PyQt window created within a class __init__ does not display [duplicate] - pyqt

Question

This question already has answers here:
Understanding Python super() with __init__() methods [duplicate]
(7 answers)
Variable scopes in Python classes
(4 answers)
Closed 2 years ago.
This code from a tutorial works fine in PyQt5 if I call it from __main__
window = QWidget()
layout = QVBoxLayout()
layout.addWidget(QPushButton('Top'))
layout.addWidget(QPushButton('Bottom'))
window.setLayout(layout)
window.show()
If I put the code class in the __init__ of a class:
class MainWindow (QMainWindow):
def __init__ (self):
print ("HERE 1")
window = QWidget()
layout = QVBoxLayout()
layout.addWidget(QPushButton('Top'))
layout.addWidget(QPushButton('Bottom'))
window.setLayout(layout)
window.show()
print ("HERE 2")
return
then in __main__:
app = QApplication (args)
mw = MainWindow.MainWindow ()
return app.exec_ ()
Then the print statemets happen but no widgets display.
Why would window not display in the second case? (The corresponding C++ code would display it.)

Answer 1

You need a bit more code to make this run. You are trying to show the widget window but you are not showing the QmainWindow. If the widget is on the QMainWindow it will be visible when you show the main window. You also need to call init from the QMainWindow class to set everything up. Finally, with a QMainWindow, you need to set a central widget.
I generally take a different approach in main. Take a look at my code below:
import sys
from PyQt5.QtWidgets import QMainWindow, QApplication, QWidget, QVBoxLayout, QPushButton
class MainWindow (QMainWindow):
def __init__ (self):
QMainWindow.__init__(self, parent=None)
print ("HERE 1")
window = QWidget()
layout = QVBoxLayout()
layout.addWidget(QPushButton('Top'))
layout.addWidget(QPushButton('Bottom'))
window.setLayout(layout)
self.setCentralWidget(window)
#window.show()
print ("HERE 2")
return
if __name__=="__main__":
app = QApplication(sys.argv)
mw = MainWindow ()
mw.show()
sys.exit(app.exec_())
If you want to show only the widget not on a QmainWindow, try this:
import sys
from PyQt5.QtWidgets import QMainWindow, QApplication, QWidget, QVBoxLayout, QPushButton
class MainWindow (QWidget):
def __init__ (self):
QWidget.__init__(self, parent=None)
print ("HERE 1")
layout = QVBoxLayout()
layout.addWidget(QPushButton('Top'))
layout.addWidget(QPushButton('Bottom'))
self.setLayout(layout)
print ("HERE 2")
if __name__=="__main__":
app = QApplication(sys.argv)
mw = MainWindow ()
mw.show()
sys.exit(app.exec_())