Kivy: maintain aspect ratio

aspecttest.py

import kivy
kivy.require('1.8.1')

from kivy.app import App
from kivy.lang import Builder

root = Builder.load_string('''
Button:
    text: 'hi'
    size_hint_y: None
    height: .5*self.width
''')

class TestApp(App):
    def build(self):
        return root

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