import sys

 from PyQt5.QtWidgets import *

 from PyQt5.QtCore import *

 from PyQt5.QtGui import *

 ################################################

 #######创建主窗口

 ################################################

 class FirstMainWindow(QMainWindow):

     def __init__(self, *args, **kwargs):

         super().__init__(*args, **kwargs)

         self.setWindowTitle('主界面')

         ###### 创建界面 ######

         self.centralwidget = QWidget()

         self.setCentralWidget(self.centralwidget)

         self.Layout = QVBoxLayout(self.centralwidget)

         # 设置顶部三个按钮

         self.topwidget = QWidget()

         self.Layout.addWidget(self.topwidget)

         self.buttonLayout = QHBoxLayout(self.topwidget)

         self.pushButton1 = QPushButton()

         self.pushButton1.setText("打开主界面")

         self.buttonLayout.addWidget(self.pushButton1)

         self.pushButton2 = QPushButton()

         self.pushButton2.setText("打开对话框")

         self.buttonLayout.addWidget(self.pushButton2)

         self.pushButton3 = QPushButton()

         self.pushButton3.setText("打开提示框")

         self.buttonLayout.addWidget(self.pushButton3)

         # 设置中间文本

         self.label = QLabel()

         self.label.setText("第一个主界面")

         self.label.setSizePolicy(QSizePolicy(QSizePolicy.Expanding, QSizePolicy.Expanding))

         self.label.setAlignment(Qt.AlignCenter)

         self.label.setFont(QFont("Roman times", 50, QFont.Bold))

         self.Layout.addWidget(self.label)

         # 设置状态栏

         self.statusBar().showMessage("当前用户：一心狮")

         # 窗口最大化

         self.showMaximized()

         ###### 三个按钮事件 ######

         self.pushButton1.clicked.connect(self.on_pushButton1_clicked)

         self.pushButton2.clicked.connect(self.on_pushButton2_clicked)

         self.pushButton3.clicked.connect(self.on_pushButton3_clicked)

     # 按钮一：打开主界面

     windowList = []

     def on_pushButton1_clicked(self):

         the_window =SecondWindow()

         self.windowList.append(the_window)   ##注：没有这句，是不打开另一个主界面的！

         self.close()

         the_window.show()

     # 按钮二：打开对话框

     def on_pushButton2_clicked(self):

         the_dialog = TestdemoDialog()

         if the_dialog.exec_() == QDialog.Accepted:

             pass

     # 按钮三：打开提示框

     def on_pushButton3_clicked(self):

         QMessageBox.information(self, "提示", "这是information框！")

         #QMessageBox.question(self, "提示", "这是question框！")

         #QMessageBox.warning(self, "提示", "这是warning框！")

         #QMessageBox.about(self, "提示", "这是about框！")

 ################################################

 #######第二个主界面

 ################################################

 class SecondWindow(QMainWindow):

     def __init__(self, *args, **kwargs):

         super().__init__(*args, **kwargs)

         self.setWindowTitle('第二主界面')

         # 设置中间文本

         self.label = QLabel()

         self.label.setText("第二个主界面")

         self.label.setSizePolicy(QSizePolicy(QSizePolicy.Expanding, QSizePolicy.Expanding))

         self.label.setAlignment(Qt.AlignCenter)

         self.label.setFont(QFont("Roman times", 50, QFont.Bold))

         self.setCentralWidget(self.label)

         # 设置状态栏

         self.statusBar().showMessage("当前用户：一心狮")

         # 窗口最大化

         self.showMaximized()

     ###### 重写关闭事件，回到第一界面

     windowList = []

     def closeEvent(self, event):

         the_window = FirstMainWindow()

         self.windowList.append(the_window)  ##注：没有这句，是不打开另一个主界面的！

         the_window.show()

         event.accept()

 ################################################

 #######对话框

 ################################################

 class TestdemoDialog(QDialog):

     def __init__(self, *args, **kwargs):

         super().__init__(*args, **kwargs)

         self.setWindowTitle('对话框')

         ### 设置对话框类型

         self.setWindowFlags(Qt.Tool)

 ################################################

 #######程序入门

 ################################################

 if __name__ == "__main__":

     app = QApplication(sys.argv)

     the_mainwindow = FirstMainWindow()

     the_mainwindow.show()

     sys.exit(app.exec_())