import sys
from PyQt5.QtWidgets import *
from PyQt5.QtGui import QIcon

class MyWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.title = 'SWH Academy Window.'
        self.left = 200
        self.top = 200
        self.width = 400
        self.height = 300
        self.initUI()
    def initUI(self):
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)
        self.window2 = MyWindowSecond(self)

        button = QPushButton("눌러봐", self)
        button.move(50, 30)
        button.clicked.connect(self.pressed)

    def pressed(self):
        self.window2.show()
        self.hide()

class MyWindowSecond(QMainWindow):
    def __init__(self, firstWindow):
        super().__init__()
        self.title = 'Second'
        self.left = 400
        self.top = 400
        self.width = 400
        self.height = 300
        self.firstWindow = firstWindow
        self.initUI()

    def initUI(self):
        self.setWindowTitle(self.title)
        self.setWindowIcon(QIcon('favicon.ico'))
        self.setGeometry(self.left, self.top, self.width, self.height)

        button3 = QPushButton("종료", self)
        button3.move(240, 50)
        # self.button3.clicked.connect(QCoreApplication.instance().quit)
        button3.clicked.connect(self.button3Clicked)

    def button3Clicked(self):
        self.firstWindow.show()
        self.hide()

if __name__ == '__main__':
    app = QApplication(sys.argv)

    window = MyWindow()
    window.show()

    sys.exit(app.exec_())