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I am building a multiple screen App with Kivy and I would like to use the ScreenManager to navigate between the multiple screens. I have seen examples and documentation for how to create the screens within a .kv file, but I want to know how to create them within the .py file.

• Problem: When I create the screen subclasses as shown below, my app
  window returns a blank screen.


• Question: What is the correct way to
  create the Screen subclasses within a .py file?

Right now I have two Screen subclasses defined: 'welcomeScreen', and 'functionScreen'. Each consists of a layout with some widgets.

kivy.require('1.9.1')

import kivy

from kivy.app import App

from kivy.uix.label import Label

from kivy.uix.floatlayout import FloatLayout

import kivy.uix.boxlayout

import kivy.uix.button

from kivy.uix.screenmanager import ScreenManager, Screen



class PanelBuilderApp(App):  # display the welcome screen



    def build(self):

        # Create the screen manager and add widgets to the base sm widget

        sm = kivy.uix.screenmanager.ScreenManager()

        sm.add_widget(Screen(name='welcomeScreen'))

        sm.add_widget(Screen(name='functionScreen'))

        # sm.current= 'welcomeScreen'

        return sm



class welcomeScreen(Screen): #welcomeScreen subclass



    def __init__(self, **kwargs): #constructor method

        super(welcomeScreen, self).__init__(**kwargs) #init parent

        welcomePage = FloatLayout()

        box = kivy.uix.boxlayout.BoxLayout(orientation='vertical', size_hint=(0.4, 0.3),

                                           padding=8, pos_hint={'top': 0.5, 'center_x': 0.5})



        welcomeLabel = Label(text='Hello and welcome to the Panel Builder version 1.0.nApp by John VorstennClick below to continue',

            halign= 'center', valign= 'center', size_hint= (0.4, 0.2), pos_hint= {'top': 1, 'center_x': 0.5})

        welcomeBox = kivy.uix.button.Button(text= 'Click to continue')

        welcomeBox.bind(on_press= self.callback)

        welcomeBox2 = kivy.uix.button.Button(text='not used')



        welcomePage.add_widget(welcomeLabel)

        box.add_widget(welcomeBox)

        box.add_widget(welcomeBox2)

        welcomePage.add_widget(box)

        self.add_widget(welcomePage)



    def callback(instance):

        print('The button has been pressed')

        sm.switch_to(Screen(name= 'functionScreen'))

        # sm.current = Screen(name= 'functionScreen')



class functionScreen(Screen):  #For later function navigation



    def __init__(self, **kwargs): #constructor method

        super(functionScreen, self).__init__(**kwargs) #init parent

        functionPage = kivy.uix.floatlayout.FloatLayout()



        functionLabel = Label(text='Welcome to the function page. Here you will choose what functions to use',

                              halign='center', valign='center', size_hint=(0.4,0.2), pox_hint={'top': 1, 'center_x': 0.5})



        functionPage.add_widget(functionLabel)

        self.add_widget(functionPage)



# sm.add_widget('Name') #Add more names later when you create more screens

# OR#

# for i in ScreenDirectory:

#     sm.add_widget(ScreenDirectory[i])



PanelBuilderApp().run()

if __name__ == '__main__':

    pass

I understand I can add the definitions to a .kv file, and I will probably do this as the app grows. However, I like being explicit as I am learning how to use kivy.

I think you think using Screen(name='welcomeScreen') you are using welcomeScreen but that is not true, if you want to use welcomeScreen you should use it directly.

On the other hand you have typographical errors so I have corrected, I recommend you follow the kivy tutorials, obviously you must have a solid OOP base (and I think you do not have it so your task is to reinforce).

import kivy

kivy.require('1.9.1')

from kivy.app import App

from kivy.uix.label import Label

from kivy.uix.floatlayout import FloatLayout

from kivy.uix.boxlayout import BoxLayout

from kivy.uix.button import Button

from kivy.uix.screenmanager import ScreenManager, Screen



class PanelBuilderApp(App):  # display the welcome screen

    def build(self):

        sm = ScreenManager()

        sm.add_widget(WelcomeScreen(name='welcomeScreen'))

        sm.add_widget(FunctionScreen(name='functionScreen'))

        return sm



class WelcomeScreen(Screen): #welcomeScreen subclass

    def __init__(self, **kwargs): #constructor method

        super(WelcomeScreen, self).__init__(**kwargs) #init parent

        welcomePage = FloatLayout()

        box = BoxLayout(orientation='vertical', size_hint=(0.4, 0.3),

                                           padding=8, pos_hint={'top': 0.5, 'center_x': 0.5})

        welcomeLabel = Label(text='Hello and welcome to the Panel Builder version 1.0.nApp by John VorstennClick below to continue',

            halign= 'center', valign= 'center', size_hint= (0.4, 0.2), pos_hint= {'top': 1, 'center_x': 0.5})

        welcomeBox = Button(text= 'Click to continue', on_press=self.callback)

        welcomeBox2 = Button(text='not used')



        welcomePage.add_widget(welcomeLabel)

        box.add_widget(welcomeBox)

        box.add_widget(welcomeBox2)

        welcomePage.add_widget(box)

        self.add_widget(welcomePage)



    def callback(self, instance):

        print('The button has been pressed')

        self.manager.current = 'functionScreen'



class FunctionScreen(Screen):  #For later function navigation

    def __init__(self, **kwargs): #constructor method

        super(FunctionScreen, self).__init__(**kwargs) #init parent

        functionPage = FloatLayout()

        functionLabel = Label(text='Welcome to the function page. Here you will choose what functions to use',

                              halign='center', valign='center', size_hint=(0.4,0.2), pos_hint={'top': 1, 'center_x': 0.5})

        functionPage.add_widget(functionLabel)

        self.add_widget(functionPage)



if __name__ == '__main__':

    PanelBuilderApp().run()