code: def showHeaderMenu( self, pos):
          
          header = self.header()
          index  = header.logicalIndexAt(pos)
          self._headerIndex = index
          
          # show a pre-set menu
          if self._headerMenu:
              menu = self._headerMenu
          else:
              menu = self.createHeaderMenu(index)
          
          # determine the point to show the menu from
          if pos is not None:
              point = header.mapToGlobal(pos)
          else:
              point = QtGui.QCursor.pos()
          
          self.headerMenuAboutToShow.emit(menu, index)
          menu.exec_(point)
docs: Displays the header menu for this tree widget.
          
          :param      pos | <QtCore.QPoint> || None