After jointing a range of cells horizontally, and then I joint the cells generally, the selected cells go mad

from PySide2 import QtCore
from PySide2 import QtGui
from PySide2 import QtWidgets
import PySide2
import os
import sys
class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent=None)
        menuBar = self.menuBar()
        joint_cell_menu = QtWidgets.QMenu("Joint")
        joint_cell_action = QtWidgets.QAction("Joint Cells", joint_cell_menu)
        self.connect(joint_cell_action, QtCore.SIGNAL("triggered(bool)"), self.joint_cell)
        horizontal_joint_cell_action = QtWidgets.QAction("Joint Cells Horizontally", joint_cell_menu)
        self.connect(horizontal_joint_cell_action, QtCore.SIGNAL("triggered(bool)"), self.horizontal_joint_cell_action)             
        joint_cell_menu.addAction(joint_cell_action)
        joint_cell_menu.addAction(horizontal_joint_cell_action)
        menuBar.addMenu(joint＿cell_menu)               
        self.view = View()               
        self.setCentralWidget(self.view)
    def joint_cell(self):
        indexes = self.view.excel.selectedIndexes()
        first_index = indexes[0]
        last_index = indexes[-1]        
        first_row = first_index.row()
        first_column = first_index.column()
        last_row = last_index.row()
        last_column = last_index.column()
        self.view.excel.setSpan(first_row, first_column, last_row-first_row+1, last_column-first_column+1)
    def horizontal_joint_cell_action(self):
        indexes = self.view.excel.selectedIndexes()        
        first_index = indexes[0]
        last_index = indexes[-1]        
        first_row = first_index.row()
        first_column = first_index.column()
        last_row = last_index.row()
        last_column = last_index.column()
        columns = last_column-first_column+1
        for r in range(first_row, last_row+1, 1):
            self.view.excel.setSpan(r, first_column, 1, columns)
class View(QtWidgets.QGraphicsView):
    def __init__(self, parent=None):
        super(View, self).__init__(parent=None)
        desktop = QtWidgets.QDesktopWidget()
        self.scene = Scene()        
        self.excel = TableWidget(10, 10)     
        self.setScene(self.scene)  
        p = self.scene.addWidget(self.excel)
        self.scene.setSceneRect(p.sceneBoundingRect().x(), p.sceneBoundingRect().y(), desktop.width(), desktop.height()+100)   
class Scene(QtWidgets.QGraphicsScene):
    def __init__(self, parent=None):
        super(Scene, self).__init__(parent=None)
class TableWidget(QtWidgets.QTableWidget):
    def __init__(self, rows, columns, parent=None):
        super(TableWidget, self).__init__(parent=None)
        self.setParent(parent)              
        self.setRowCount(rows)
        self.setColumnCount(columns)           
        self.resize(3000, 3000)def main():
    app = QtWidgets.QApplication([])
    mainwindow = MainWindow()
    mainwindow.show()
    sys.exit(app.exec_())
if __name__ == "__main__":
    main()