def paintEvent(self, event):
        """Override Qt method."""
        painter = QPainter(self)

        color = QColor(self.color)
        color.setAlphaF(.5)
        painter.setPen(color)
        offset = self.editor.document().documentMargin() + \
            self.editor.contentOffset().x()

        for _, line_number, block in self.editor.visible_blocks:

            indentation = TextBlockHelper.get_fold_lvl(block)
            ref_lvl = indentation
            block = block.next()
            last_line = block.blockNumber()
            lvl = TextBlockHelper.get_fold_lvl(block)
            if ref_lvl == lvl:  # for zone set programmatically such as imports
                # in pyqode.python
                ref_lvl -= 1

            while (block.isValid() and
                   TextBlockHelper.get_fold_lvl(block) > ref_lvl):
                last_line = block.blockNumber()
                block = block.next()

            end_of_sub_fold = block
            if last_line:
                block = block.document().findBlockByNumber(last_line)
                while ((block.blockNumber()) and (block.text().strip() == ''
                       or block.text().strip().startswith('#'))):
                    block = block.previous()
                    last_line = block.blockNumber()

            block = self.editor.document().findBlockByNumber(line_number)
            top = int(self.editor.blockBoundingGeometry(block).translated(
                self.editor.contentOffset()).top())
            bottom = top + int(self.editor.blockBoundingRect(block).height())

            indentation = TextBlockHelper.get_fold_lvl(block)

            for i in range(1, indentation):
                if (line_number > last_line and
                        TextBlockHelper.get_fold_lvl(end_of_sub_fold) <= i):
                    continue
                else:
                    x = self.editor.fontMetrics().width(i * self.i_width *
                                                        '9') + offset
                    painter.drawLine(x, top, x, bottom)