'Override Qt method.'

def paintEvent(self, event):

    painter = QPainter(self)
    color = QColor(self.color)
    color.setAlphaF(0.5)
    painter.setPen(color)
    for (top, line_number, block) in self.editor.visible_blocks:
        bottom = (top + int(self.editor.blockBoundingRect(block).height()))
        indentation = TextBlockHelper.get_fold_lvl(block)
        for i in range(1, indentation):
            x = self.editor.fontMetrics().width(((i * self.i_width) * '9'))
            painter.drawLine(x, top, x, bottom)
