code: def drawBackground(self, painter, rect):
          
          if self._dirty:
              self.rebuild()
          
          # draw the alternating rects
          gantt   = self.ganttWidget()
          
          # draw the alternating rects
          painter.setPen(Qt.NoPen)
          painter.setBrush(gantt.alternateBrush())
          for rect in self._alternateRects:
              painter.drawRect(rect)
          
          # draw the weekends
          painter.setBrush(gantt.weekendBrush())
          for rect in self._weekendRects:
              painter.drawRect(rect)
          
          # draw the default background
          painter.setPen(gantt.gridPen())
          painter.drawLines(self._hlines + self._vlines)
docs: Draws the background for this scene.
          
          :param      painter | <QPainter>
                      rect    | <QRect>