import sys
from PyQt5.QtWidgets import QMainWindow, QPushButton, QApplication
  
  
class First(QMainWindow):
  
  def __init__(self):
    super().__init__()
  
    self.initUI()
  
  def initUI(self):
    self.btn = QPushButton("进入页面2", self)
    self.btn.move(30, 50)
     
  
  
    self.setGeometry(300, 200, 500, 500)
    self.setWindowTitle('页面1')
 
  
class Second(QMainWindow):
  def __init__(self):
    super().__init__()
  
    self.initUI()
  
  def initUI(self):
    self.btn = QPushButton("进入页面3", self)
    self.btn.move(30, 50)
    self.setGeometry(300, 200, 290, 150)
    self.setWindowTitle('页面2')
 
 
class Third(QMainWindow):
  def __init__(self):
    super().__init__()
  
    self.initUI()
  
  def initUI(self):
    self.btn = QPushButton("返回页面1", self)
    self.btn.move(30, 50)
 
    self.btn2 = QPushButton("功能函数", self)
    self.btn2.move(30, 90)
 
    self.setGeometry(500, 200, 290, 350)
    self.setWindowTitle('页面3')
    self.btn2.clicked.connect(self.action)
  def action(self):
    print("这是功能函数")
   
  
  
if __name__ == '__main__':
  app = QApplication(sys.argv)
  a = First()
  b = Second()
  c = Third()
  a.show()
  a.btn.clicked.connect(b.show)
  a.btn.clicked.connect(a.close)
  b.btn.clicked.connect(c.show)
  b.btn.clicked.connect(b.close)
  c.btn.clicked.connect(a.show)
  c.btn.clicked.connect(c.close)
  sys.exit(app.exec_())

from PyQt5 import QtCore, QtGui, QtWidgets
import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, QLabel,\
                     QPushButton, QLineEdit, QMenuBar, QStatusBar
from PyQt5.QtCore import *
 
class FirstWindow(QWidget):
 
  def __init__(self, parent=None):
    # super这个用法是调用父类的构造函数
    # parent=None表示默认没有父Widget，如果指定父亲Widget，则调用之
    super(FirstWindow, self).__init__(parent)
    self.setGeometry(500, 500, 500, 500)
    self.setWindowTitle('显示')
    self.btn = QPushButton(self)
    self.btn.setText('进入页面2')
    self.btn.move(150, 50)
 
 
#下面的一部分是Qtdesigner的代码
 
class Ui_MainWindow(object):
  def setupUi(self, MainWindow):
    MainWindow.setObjectName("MainWindow")
    MainWindow.resize(624, 479)
    self.centralwidget = QtWidgets.QWidget(MainWindow)
    self.centralwidget.setObjectName("centralwidget")
    self.label = QtWidgets.QLabel(self.centralwidget)
    self.label.setGeometry(QtCore.QRect(190, 130, 51, 31))
    self.label.setObjectName("label")
    self.pushButton = QtWidgets.QPushButton(self.centralwidget)
    self.pushButton.setGeometry(QtCore.QRect(330, 260, 75, 23))
    self.pushButton.setObjectName("pushButton")
    self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
    self.lineEdit.setGeometry(QtCore.QRect(280, 130, 151, 21))
    self.lineEdit.setObjectName("lineEdit")
    self.label_2 = QtWidgets.QLabel(self.centralwidget)
    self.label_2.setGeometry(QtCore.QRect(190, 180, 51, 31))
    self.label_2.setObjectName("label_2")
    self.lineEdit_2 = QtWidgets.QLineEdit(self.centralwidget)
    self.lineEdit_2.setGeometry(QtCore.QRect(280, 190, 151, 21))
    self.lineEdit_2.setObjectName("lineEdit_2")
    self.pushButton_2 = QtWidgets.QPushButton(self.centralwidget)
    self.pushButton_2.setGeometry(QtCore.QRect(200, 260, 75, 23))
    self.pushButton_2.setObjectName("pushButton_2")
    MainWindow.setCentralWidget(self.centralwidget)
    self.menubar = QtWidgets.QMenuBar(MainWindow)
    self.menubar.setGeometry(QtCore.QRect(0, 0, 624, 23))
    self.menubar.setObjectName("menubar")
    MainWindow.setMenuBar(self.menubar)
    self.statusbar = QtWidgets.QStatusBar(MainWindow)
    self.statusbar.setObjectName("statusbar")
    MainWindow.setStatusBar(self.statusbar)
 
    self.retranslateUi(MainWindow)
    QtCore.QMetaObject.connectSlotsByName(MainWindow)
 
  def retranslateUi(self, MainWindow):
    _translate = QtCore.QCoreApplication.translate
    MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
    self.label.setText(_translate("MainWindow", "  长"))
    self.pushButton.setText(_translate("MainWindow", "返回"))
    self.label_2.setText(_translate("MainWindow", "  宽"))
    self.pushButton_2.setText(_translate("MainWindow", "确定"))
 
if __name__ == "__main__":
  app = QApplication(sys.argv)
  MainWindow = QMainWindow()
  ui = Ui_MainWindow()
  ui.setupUi(MainWindow)
  ex = FirstWindow()
  ex.btn.clicked.connect(MainWindow.show)
  ex.btn.clicked.connect(ex.close)
  ui.pushButton.clicked.connect(ex.show)
  ui.pushButton.clicked.connect(MainWindow.close)
  ex.show()
  sys.exit(app.exec_())