Simple kivy app with text input

my.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.textinput import TextInput

kv = """
<Test>:
    # orientation: 'horizontal'
    orientation: 'vertical'
    Button:
        size_hint: 1, 0.95
        text: 'test button'
    MyTextInput:
        id: text_input
        size_hint: 0.5, None
        text_hint: 'input message...'
        multiline: True
        height: self.minimum_height
"""

Builder.load_string(kv)


class MyTextInput(TextInput):
    def on_text_validate(self):
        App.get_running_app().root.ids.text_input.text = ''

    def insert_text(self, substring, from_undo=False):
        if substring == '\n':
            substring = ''
            self.dispatch('on_text_validate')
        return super(MyTextInput, self).insert_text(substring, from_undo)


class Test(BoxLayout):
    pass


class TestApp(App):
    def build(self):
        t = Test()
        return t


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