[FIXED] How to erase point in PyQt5 QPainter?

Issue

I want to display the x and y coordinates from an accelerometer on an image. I used QPainter to set up the image and the drawPoint function to draw the coordinate at the given point. When the new coordinate is drawn, I need to erase the old coordinate so that there is only one at a time. In the code below, I called drawCoordinate twice, what happens is the target image ends up with 2 coordinate points at 0,0 and 0.5,0.5 but ideally I would be left with the last drawn coordinate.

This is my first question on here so I hope my format is correct! Let me know if I need to fix anything to help with clarity.

class Target(QWidget):

    def __init__(self):
        super().__init__()
        self.drawing = False
        self.image = QPixmap(r"Pictures\target_png_300.png")
        self.setGeometry(0, 0, 300, 300)
        self.resize(self.image.width(), self.image.height())
        self.show()

    def paintEvent(self, event):
        painter = QPainter(self)
        painter.drawPixmap(self.rect(), self.image)
    
    def paintCoordinate(self, x, y):
        painter = QPainter(self.image)
        r = QRect(-1, -1, 2, 2)
        painter.setWindow(r)
        pen = QPen(Qt.black, 0.06, Qt.DotLine, Qt.RoundCap)
        painter.setPen(pen)
        painter.drawPoint(QPointF(x, y))

if __name__ == '__main__':
    
    app = QApplication(sys.argv)
    ex = Target()
    ex.paintCoordinate(0, 0)
    ex.paintCoordinate(0.5, 0.5)
    sys.exit(app.exec_())

Solution

If you want only one point then don't modify the QPixmap but just do the painting in the paintEvent method:

class Target(QWidget):
    def __init__(self):
        super().__init__()
        self._pixmap = QPixmap()
        self._coordinate = QPointF()

    @property
    def pixmap(self):
        return self._pixmap

    @pixmap.setter
    def pixmap(self, pixmap):
        self._pixmap = pixmap.copy()
        self.update()
        size = self.pixmap.size()
        if size.isValid():
            self.resize(size)
        else:
            self.resize(300, 300)

    @property
    def coordinate(self):
        return self._coordinate

    @coordinate.setter
    def coordinate(self, point):
        self._coordinate = point
        self.update()

    def paintEvent(self, event):
        painter = QPainter(self)
        painter.drawPixmap(self.rect(), self.pixmap)
        r = QRect(-1, -1, 2, 2)
        painter.setWindow(r)
        pen = QPen(Qt.black, 0.06, Qt.DotLine, Qt.RoundCap)
        painter.setPen(pen)
        painter.drawPoint(self.coordinate)


if __name__ == "__main__":

    app = QApplication(sys.argv)
    ex = Target()
    ex.pixmap = QPixmap(r"Pictures\target_png_300.png")
    ex.coordinate = QPointF(0, 0)
    ex.coordinate = QPointF(0.5, 0.5)
    ex.show()
    sys.exit(app.exec_())

Answered By - eyllanesc