pyqt add widget to window

1# first we need to import  QVBoxLayout, QWidget from  PyQt5.QtWidgets
2import sys
3from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, \
4    QPushButton, QVBoxLayout
5# you can copy and run this code
6class MainWindow(QMainWindow):
7    def __init__(self, parent=None):
8        super(MainWindow, self).__init__(parent)
9        
10        layout = QVBoxLayout()
11        btn = QPushButton("Click")
12
13        layout.addWidget(btn) # add the widget to our layout
14        
15        w = QWidget()
16        w.setLayout(layout) # set layout to our central widget
17        self.setCentralWidget(w) # set w as central widget
18        self.show()
19
20if __name__ == "__main__":
21    app = QApplication(sys.argv)
22    window = MainWindow()
23    sys.exit(app.exec_())