Pyqt5 learning pit encounter and pit drainage (2) buttons in qformlayout layout cannot be displayed

At first, the source code is as follows ：

import sys
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

#Qmessagebox
# class MessageBox(QMainWindow):
# def __init__(self, parent=None):
# super(MessageBox, self).__init__(parent)
# self.setWindowTitle("QDialog Demo")
# self.resize(350,300)
#
# self.btn = QPushButton(self)
# self.btn.setText(" Pop-up dialog box ")
# self.btn.clicked.connect(self.showmsg1)
#
# def showmsg1(self):
# message = QMessageBox.critical(self,"messagebox"," Confirmation window ",QMessageBox.Ok | QMessageBox.Cancel,QMessageBox.Ok)
#
#
# if __name__ == "__main__":
# app = QApplication(sys.argv)
# box = MessageBox()
# box.show()
# sys.exit(app.exec_())

class InputDialog(QMainWindow):
    def __init__(self, parent=None):
        super(InputDialog, self).__init__(parent)
        self.setWindowTitle("InputDialog Demo")

        layout = QFormLayout()

        self.btn1 = QPushButton(" Get the options in the list ")
        self.btn1.clicked.connect(self.getItem)
        self.lb1 = QLineEdit()
        layout.addRow(self.btn1,self.lb1)

        self.btn2 = QPushButton(" Get string ")
        self.btn2.clicked.connect(self.getText)
        self.lb2 = QLineEdit()
        layout.addRow(self.btn2,self.lb2)

        self.btn3 = QPushButton(" Get an integer ")
        self.btn3.clicked.connect(self.getInt)
        self.lb3 = QLineEdit()
        layout.addRow(self.btn3,self.lb3)

        self.setLayout(layout)

    def getItem(self):
        items = {
    "C","c++","JAVA",'Python'}
        item , ok = QInputDialog.getItem(self,"select inputdialog"," list ",items,0,False)
        if ok and item:
            self.lb1.setText(item)

    def getText(self):
        text , ok = QInputDialog.getText(self,"Text inputdialog"," Enter a name ")
        if ok:
            self.lb2.setText(str(text))


    def getInt(self):
        num , ok = QInputDialog.getInt(self,"Int inputdialog"," Input integer ")
        if ok:
            self.lb2.setText(str(num))

if __name__ == "__main__":
    app = QApplication(sys.argv)
    box = InputDialog()
    box.show()
    sys.exit(app.exec_())

After searching online for a long time, I couldn't find a solution , And then it came to me , It seems that the inherited things in the class are not quite right , Then I tried to change it to QWidget, It works . Although I don't know why , But the problem was finally solved .
If someone knows why, I hope you can leave me a message .