Qtdesigner designs the interface, saves it as a UI file, converts it into a Python file through PyUIC, and imports the classes of this file into another python file, main.py. (in order to separate the UI, it is convenient to edit UI),

from mainUI import Ui_MainWindow

1. how to continue writing code to make the page appear after
2. the details of how to implement the slot function openFile (), in main.py are customized during the QtDesigner design

add the contents of the python file converted by PyUIC:

-sharp -*- coding: utf-8 -*-

-sharp Form implementation generated from reading ui file "mainUI.ui"
-sharp
-sharp Created by: PyQt5 UI code generator 5.10
-sharp
-sharp WARNING! All changes made in this file will be lost!

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayoutWidget = QtWidgets.QWidget(self.centralwidget)
        self.gridLayoutWidget.setGeometry(QtCore.QRect(230, 160, 160, 181))
        self.gridLayoutWidget.setObjectName("gridLayoutWidget")
        self.gridLayout = QtWidgets.QGridLayout(self.gridLayoutWidget)
        self.gridLayout.setContentsMargins(0, 0, 0, 0)
        self.gridLayout.setObjectName("gridLayout")
        self.toolButton_4 = QtWidgets.QToolButton(self.gridLayoutWidget)
        self.toolButton_4.setObjectName("toolButton_4")
        self.gridLayout.addWidget(self.toolButton_4, 2, 0, 1, 1)
        self.toolButton_3 = QtWidgets.QToolButton(self.gridLayoutWidget)
        self.toolButton_3.setObjectName("toolButton_3")
        self.gridLayout.addWidget(self.toolButton_3, 1, 0, 1, 1)
        self.toolButton = QtWidgets.QToolButton(self.gridLayoutWidget)
        self.toolButton.setObjectName("toolButton")
        self.gridLayout.addWidget(self.toolButton, 0, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 30))
        self.menubar.setObjectName("menubar")
        self.menuFile = QtWidgets.QMenu(self.menubar)
        self.menuFile.setObjectName("menuFile")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.actionNew = QtWidgets.QAction(MainWindow)
        self.actionNew.setObjectName("actionNew")
        self.actionOpen = QtWidgets.QAction(MainWindow)
        self.actionOpen.setObjectName("actionOpen")
        self.actionSave = QtWidgets.QAction(MainWindow)
        self.actionSave.setObjectName("actionSave")
        self.actionClose = QtWidgets.QAction(MainWindow)
        self.actionClose.setObjectName("actionClose")
        self.actionQuite = QtWidgets.QAction(MainWindow)
        self.actionQuite.setObjectName("actionQuite")
        self.menuFile.addAction(self.actionNew)
        self.menuFile.addAction(self.actionOpen)
        self.menuFile.addSeparator()
        self.menuFile.addAction(self.actionSave)
        self.menuFile.addSeparator()
        self.menuFile.addAction(self.actionClose)
        self.menuFile.addSeparator()
        self.menuFile.addAction(self.actionQuite)
        self.menubar.addAction(self.menuFile.menuAction())

        self.retranslateUi(MainWindow)
        self.toolButton_4.clicked.connect(MainWindow.close)
        self.toolButton_3.clicked.connect(MainWindow.openFile)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.toolButton_4.setText(_translate("MainWindow", "Quit"))
        self.toolButton_3.setText(_translate("MainWindow", "Open"))
        self.toolButton.setText(_translate("MainWindow", "New"))
        self.menuFile.setTitle(_translate("MainWindow", "Fi&le"))
        self.actionNew.setText(_translate("MainWindow", "New"))
        self.actionOpen.setText(_translate("MainWindow", "Open"))
        self.actionSave.setText(_translate("MainWindow", "Save"))
        self.actionClose.setText(_translate("MainWindow", "Close"))
        self.actionQuite.setText(_translate("MainWindow", "Quit"))