Как обработать сигнал нажатия на кнопку QPushButton?

import sys
import sqlite3
from PyQt5 import QtWidgets, QtCore
from PyQt5.QtWidgets import (QMainWindow, QDesktopWidget, QDialog, QLabel, QHBoxLayout, 
							QVBoxLayout, QWidget, QLineEdit, QApplication, QPushButton)
from PyQt5.QtCore import pyqtSlot


class Singleton():
  _instance = None
  def __new__(class_, *args, **kwargs):
    if not isinstance(class_._instance, class_):
        class_._instance = QWidget.__new__(class_, *args, **kwargs)
    return class_._instance


class MainWindow(Singleton, QtWidgets.QWidget):
	def __init__(self, parent=None):
		QtWidgets.QWidget.__init__(self, parent)
		self.count_teached_word = QtWidgets.QLabel("Count teached word : Y")
		self.count_teached_word.setAlignment(QtCore.Qt.AlignCenter)
		self.count_word = QtWidgets.QLabel("Count word : X")
		self.count_word.setAlignment(QtCore.Qt.AlignCenter)
		self.btn_teach = QtWidgets.QPushButton("TEACH WORD")
		self.btn_teach.clicked.connect(self.fun_teach)
		self.btn_edit = QtWidgets.QPushButton("EDIT WORD")
		self.btn_edit.clicked.connect(self.fun_edit)
		self.btn_test = QtWidgets.QPushButton("TEST")
		self.btn_test.clicked.connect(self.fun_test)


		vertical_layout = QVBoxLayout()
		vertical_layout.addWidget(self.count_word, QtCore.Qt.AlignCenter)
		vertical_layout.addWidget(self.count_teached_word, QtCore.Qt.AlignVCenter)
		vertical_layout.addWidget(self.btn_teach)
		vertical_layout.addWidget(self.btn_edit)
		vertical_layout.addWidget(self.btn_test)
		self.setLayout(vertical_layout)
		vertical_layout.setContentsMargins(0, 0, 0, 0)

		self.setWindowTitle("tech word eng")
		self.resize(180, 210)
		self.center()
		self.show()


	def center(self):
		qr = self.frameGeometry()
		cp = QDesktopWidget().availableGeometry().center()
		qr.moveCenter(cp)
		self.move(qr.topLeft())


	def fun_teach(self):
		self.close()
		return TeachingWindow()


	def fun_edit(self):
		self.close()
		return EditWindow()


	def fun_test(self):
		self.close()
		return TestWindow()



class TeachingWindow(Singleton, QtWidgets.QWidget):
	"""on page show word for teach"""
	def __init__(self, parent=None):
		QtWidgets.QWidget.__init__(self, parent)

		self.english_show_word = QtWidgets.QLabel("english word")
		self.english_show_word.setAlignment(QtCore.Qt.AlignCenter)
		self.option_one = QtWidgets.QPushButton("Option 1")
		#self.option_one.clicked.connect(self)
		self.option_two = QtWidgets.QPushButton("Option 2")
		#self.option_two.clicked.connect(self)
		self.option_three = QtWidgets.QPushButton("Option 3")
		#self.option_three.clicked.connect(self)
		self.option_four = QtWidgets.QPushButton("Option 4")
		#self.option_four.clicked.connect(self)
		self.btn_exit = QtWidgets.QPushButton("MAIN WINDOW")
		#self.btn_exit.clicked.connect(self.fun_exit)


		vertical_layout = QVBoxLayout()
		vertical_layout.addWidget(self.english_show_word)
		vertical_layout.addWidget(self.option_one)
		vertical_layout.addWidget(self.option_two)
		vertical_layout.addWidget(self.option_three)
		vertical_layout.addWidget(self.option_four)
		vertical_layout.addWidget(self.btn_exit)
		self.setLayout(vertical_layout)
		vertical_layout.setContentsMargins(0, 0, 0, 0)

		self.setWindowTitle("Teaching")
		self.resize(180, 210)
		self.show()


		def fun_learned_word(self):
			pass


		def fun_not_learned_word(self):
			pass


		def fun_exit(self):
			self.close()
      return MainWindow()

		
if __name__ == '__main__':
	app = QApplication(sys.argv)
	#mainn_window = TeachingWindow()
	#main_window = EditWindow()
	mainnn_window = MainWindow()
	sys.exit(app.exec_())

При нажатии на кнопку self.btn_exit = QtWidgets.QPushButton("MAIN WINDOW") мне нужно вернуться в MainWindow. Я обрабатываю событие нажатия self.btn_exit.clicked.connect(self.fun_exit) и вместо возвращения на главную получаю ошибку AttributeError: 'TeachingWindow' object has no attribute 'fun_exit'. Много статей/форумов посмотрел, но так и не понял почему она возникла.
Помогите плз) Заранее огромное спасибо!