from PyQt5 import QtWidgets
from w import W
import sys
class D(QtWidgets.QDialog):
    def __init__(self):
        super(D, self).__init__()
        self.btn = QtWidgets.QPushButton('Click me')
        layout = QtWidgets.QHBoxLayout()
        layout.addWidget(self.btn)
        self.setLayout(layout)
        self.btn.clicked.connect(self.runWin)
    def runWin(self):
        win = W('Test')
        win.show()
        self.close()
if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    login = D()
    login.show()
    sys.exit(app.exec_())

 from PyQt5 import QtWidgets
class W(QtWidgets.QMainWindow):
    def __init__(self, text):
        super(W, self).__init__()
        label = QtWidgets.QLabel(text)
        self.setCentralWidget(label)