kivy: SyntaxError: invalid syntax при создании функции

Изучаю библиотеку kivy. Столкнулся с ошибкой, которую не могу исправить.

Code:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Rectangle
from kivy.core.window import Window

class GameWidget(Widget):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self._keyboard = Window.request_keyboard(self._on_keyboard_closed, self)
        self._keyboard.bind(on_key_down=self._on_key_down

    def _on_keyboard_closed(self):
        self._keyboard.unbind(on_key_down=self._on_key_down)
        self._keyboard = None
    def _on_key_down(self, keyboard,keycode,text,modifiers):
        print('button')
        
        
        
        with self.canvas:
            Rectangle(source='player.png', pos=(0,0),size=(100,100))

class MyApp(App):
    def build(self):
        return GameWidget()
        
if __name__ == "__main__":
    app = MyApp()
    app.run()

Error:

  File "main.py", line 12
    def _on_keyboard_closed():
    ^
SyntaxError: invalid syntax