circle.py

from kivy.uix.widget import Widget
from kivy.properties import BoundedNumericProperty, ListProperty
from kivy.lang import Builder

Builder.load_string('''
<Circle>:
    r: 100
    color: 0, 0, 1, 1
    canvas:
        Color:
            rgba: self.color
        Line:
            circle: self.center_x, self.center_y, self.r
            width: 10
''')

class Circle(Widget):
    r = BoundedNumericProperty(100, min=0)
    color = ListProperty()