Kivy: Label use maximum font_size

fontsize.py

import kivy
kivy.require('1.8.1')

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

root = Builder.load_string('''
Label:
    text: 'hello'
''')

class TestApp(App):
    def build(self):
        self.trigger_update_size = Clock.create_trigger(self.update_size)
        root.bind(size=self.trigger_update_size)

        # this makes sure the callback is run once when the texture is first created
        root.bind(texture=self.trigger_update_size)
        root.bind(texture=lambda *args: root.unbind(texture=self.trigger_update_size))

        return root

    def update_size(self, *args):
        if self.root.texture and self.root.texture_size[0]:
            if abs(self.root.texture_size[0] - self.root.width) > (self.root.texture_size[0] * 0.001):
                xratio = float(self.root.width) / float(self.root.texture_size[0])
                yratio = float(self.root.height) / float(self.root.texture_size[1])
                ratio = min(xratio, yratio)
                ofs = float(self.root.font_size)
                fs = self.root.font_size * ratio
                if abs(ofs - fs) > (ofs * 0.001):
                    self.root.font_size = max(1, fs)
                    print 'set font size to', self.root.font_size

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