Есть код рисовалки, подскажите как сделать так, чтобы принажатии кнопки "1" менялось значение rad?
```
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button

from kivy.graphics import (Color, Ellipse, Rectangle, Line)

class PainterWidget(Widget):

global rad
rad = 20
def on_touch_down(self, touch):
    with self.canvas:
        Color(0, 1, 0, 1)

        Ellipse(pos = (touch.x - rad/2, touch.y - rad/2), size = (rad, rad))
        touch.ud['line'] = Line(points = (touch.x, touch.y), width = 15)

def on_touch_move(self, touch):
    touch.ud['line'].points += (touch.x, touch.y)

def add_widget(self,event):
    Button.text = '13334'
    Button.size = (100, 100)
    Button.pos = (100, 100)

class PaintApp(App):
def build(self):

    parent = Widget()
    self.painter = PainterWidget()
    parent.add_widget(self.painter)
    parent.add_widget(Button(text = 'clear', on_press = self.clear_canvas, size = (100, 100)))
    parent.add_widget(Button(text = '1', on_press = self.size_1, size = (100, 100), pos = (100, 0)))
    parent.add_widget(Button(text = '2', on_press = self.size_2, size = (100, 100), pos = (200, 0)))
    parent.add_widget(Button(text = '3', on_press = self.size_3, size = (100, 100), pos = (300, 0)))

    return parent

def clear_canvas(self, instance):
    self.painter.canvas.clear()

def size_1(self, event):
    self.painter.rad=50

def size_2(self, instance):
    rad = 20

def size_3(self, instance):
    rad = 30

if name == 'main':
PaintApp().run()
```