PyQT5_Open_Window

import sys
from importlib._bootstrap_external import _check_name

from PyQt5.QtWidgets import QMainWindow, QPushButton, QLabel, QMenu, qApp, QApplication,  QWidget
from PyQt5.QtCore import QObject, Qt, pyqtSignal
from PyQt5.QtGui import QPainter, QFont, QColor, QPen

class MainWindow(QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__()

        btn = QPushButton('Click me!', self)
        btn.clicked.connect(self.onClick)


    def onClick(self):
        self.SW = SecondWindow()
        self.SW.show()



class SecondWindow(QWidget):
    def __init__(self):
        super(SecondWindow, self).__init__()
        lbl = QLabel('Second Window', self)



if __name__ == "__main__":

    app = QApplication(sys.argv)
    MW = MainWindow()
    MW.show()
    sys.exit(app.exec_())