bind event to every class instance?

main.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.label import Label

def before(self, *_):
    self.bind(**{'on_touch_down' : test})

def test(*_):
    print ('cool!')


def my_decorator(func, before):

    def inner_before(*args, **kwargs): 
        before(*args)
        func(*args, **kwargs)

    return inner_before


KV = """
BoxLayout
    MyLabel
    MyLabel
    MyLabel


<MyLabel>
    text: 'touch me'
"""

class MyLabel(Label):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

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

        MyLabel.__init__ = my_decorator(MyLabel.__init__, before = before)
        return Builder.load_string(KV)


MyApp().run()