how to switch between two screens in kivy python?

Question

I'm trying to switch between two screens.. but it doesn't work.. it didn't show anything or any error I don't know what is the problem, here's my code

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


class mainWindow(Screen):
    pass


class secondWindow(Screen):
    pass


class windowManager(ScreenManager):
    pass

sm = ScreenManager()
sm.add_widget(mainWindow(name='main'))
sm.add_widget(secondWindow(name='second'))


class multApp(App):
    def build(self):
        return mainWindow()


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

and mult.kv

windowManager:
    mainWindow:
    secondWindow:

<mainWindow>:
    name: "main"

    Button:
        text: "Submit"
        on_press : root.manager.current = "second"

<secondWindow>:
    name: "second"

    Button:
        text: "go back"
        on_press : root.manager.current = "main"

Answer 1

You have several issues in your code.

• Your build() method returns mainWindow(). This means that your app GUI consists of just a Screen (mainwindow) with no ScreenManager. So the root.manager.current = lines in your kv will fail because there is no manager.
• Your mult.kv file contains a rule (windowManager:) for building the GUI, but this rule is over-ridden by your build() method.
• In your code, you have three lines (starting with sm = ScreenManager()) that also build the GUI, but you do nothing with the results of these lines. So those lines have no effect.
• Your class names should start with an upper case character. When using kv, as you are, this is a requirement.

Taking all this into account, here is a modified version of your code that should work:

python code:

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

class MainWindow(Screen):
    pass


class SecondWindow(Screen):
    pass


class WindowManager(ScreenManager):
    pass

# sm = ScreenManager()
# sm.add_widget(mainWindow(name='main'))
# sm.add_widget(secondWindow(name='second'))


class multApp(App):
    pass
    # def build(self):
    #     return mainWindow()


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

mult.kv:

WindowManager:
    MainWindow:
    SecondWindow:

<MainWindow>:
    name: "main"

    Button:
        text: "Submit"
        on_press : root.manager.current = "second"

<SecondWindow>:
    name: "second"

    Button:
        text: "go back"
        on_press : root.manager.current = "main"

Answer 2

I agree with John's answers, just to add to that. If you are declaring manager as a class I always return manager in the build methods, also its good practice to create ids for your screens under your root manager, as such will make them easier to reference

<WindowManager>:
    id: screen_manager

    MainWindow:

         id: main_window
         name: "main_window"
         manager: screen_manager   

    SecondWindow:

         id: second_window
         name: "second_window"
         manager: screen_manager