from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.textinput import TextInput

class LoginScreen(GridLayout):

    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.cols = 2
        self.add_widget(Label(text='User Name'))
        username = TextInput(multiline=False)
        self.add_widget(self.username)
        self.add_widget(Label(text='password'))
        password = TextInput(password=True, multiline=False)
        self.add_widget(self.password)

class MyApp(App):
    def build(self):
        return LoginScreen()

if __name__ == '__main__':
    LoginScreen().run()

O error que aparece foi:

    File "D:/Users/Usuario/PycharmProjects/eXcript/main.py", line 40, in <module>
     LoginScreen().run()
   File "D:/Users/Usuario/PycharmProjects/eXcript/main.py", line 30, in __init__
     self.add_widget(self.username)

 AttributeError: 'LoginScreen' object has no attribute 'username'