Pass extra information to a kv-defined widget

gistfile1.py

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory
from kivy.uix.boxlayout import BoxLayout

kv = """

<Test>:
    orientation: 'vertical'
    Button:
        text: 'Add a new button'
        on_press: root.callback()
		
<MyButton@Button>:
	amount: self.amount
	text: self.text
	on_press: print(self.amount)
"""

Builder.load_string(kv)


class Test(BoxLayout):
    def __init__(self, *args, **kwargs):
        super(Test, self).__init__(*args, **kwargs)
        self.callback()

    def callback(self):
        self.add_widget(Factory.MyButton(text="Hello",amount=12))


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

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

useful stuff is on lines 15,16,and 29