gistfile1.py

class NumberChooser(BoxLayout):
    number = NumericProperty(0)
    number_max = NumericProperty(9)
    number_min = NumericProperty(0)
    def increment(self):
        if self.number < self.number_max:
            self.number += 1
    def decrement(self):
        if self.number > self.number_min:
            self.number -= 1

kv part

NumberChooser>:
    orientation: 'horizontal'
    Button:
        size_hint_x: None
        width: self.parent.height
        on_press: root.decrement()
        canvas:
            Line:
                points: [self.x + 0.15*self.width,self.y+0.5*self.height,self.x+0.85*self.width,self.y+0.5*self.height]
                width: 2
                cap: 'square'
    Label:
        font_size: (0.9*self.height,'px')
        text: str(root.number)
    Button:
        size_hint_x: None
        width: self.parent.height
        on_press: root.increment()
        canvas:
            Line:
                points: [self.x + 0.15*self.width,self.y+0.5*self.height,self.x+0.85*self.width,self.y+0.5*self.height]
                width: 2
                cap: 'square'
            Line:
                points: [self.x + 0.5*self.width,self.y+0.15*self.height,self.x+0.5*self.width,self.y+0.85*self.height]
                width: 2
                cap: 'square'