import sys
from PyQt5.QtWidgets import *

class MyWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.windowB = MyWindow2(self)
        self.initUI()
    def initUI(self):
        self.setWindowTitle("누르지 마")
        self.label = QLabel("홍길동", self)
        self.label.move(100, 10)
        self.label.resize(150, 30)
        button = QPushButton("click!", self)
        button.move(50, 30)
        button.clicked.connect(self.pressed)
    def pressed(self):
        self.windowB.show()
        self.hide()

class MyWindow2(QMainWindow):
    def __init__(self, w):
        super().__init__()
        self.windowA = w
        self.initUI()
    def initUI(self):
        self.setWindowTitle("위험")
        print(self.windowA.label.text())
        button = QPushButton("don't click!", self)
        button.move(50, 30)
        button.clicked.connect(self.pressed)
    def pressed(self):
        self.windowA.show()
        # self.b=MyWindow()
        # self.b.show()
        self.hide()
if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = MyWindow()
    window.show()
    sys.exit(app.exec_())