Rewriting code in python with use of kivy gone wrong

Question

I am trying to rewrite some code. My main problem is that I don't understand the build(self) function in Test(App). Does self.swiper_manager reference swiper_manager = None and start_screen.ids.swiper_manager reference MDSwiperManager: id: swiper_manager in the .kv file?
Code that I am trying to rewrite.

class MySwiperManager(BoxLayout):
    pass

class Test(App):
    theme_cls = ThemeManager()
    theme_cls.primary_palette = 'Indigo'
    swiper_manager = None

    def build(self):
        start_screen = MySwiperManager()
        self.swiper_manager = start_screen.ids.swiper_manager
        paginator = MDSwiperPagination()
        paginator.screens = self.swiper_manager.screen_names
        paginator.manager = self.swiper_manager
        self.swiper_manager.paginator = paginator
        start_screen.add_widget(paginator)

        return start_screen

The code below is my best attempt to rewrite Name of file: Prelogin/prescreenmanager.py

from kivy.app import App
from kivy.core.window import Window
from kivy.metrics import dp
from kivy.properties import StringProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.screenmanager import Screen
from kivy.lang import Builder

from kivymd.uix.card import MDCard
from kivymd.uix.managerswiper import MDSwiperPagination
from kivymd.theming import ThemeManager

class MySwiperManager(BoxLayout):
    pass

class PreScreenManager(Screen):
    theme_cls = ThemeManager()
    theme_cls.primary_palette = 'Indigo'
    swiper_manager = None
    start_screen = MySwiperManager()
    swiper_manager = start_screen.ids.swiper_manager
    paginator = MDSwiperPagination()
    paginator.screens = swiper_manager.screen_names
    paginator.manager = swiper_manager
    swiper_manager.paginator = paginator
    start_screen.add_widget(paginator)

The context of my code is as follows: Name of file: Prelogin/prescreenmanager.kv

<ScreenOne@Screen>:
    name: 'screen one'
    Label: 
        text: 'Hello World'

<ScreenTwo@Screen>:
    name: 'screen two'


<ScreenThree@Screen>:
    name: 'screen three'


<ScreenFour@Screen>:
    name: 'screen four'



<ScreenFive@Screen>:
    name: 'screen five'


<PreScreenManager>:
    MySwiperManager:
        orientation: 'vertical'

        MDSwiperManager:
            id: swiper_manager

            ScreenOne: 

            ScreenTwo:

            ScreenThree:

            ScreenFour:

            ScreenFive:

main.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.anchorlayout import AnchorLayout
from kivy.factory import Factory
from kivymd.theming import ThemeManager

# Load the kv files
Builder.load_file("Prelogin/prescreenmanger.kv")

from Prelogin.prescreenmanger import PreScreenManager


class MainApp(App):
    pass

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

main.kv

AnchorLayout:
    canvas.before:
        Rectangle:
            size: self.size
            pos: self.pos
            source: "Start/Background.png"   
    anchor_x: 'center'
    anchor_y: 'top'
    Image:
        source: 'Start/Loginlogo.png'
        keep_ratio: False
        allow_stretch: True
        opacity: 1
        size_hint: 1, 0.25
        pos_hint: (0,0)  
    ScreenManager:
        id: screen_manager
        PreScreenManager:
            id: PreScreenManager

Error (That I don't understand)

[INFO   ] [Logger      ] Record log in C:\Users\Bruger\.kivy\logs\kivy_19-11-23_9.txt
[INFO   ] [deps        ] Successfully imported "kivy_deps.gstreamer" 0.1.17
[INFO   ] [deps        ] Successfully imported "kivy_deps.glew" 0.1.12
[INFO   ] [deps        ] Successfully imported "kivy_deps.sdl2" 0.1.22
[INFO   ] [Kivy        ] v1.11.1
[INFO   ] [Kivy        ] Installed at "C:\Program Files\Python37\lib\site-packages\kivy\__init__.py"
[INFO   ] [Python      ] v3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)]
[INFO   ] [Python      ] Interpreter at "C:\Program Files\Python37\python.exe"
[INFO   ] [Factory     ] 184 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil, img_gif (img_ffpyplayer ignored)
[INFO   ] [KivyMD      ] v0.102.0
[INFO   ] [Window      ] Provider: sdl2
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] GLEW initialization succeeded
[INFO   ] [GL          ] Backend used <glew>
[INFO   ] [GL          ] OpenGL version <b'4.5.0 - Build 24.20.100.6286'>
[INFO   ] [GL          ] OpenGL vendor <b'Intel'>
[INFO   ] [GL          ] OpenGL renderer <b'Intel(R) HD Graphics 620'>
[INFO   ] [GL          ] OpenGL parsed version: 4, 5
[INFO   ] [GL          ] Shading version <b'4.50 - Build 24.20.100.6286'>
[INFO   ] [GL          ] Texture max size <16384>
[INFO   ] [GL          ] Texture max units <32>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
[INFO   ] [Text        ] Provider: sdl2
[INFO   ] [GL          ] NPOT texture support is available
 Traceback (most recent call last):
   File "kivy\properties.pyx", line 860, in kivy.properties.ObservableDict.__getattr__
 KeyError: 'swiper_manager'

 During handling of the above exception, another exception occurred:

 Traceback (most recent call last):
   File "c:\Users\Bruger\OneDrive\Dokumenter\Python - Java - kivy\Kivy2\Initium\Start\main.py", line 10, in <module>
     from Prelogin.prescreenmanger import PreScreenManager
   File "C:\Users\Bruger\OneDrive\Dokumenter\Python - Java - kivy\Kivy2\Initium\Prelogin\prescreenmanger.py", line 16, in <module>
     class PreScreenManager(Screen):
   File "C:\Users\Bruger\OneDrive\Dokumenter\Python - Java - kivy\Kivy2\Initium\Prelogin\prescreenmanger.py", line 21, in PreScreenManager
     swiper_manager = start_screen.ids.swiper_manager
   File "kivy\properties.pyx", line 863, in kivy.properties.ObservableDict.__getattr__
 AttributeError: 'super' object has no attribute '__getattr__'

Answer 1

The build() method of an App returns the Widget that will be the root of the App display, The lines in the class TestApp before the def build()

theme_cls = ThemeManager()
theme_cls.primary_palette = 'Indigo'
swiper_manager = None

create class variables that are shared by all instances of the Test class (but since it is an App, there should only be one instance).

So, in the build() method, self.swiper_manager references the class variable named swiper_manager. And the line with ids references the id in the kv file.

That is all fine, but in your prescreenmanager.py, you have a bunch of code that is not in any method. That code, like the class variable creation in Test, is run when Python loads the class, even before the App is built. So, at that point, the ids have not yet been defined, so you get the error you see.

And I still don't know what you are trying to do with that dangling code in prescreenmanager.py.