class Ui_MainWindow(object):
def __init__(self):
<skipped>
<skipped>

def check_userPass(self):  # an internal method/function from `Ui_MainWindow`.
username = self.txt_user.text()
password = self.txt_pass.text()
if len(username) < 2  and  len(password) < 2:
QtWidgets.QMessageBox.question(self, "Checkk", "Helllooo", QtWidgets.QMessageBox.Ok, QtWidgets.QMessageBox.Cancel)
###error for above line:
##Traceback (most recent call last):
##  File ".demo_codes.py", line 215, in check_userPass
##    QtWidgets.QMessageBox.question(self, 'List manipulation', "Helllooo", ##QtWidgets.QMessageBox.Ok, QtWidgets.QMessageBox.Ok)
##TypeError: question(QWidget, str, str, buttons: ##Union[QMessageBox.StandardButtons, QMessageBox.StandardButton] = ##QMessageBox.StandardButtons(QMessageBox.Yes|QMessageBox.No), defaultButton: ##QMessageBox.StandardButton = QMessageBox.NoButton): argument 1 has unexpected #type 'Ui_MainWindow'

QtWidgets.QMessageBox.question("Checkk", "Helllooo", QtWidgets.QMessageBox.Ok, QtWidgets.QMessageBox.Cancel)
###error for above line:
##Traceback (most recent call last):
##  File ".demo_codes.py", line 215, in check_userPass
##    QtWidgets.QMessageBox.question('List manipulation', "Helllooo", ##QtWidgets.QMessageBox.Ok, QtWidgets.QMessageBox.Ok)
##TypeError: question(QWidget, str, str, buttons: ##Union[QMessageBox.StandardButtons, QMessageBox.StandardButton] = ##QMessageBox.StandardButtons(QMessageBox.Yes|QMessageBox.No), defaultButton: ##QMessageBox.StandardButton = QMessageBox.NoButton): argument 1 has unexpected type 'str'

if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
#ui.setupUi(MainWindow) # i renamed `setupUi` to __init__.
MainWindow.show()
sys.exit(app.exec_())