...

src/kr/co/swh/lecture/python/pyqt5/mainwindow3.py

 from PyQt5.QtGui import QIcon
 from PyQt5.QtCore import Qt
 
-class MyWindow(QWidget):
+class MyWindow(QMainWindow):
     def __init__(self):
         super().__init__()
         self.title = 'SWH Academy Window.'