An example of overloading open() in kivy/uix/dropdown.py to allow the function to be bound to a TextInput object

dropdown.py

from kivy.app import App 
from kivy.uix.button import Button
from kivy.uix.dropdown import DropDown
from kivy.uix.textinput import TextInput

class TextDropDown(DropDown):
    def __init__(self, *args, **kwargs):
        super(TextDropDown, self).__init__(*args, **kwargs)
        
    def open(self, widget, text):
        # ensure we are not already attached
        if self.attach_to is not None:
            self.dismiss()

        # we will attach ourself to the main window, so ensure the widget we are
        # looking for have a window
        self._win = widget.get_parent_window()
        if self._win is None:
            raise DropDownException(
                'Cannot open a dropdown list on a hidden widget')

        self.attach_to = widget
        widget.bind(pos=self._reposition, size=self._reposition)
        self._reposition()

        # attach ourself to the main window
        self._win.add_widget(self)

class Test(App):

    def build(self):
        main = TextInput(text='Television', size_hint=(None, None))
        dropdown = TextDropDown()

        dropdown.add_widget(Button(text='Breaking Bad', size_hint_y=None))
        dropdown.add_widget(Button(text='Homeland', size_hint_y=None))

        main.bind(text=dropdown.open)

        return main

Test().run()