I want this button to open up that ListView but it won't.

droplisttest.py

from kivy.app import App
from kivy.adapters.simplelistadapter import SimpleListAdapter
from kivy.lang import Builder
from kivy.properties import ObjectProperty
from kivy.uix.button import Button
from kivy.uix.dropdown import DropDown


class ListDropDown(DropDown):
    adapter = ObjectProperty()


class ListDropDownButton(Button):
    mainbut = ObjectProperty()

    def on_release(self, *args):
        self.mainbut.text = self.text


class DropListTestApp(App):
    def build(self):
        b = Button(
            text='Press me',
            size_hint_y=None,
            pos_hint={'top': 1},
            height=100
        )
        a = SimpleListAdapter(
            data=['spam', 'eggs', 'ham'],
            cls=ListDropDownButton,
            args_converter=lambda i, v: {
                'text': v,
                'mainbut': b
            }
        )
        ldd = ListDropDown(adapter=a)
        b.bind(on_release=ldd.open)
        return b


Builder.load_string("""
<ListDropDown>:
    ListView:
        adapter: root.adapter
""")


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