Kivy使用来自KV文件的ScreenManager

Question

I'm writing a kivy program/game. I've made one before, but was only 1 screen in total. I've started developing a new program with ideas of implementing a Screen Manager . I managed to build the Screen Manager inside of the python file, but it wasn't suitable for my long term usage, and I wanted to try my hand at a Screen Manager from the .kv file .

Any guidance will be greatly appreciated, not sure what I missed. I looked at this questionv( Kivy - Screen Manager - Accessing attribute in other class ), and pieced together what I thought was right, but still can't get a screen to load - I feel this is a simple answer and I'm being blind...

main.py;

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition


class MenuScreen(Screen):
    pass

class NewGameScreen(Screen):
    pass

class LoadGameScreen(Screen):
    pass

class ASCIILifeApp(App):
    pass

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

ASCIILife.kv;

#: kivy 1.9
#: import ScreenManager kivy.uix.screenmanager.ScreenManager
#: import Screen kivy.uix.screenmanager.ScreenManager
#: import NewGameScreen screen
#: import LoadGameScreen screen

ScreenManager:
    id: screen_manager
    #transition: FadeTransition()
    MenuScreen:
        id: menu_screen
        name: 'MenuScreen'
        manager: 'screen_manager'
    NewGameScreen:
        id: newgame_screen
        name: 'NewGameScreen'
        manager: 'screen_manager'
    LoadGameScreen: 
        id: loadgame_screen
        name: 'LoadGameScreen'
        manager: 'screen_manager'

<MenuScreen>:
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'ASCII Life'
            font_size: 50
        Button:
            text: 'New Game'
            font_size: 30
            on_release: app.root.current = 'newgame'
        Button:
            text: 'Load Game'
            font_size: 30
            on_release: app.root.current = 'loadgame'
        Button:
            text: 'Settings'
            font_size: 30
            on_release: app.root.current = 'something'
<NewGameScreen>:
    BoxLayout:
        orientation: 'vertical'
        BoxLayout:
            orientation: 'vertical'
            Label:
                text: 'Game Length in Days'
                font_size: 30
            BoxLayout:
                orientation: 'horizontal'
                ToggleButton:
                    text: '100'
                    state: 'down'
                    group: 'newgame_days'
                    font_size: 30
                ToggleButton:
                    text: '200'
                    group: 'newgame_days'
                    font_size: 30
            BoxLayout:
                orientation: 'horizontal'
                ToggleButton:
                    text: '365'
                    group: 'newgame_days'
                    font_size: 30
                ToggleButton:
                    text: '3650'
                    group: 'newgame_days'
                    font_size: 30
            ToggleButton:
                text: 'Unlimited'
                group: 'newgame_days'
                font_size: 30
        BoxLayout:
            orientation: 'vertical'
            size_hint: (1, .5)
            Label:
                text: 'Difficulty (Score Multiplier)'
                font_size: 30
            BoxLayout:
                orientation: 'horizontal'
                ToggleButton:
                    text: 'Easy (x1)'
                    state: 'down'
                    group: 'newgame_difficulty'
                    font_size: 30
                ToggleButton:
                    text: 'Medium (x2.5)'
                    group: 'newgame_difficulty'
                    font_size: 30
                ToggleButton:
                    text: 'Hard (x5)'
                    group: 'newgame_difficulty'
                    font_size: 30
<LoadGameScreen>:
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'load a game'
            font_size: 30
        Widget:
            canvas:
                Ellipse:
                    pos: self.pos
                    size: self.size
        BoxLayout:
            Button:
                text: 'Menu'
                font_size: 30
                on_release: app.root.current = 'menu'
            Button:
                text: 'text'
                font_size: 30

Edit: Truncated kv file

Answer 1

The answer was using a builder to build the kv file. Knew it was something simple. File below;

main.py

#!/usr/bin/kivy
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition
from kivy.properties import ObjectProperty


class MenuScreen(Screen):
    pass

class NewGameScreen(Screen):
    pass

class LoadGameScreen(Screen):
    pass

class ScreenManager(ScreenManager):
    pass

buildKV = Builder.load_file("ASCIILife.kv")

class ASCIILifeApp(App):
    def build(self):
        return buildKV

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

Answer 2

To elaborate on Chazara's findings:

Either rename the kv code file according to the naming convension:

Kivy looks for a Kv file with the same name as your App class in lowercase, minus “App” if it ends with 'App'.

From the documentation: how to load kv language

Or just make it explicit with the builder function:

Builder.load_file('path/to/file.kv')

Answer 3

To elaborate on Chazara's findings even more: i tested the code from Chazara with kivy 1.10.1dev0 and Python 3.6; i got an error saying:

 kivy.uix.screenmanager.ScreenManagerException: No Screen with name "something".

to fix it, i changed in the .kv file

[...]    
Button:
   text: 'New Game'
   font_size: 30
   on_release: app.root.current = 'newgame'
[...]

to:

[...]    
Button:
   text: 'New Game'
   font_size: 30
   on_release: app.root.current = 'NewGameScreen'
[...]

it references the name of NewGameScreen given in the ScreenManager

    ScreenManager:
[...]
         NewGameScreen:
            id: newgame_screen
            *name: 'NewGameScreen'*
            manager: 'screen_manager'