Python - 登录代码在Kivy中不起作用

Question

尝试运行此代码时出现以下错误。我试过两个版本的Python（2和3）：

  

语法错误：语法无效

from Kivy.app import App
from Kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
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="Username:"))
        self.username = TextInput(multiline=False)
        self.add_widget(self.username)

class SimpleKivy(App):
    def build(sef):
        return LoginScreen()