Kivy Screen isn't loading

Question

I'm trying to create a simple Screen with Kivy.

Here's my code:

class invoice_screen(Screen):
    def __init__(self, **kwargs):
        super(invoice_screen, self).__init__(**kwargs)

        self._window = BoxLayout(orientation='vertical')

        self._window.add_widget(Label(text="something or another"))

        scroll = ScrollView()
        box = BoxLayout(orientation='vertical')
        top_row = BoxLayout(orientation='horizontal')
        top_row.add_widget(
            Label(
                text="item",
                size_hint=(.8, 1)    
            )
        )
        top_row.add_widget(
            Label(
                text="quantity",
                size_hint=(.2, 1)    
            )
        )
        box.add_widget(bot_row)
        scroll.add_widget(box)
        self._window.add_widget(scroll)

        buttons = BoxLayout(orientation='horizontal')
        button1 = Button(text="Complete!")
        button1.bind(on_release=self.forward)
        button2 = Button(text="Go Back")
        button2.bind(on_release=self.back)
        buttons.add_widget(button1, button2)
        self._window.add_widget(buttons)
         
    def back(self):
        pass

    def forward(self):
        Popup(
            title="Completed",
            content=Label(text="You can close this NOW!"),
            size=(400,400)
        ) 

class app(App):
    def build(self):
        mane_chan = ScreenManager()
        mane_chan.add_widget(invoice_screen(name="invoice"))
        mane_chan.current = "invoice"
        return mane_chan

if __name__ == "__main__":
    app().run()

But when I run it, only the Kivy window shows up. There aren't any of the elements I've included in the Screen. There weren't even any errors.

I've got no idea what I'm doing wrong here.

Answer 1

You add all widgets to self._window ( BoxLayout ) but you forgot to add this self._window to screen ( self )

self.add_widget(self._window)

That's all.

---

Full working code with other small fixes - see comments in code

# https://kivy.org/doc/stable/guide/basic.html

#import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen
 
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.scrollview import ScrollView

from kivy.uix.popup import Popup

class InvoiceScreen(Screen):
    
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self._window = BoxLayout(orientation='vertical')
        self.add_widget(self._window)  # <--- need to add `boxlayout` to `screen`

        self._window.add_widget(Label(text="something or another"))

        scroll = ScrollView()
        
        box = BoxLayout(orientation='vertical')
        
        top_row = BoxLayout(orientation='horizontal')
        top_row.add_widget(
            Label(
                text="item",
                size_hint=(.8, 1)    
            )
        )
        top_row.add_widget(
            Label(
                text="quantity",
                size_hint=(.2, 1)    
            )
        )
        box.add_widget(top_row)   # <-- it has to be `top_row` instead of `bot_row`
        
        scroll.add_widget(box)
        self._window.add_widget(scroll)

        buttons = BoxLayout(orientation='horizontal')
        
        button1 = Button(text="Complete!")
        button1.bind(on_release=self.forward)
        
        button2 = Button(text="Go Back")
        button2.bind(on_release=self.back)
        
        #buttons.add_widget(button1, button2)  # <-- wrong
        buttons.add_widget(button1)  # <-- it has to add only one widget
        buttons.add_widget(button2)  # <-- it has to add only one widget
        
        self._window.add_widget(buttons)
        
    def back(self, event):
        pass

    def forward(self, event):
        Popup(
            title="Completed",
            content=Label(text="You can close this NOW!"),
            size=(400,400)
        ).open()   # <-- forgot `.open()`

class app(App):
    def build(self):
        mane_chan = ScreenManager()
        mane_chan.add_widget(InvoiceScreen(name="invoice"))
        mane_chan.current = "invoice"
        return mane_chan

if __name__ == "__main__":
    app().run()