Binding lambda function to button doesn't work in a for loop

Question

I'm trying to generate a list of buttons in Kivy, binding each on_release event to the action() function of an instance of my Tracks class (stored in track_list). I have made a small example to show the problem. The first 3 buttons print the correct number, but they do not use a for loop... The last 7 buttons do not work, and they always print 9.

bindtest.py

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.widget import Widget

track_list = []
button_list = []


class BindTest(Widget):
    global track_list
    global button_list

    def initialize(self):
        for x in range(0, 10):
            track_list.append(Tracks(x))

        #
        # this works, and each button prints its unique 'num' when clicked
        #

        button_list.append(TrackButton())
        button_list[len(button_list) - 1].bind(on_release=lambda y: track_list[0].action())
        self.ids.buttons.add_widget(button_list[len(button_list)-1])

        button_list.append(TrackButton())
        button_list[len(button_list) - 1].bind(on_release=lambda y: track_list[1].action())
        self.ids.buttons.add_widget(button_list[len(button_list) - 1])

        button_list.append(TrackButton())
        button_list[len(button_list) - 1].bind(on_release=lambda y: track_list[2].action())
        self.ids.buttons.add_widget(button_list[len(button_list) - 1])

        #
        # this does not work. each button prints '9' when clicked
        #

        for x in range(3, 10):
            button_list.append(TrackButton())
            button_list[len(button_list)-1].bind(on_release=lambda y: track_list[x].action())
            self.ids.buttons.add_widget(button_list[len(button_list)-1])


class TrackButton(Button):
    pass


class Tracks:
    num = ''

    def action(self):
        print(self.num)

    def __init__(self, num):
        self.num = num


class BindTestApp(App):
    def build(self):
        return BindTest()


bindTest = BindTestApp()
bindTest.run()

bindtest.kv

<TrackButton>:
    text: 'button'

<BindTest>:
    __init__: root.initialize()
    BoxLayout:
        size: root.size
        id: buttons
    orientation: 'vertical'