Adding widget results in "AttributeError: 'HomeScreen' object has no attribute '_trigger_layout'"

Question

Why does this code generate the following error?

AttributeError: 'HomeScreen' object has no attribute '_trigger_layout'

class HomeScreen(BoxLayout):

    def __init__(self):

        print "homescreen"

        topbar = BoxLayout(spacing = -2, size = (64, 64), size_hint = (1, None))

        back_button = Button(size = (32, 64), size_hint = (None, 1), text = '>', font_size = 15 + 0.015 * self.height, background_color = (0.239, 0.815, 0.552, 1))
        home_button = Button(text = "HOME", font_size = 10 + 0.015 * self.width, background_color = (0.239, 0.815, 0.552, 1))
        more_button = Button(size = (32, 64), size_hint = (None, 1), text = '...', font_size = 15 + 0.015 * self.height, background_color = (0.239, 0.815, 0.552, 1))

        topbar.add_widget(back_button)
        topbar.add_widget(home_button)
        topbar.add_widget(more_button)

        self.add_widget(topbar)

Answer 1

Because you have overwritten the init method, which is needed. In the code below I am also overwriting the init method, but I am also calling the init method before it was overwritten.

class HomeScreen(BoxLayout):
    def __init__(self, **kwargs):
        super(HomeScreen, self).__init__(**kwargs)
        ...

---

from kivy.app import App
from kivy.uix.slider import Slider
from kivy.uix.treeview import TreeView, TreeViewNode
from kivy.uix.button import Button
from kivy.uix.slider import Slider
from kivy.uix.label import Label
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder

class HomeScreen(BoxLayout):
    def __init__(self, **kwargs):
        super(HomeScreen, self).__init__(**kwargs)


        print("homescreen")

        topbar = BoxLayout(spacing = 0, size = (64, 64), size_hint = (1, None))

        back_button = Button(size = (32, 64), size_hint = (None, 1), text = '>', font_size = 15 + 0.015 * self.height, background_color = (0.239, 0.815, 0.552, 1))
        home_button = Button(text = "HOME", font_size = 10 + 0.015 * self.width, background_color = (0.239, 0.815, 0.552, 1))
        more_button = Button(size = (32, 64), size_hint = (None, 1), text = '...', font_size = 15 + 0.015 * self.height, background_color = (0.239, 0.815, 0.552, 1))

        topbar.add_widget(back_button)
        topbar.add_widget(home_button)
        topbar.add_widget(more_button)
        self.add_widget(topbar)


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

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