):
        self.brushSize = 1
        self.brushStyle = Qt.SolidLine
        self.brushColor = QColor(0, 0, 0)
        self.shapeMode = None

        self.drawing = False
        self.mousePointer = None

    def mousePressEvent(self, event):
        self.drawing = True
        self.mousePointer = event.pos()

    def mouseMoveEvent(self, event):
        #if no pen mode set draw lines from event to event
        if self.drawing:
            painter = QPainter(self.image)
            painter.setPen(QPen(self.brushColor,
                                self.brushSize,
                                self.brushStyle))
            
            shape = None #i try later to assign the method Qpainter.draw<someShape> to this variable
            #             hoping it works like in tkinter.
            if self.shapeMode == None:#free shape
                painter.drawLine(self.mousePointer, event.pos())
                self.mousePointer = event.pos()

            else:
                #previous x and previous y
                ox, oy = self.mousePointer.x(), self.mousePointer.y()
                #current x and current y
                dx, dy = event.pos().x(), event.pos().y()
                width, height = dx - ox, dy - oy
                #self.shapeMode is a string corresponding to a QPainter method
                #we get the corresponding method using getattr builtin function
                drawMethod = getattr(painter, self.shapeMode)# = painter.someFunc this works fine
                shape = drawMethod(ox, oy, width, height) #assigning the method call to a variable

            self.update()
            if shape != None:
                painter.eraseRect(shape)

        """
        if self.drawing and self.shapeMode == None:
            painter = QPainter(self.image)
            painter.setPen(QPen(self.brushColor,
                                self.brushSize,
                                self.brushStyle))
            painter.drawLine(self.mousePointer, event.pos())
            self.mousePointer = event.pos()
            self.update()"""
        #otherwise if pen mode set draw shape at event then delete until release


    def mouseReleaseEvent(self, event):
        if self.shapeMode != None:
            painter = QPainter(self.image)
            painter.setPen(QPen(self.brushColor,
                                self.brushSize,
                                self.brushStyle))

            #previous x and previous y
            ox, oy = self.mousePointer.x(), self.mousePointer.y()
            #current x and current y
            dx, dy = event.pos().x(), event.pos().y()
            width, height = dx - ox, dy - oy
            #self.shapeMode is a string corresponding to a QPainter methid
            #we get the corresponding method using getattr builtin function
            drawMethod = getattr(painter, self.shapeMode)# = painter.someFunc
            shape = drawMethod(ox, oy, width, height)
            self.update()

        self.mousePointer = event.pos()
        self.drawing = False
        #TODO end registering the action

    def paintEvent(self, event):
        widgetPainter = QPainter(self)
        widgetPainter.drawImage(self.rect(), self.image, self.rect())