Setting a Widget inside the MainWindow

Hello,

I am currently using PyQt5 with Python3.
I'm having trouble getting a QTabWidget to be placed inside the main screen when called from a separate class.
Currently when the button "single" from the view menu option is selected a QTabWidget is produced but in a seperate window from my QMainWindow widget.
I believe the issue lies in my OOP but I was hoping for some input or direction to documentation that could provide me with a solution.

The structure is as follows:

__main__.py

Code:

mainscreen.App()

mainwindow.py

Code:

class App(QMainWindow):
	def __init__(self): 
		appl = QApplication(sys.argv)
 
		super().__init__() #is this line necessary
		self.title = "PyQt EVE-Application"
		self.showMaximized()
		self.initUI()
 
		sys.exit(appl.exec_())
 
	def menu(self):
		mainMenu = self.menuBar()
		fileMenu = mainMenu.addMenu('File')
		openMenu = mainMenu.addMenu('Open')
		viewMenu = mainMenu.addMenu('View')
		helpMenu = mainMenu.addMenu('Help')
 
		#FILE
		exitButton = QAction('Exit', self)
		exitButton.setShortcut('Ctrl + E')
		exitButton.setStatusTip('Exit application')
		exitButton.triggered.connect(self.close)
		fileMenu.addAction(exitButton)
 
		#OPEN
 
		#VIEW
		singleview = QAction('Single View', self)
		singleview.setShortcut ('Ctrl + S')
		singleview.setStatusTip('Change view to one widget.')
		singleview.triggered.connect(self.singlecall)
		viewMenu.addAction(singleview)
 
 
		# doubleview
		doubleview = QAction('Double View', self)
		doubleview.setShortcut('Ctrl + D')
		doubleview.setStatusTip('Change view to two wigets.')
		doubleview.triggered.connect(widgetviews.double)
		viewMenu.addAction(doubleview)
 
 
		# tripleview
		tripleview = QAction('Triple View', self)
		tripleview.setShortcut('Ctrl + T')
		tripleview.setStatusTip('Change view to three widgets.')
		tripleview.triggered.connect(widgetviews.triple)
		viewMenu.addAction(tripleview)
 
		# quadview
		quadview = QAction('Quad View', self)
		quadview.setShortcut('Ctrl + Q')
		quadview.setStatusTip('Change view to four widgets.')
		quadview.triggered.connect(widgetviews.quad)
		viewMenu.addAction(quadview)
		#HELP
	def singlecall(self):
		self.t = widgetviews.single(self)
		self.t.show()
 
	def initUI(self):
		self.setWindowTitle(self.title)
		self.menu()
		#default to quad view
		self.show()
if __name__ == "__main__":
	pass

tabwidget.py

Code:

class single(QWidget):
	def __init__(self, App):
		super().__init__()
 
		App.blankwidget = QWidget()
		App.tab1 = QTabWidget()
		App.tab1.addTab(App.blankwidget, "Tab 1")
		App.tab2 = QTabWidget()
		App.tab3 = QTabWidget()
		App.tab4 = QTabWidget()
 
		grid = QGridLayout()
		grid.addWidget(App.tab1,0,0)
		grid.addWidget(App.tab2,0,1)
		grid.addWidget(App.tab3,1,0)
		grid.addWidget(App.tab4,1,1)
		self.setLayout(grid)

Thank you in Advance