PyQt5) When I try to open Second window, it is closed without any error messages - pyqt5

Question

I'm trying to open second window on main window.
I made 2 classes for each windows, and run the code. when I click the button on main window, the main window is closed and second windows show. but after 1 seconds the second window is closed and the process ends.
Really I don't know why it does...
from PyQt5 import QtWidgets
import sys
class MainWindow(QtWidgets.QMainWindow):
def __init__(self):
super(MainWindow, self).__init__()
self.resize(400, 300)
# Button
self.button = QtWidgets.QPushButton(self)
self.button.setGeometry(0, 0, 400, 300)
self.button.setText('Open Sub Window')
self.button.setStyleSheet('font-size:40px')
self.button.clicked.connect(self.sub_show)
def sub_show(self):
self.hide()
self.sub_window = SubWindow()
self.sub_window.exec()
self.show()
class SubWindow(QtWidgets.QWidget):
def __init__(self):
super(SubWindow, self).__init__()
self.resize(400, 300)
self.button2 = QtWidgets.QPushButton(self)
self.button2.setGeometry(0, 0, 400, 300)
self.button2.setText('Back to Main window')
self.button2.setStyleSheet('font-size:40px')
self.button2.clicked.connect(self.home)
self.show()
def home(self):
self.close()
if __name__ == "__main__" :
app = QtWidgets.QApplication([])
mw = MainWindow()
mw.show()
sys.exit(app.exec())

Answer 1

You are calling show a few to many times. Also QWidget doesn't have an exec method. You will also want the first window to reappear when you close the second I assume.
I made some changes in the example below. Give it a try:
from PyQt5 import QtWidgets
import sys
class MainWindow(QtWidgets.QMainWindow):
def __init__(self):
super(MainWindow, self).__init__()
self.resize(400, 300)
self.button = QtWidgets.QPushButton(self)
self.button.setGeometry(0, 0, 400, 300)
self.button.setText('Open Sub Window')
self.button.setStyleSheet('font-size:40px')
self.button.clicked.connect(self.sub_show)
def sub_show(self):
self.hide()
self.sub_window = SubWindow()
# connect signal to show again when button2 is pressed
self.sub_window.button2.clicked.connect(self.show)
self.sub_window.show()
class SubWindow(QtWidgets.QWidget):
def __init__(self):
super(SubWindow, self).__init__()
self.resize(400, 300)
self.button2 = QtWidgets.QPushButton(self)
self.button2.setGeometry(0, 0, 400, 300)
self.button2.setText('Back to Main window')
self.button2.setStyleSheet('font-size:40px')
self.button2.clicked.connect(self.close)
if __name__ == "__main__" :
app = QtWidgets.QApplication([])
mw = MainWindow()
mw.show()
sys.exit(app.exec())