Python List iter 和下一期

如何解决Python List iter 和下一期

我有一个列表，并使用ITER和下一个试图项之间开关，但它跳过的元件。我打印了清单，所有项目都在那里，那么我哪里出错了？

        if event.key == pygame.K_UP:
            level = self.current_level
            self.current_level.set_visible(False)
            list_iter = iter(self.levels)
            
            self.current_level = next(list_iter,None)
            if self.current_level == None:
                self.current_level = level
            else:   
                focuser.switch_component(self.current_level)
                self.current_level.set_visible(True)
                print self.current_level.full_name
            
            print "Len levels = " + str(len(self.levels))   
            print self.levels
            return True
                
        if event.key == pygame.K_DOWN:
            level = self.current_level
            self.current_level.set_visible(False)
            list_iter = iter(reversed(self.levels))
            
            self.current_level = next(list_iter,None)
            if self.current_level == None:
                self.current_level = level
            else:   
                focuser.switch_component(self.current_level)
                self.current_level.set_visible(True)
                print self.current_level.full_name
            
            print "Len levels = " + str(len(self.levels))
            print self.levels   
            return True