如何在使用exec打开时关闭Qdialog

Question

我有一个qdialog在我使用exec时运行并运行但是当我使用call回调取消按钮时，我无法取消对话框。

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
      MainWindow.setObjectName(_fromUtf8("MainWindow"))
      MainWindow.resize(1200, 900)
      self.StudentButton_Add = QtGui.QPushButton(MainWindow)
      self.StudentButton_Add.setGeometry(QtCore.QRect(30, 830, 91, 23))
      self.StudentButton_Add.setObjectName(_fromUtf8("StudentButton_Add"))
      self.StudentButton_Add.clicked.connect(self.addStudentfunc)
    def retranslateUi(self, MainWindow):
      self.StudentButton_Add.setText(_translate("MainWindow", "Add Student", None))


    def addStudentfunc(self):
      #function to execute dialog
      self.homewindow = QtGui.QDialog()
      self.ui = Ui_DialogAdd()
      self.ui.setupUi(self.homewindow)
      self.homewindow.exec_() #this is where i have used exec

class Ui_DialogAdd(object):
   def setupUi(self, Dialog):
      Dialog.setObjectName(_fromUtf8("Dialog"))
      Dialog.resize(480, 640)
      self.pushButton_2 = QtGui.QPushButton(Dialog)
      self.pushButton_2.setGeometry(QtCore.QRect(140, 530, 61, 27))
      self.pushButton_2.setObjectName(_fromUtf8("pushButton_2"))
      self.pushButton_2.clicked.connect(self.cancelClicked)
      self.retranslateUi(Dialog)
      QtCore.QMetaObject.connectSlotsByName(Dialog)


   def canelClicked(self):
      Ui_dialogAdd.close() #this is where I am trying to close the dialog