railee

class LogWin(QtWidgets.QMainWindow, Login):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        self.pushButton.pressed.connect(self.login)

    def error(self):
        QMessageBox.about(self, "Error", "Логин неверный.")

    def login(self):
        login = self.lineEdit.text()
        response = requests.get('http://127.0.0.1:5000/login')
        logins = response.json()['logins']
        if login in logins:
            self.hide()
            self.window = Messenger(login = login)
            self.window.show()
        else:
            self.error()
            self.lineEdit.clear()

reg = QtWidgets.QApplication([])
regwin = LogWin()
regwin.show()
reg.exec()

class Messenger(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self, login):
        super().__init__()
        self.setupUi(self)
        self.label_3.setText(login)
        self.label_3 = login