I am facing a problem in my python kivy code

Question

So i am writing my first python kivy code and came across this problem. Tried searching up but didn't quite understand how to solve it. can u please correct my code and tell me what's wrong?

import kivy
from kivy.app import App
from kivy.config import Config
from kivy.lang.builder import Builder
from kivy.uix.screenmanager import Screen, ScreenManager
Config.set('graphics', 'width', '400')
Config.set('graphics', 'height', '150')

kv = Builder.load_file("krxls.kv")


class LoginPage(Screen):
    pass

class RegisterPage(Screen):
    pass

class WindowManager(ScreenManager):
    pass



class KrxLS(App):
    def build(self):
        return kv


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

This is my kv file

WindowManager:
    LoginPage:
    RegisterPage:

<LoginPage>:
    name: "lp"
    GridLayout:
        cols: 1
        Label:
            text: "Login!!"
        GridLayout:
            cols: 2
            Label:
                text: "Username"
            TextInput:
                multiline: "False"
            Label:
                text: "Password"
            TextInput:
                multiline: "False"
            Button:
                text: "Sign up"
                on_release: app.root.current = "rp"
            Button:
                text: "Sign in!"

<RegisterPage>:
    name: "rp"
    GridLayout:
        cols: 1
        Label:
            text: "Register!!"
        GridLayout:
            cols: 2
            Label:
                text: "Username"
            TextInput:
                multiline: "False"
            Label:
                text: "Password"
            TextInput:
                multiline: "False"
            Button:
                text: "Sign in"
                on_release: app.root.current = "lp"
            Button:
                text: "Sign Up!"

i get this error

 Traceback (most recent call last):
   File "/Users/randomman/PycharmProjects/Beginner/GUIDev/kivyGUI.py", line 12, in <module>
     kv = Builder.load_file("krxls.kv")
   File "/Users/randomman/PycharmProjects/Beginner/venv/lib/python3.7/site-packages/kivy/lang/builder.py", line 301, in load_file
     return self.load_string(data, **kwargs)
   File "/Users/randomman/PycharmProjects/Beginner/venv/lib/python3.7/site-packages/kivy/lang/builder.py", line 399, in load_string
     widget = Factory.get(parser.root.name)(__no_builder=True)
   File "/Users/randomman/PycharmProjects/Beginner/venv/lib/python3.7/site-packages/kivy/factory.py", line 131, in __getattr__
     raise FactoryException('Unknown class <%s>' % name)
 kivy.factory.FactoryException: Unknown class <WindowManager>

Please help!! i am learning kivy through youtube vids and tried following them but i got this error... I have already searched this up in google and haven't found a proper solution.

Answer 1

You need to define your WindowManager:

class WindowManager(ScreenManager):
    pass

before you load the kv file. A generally safe way to do the kv load is to call it inside the build() method, like this:

class KrxLS(App):
    def build(self):
        kv = Builder.load_file("krxls.kv")
        return kv