Python Kivy: Connect TabbePanel to Screen using Screenmanager

I’ve been working on a Python app consisting of a main.py and a gui.kv. All screens (“Main”, “First”, “Second”) are managed by the ScreenManager as shown in the example main.py.
Now I would like to link a tab to each screen. By pressing the corresponding tab the screen shall switch. What I managed so far is the addition of a TabbedPanel with 3 TabbedPanelItems (“Main”, “First”, “Second”) to the gui.kv.

The problem: The TappedPanelItem event (on_release: root.manager.current= 'first') doesn’t work as intended. When I press the tab “First” once, the program switches to a blank screen and the “Main” tab marking is still illuminated. The second time I press the tab “First” the “First” screen finally shows the “First”-Screen and the “First“ tab marking illuminates.

main.py:

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen, NoTransition 
from kivy.uix.tabbedpanel import TabbedPanel
from kivy.uix.floatlayout import FloatLayout


class TabbedTest(TabbedPanel):
    pass
class MainScreen(Screen):
    pass
class Firstscreen(Screen):
    pass
class Secondscreen(Screen):
    pass

class TabTest_with_Screenmanager_V2(App):

    def build(self):
        root = FloatLayout()
        #self.tabbedtest = TabbedTest(size_hint_y=0.5, pos_hint={'top': 1.0})
        #root.add_widget(self.tabbedtest)

        # Create the screen manager
        sm = ScreenManager(size_hint_y=0.99, pos_hint={'y': 0},transition=NoTransition())
        sm.add_widget(MainScreen(name='main'))
        sm.add_widget(Firstscreen(name='first'))
        sm.add_widget(Secondscreen(name='second'))
        root.add_widget(sm)

        return root


if __name__ == '__main__':
    TabTest_with_Screenmanager_V2().run()

TabTest_with_Screenmanager_V2.kv:

<MainScreen>:
    id: main

    TabbedPanel:
        do_default_tab: False

        TabbedPanelItem:
            text:"Main"
            BoxLayout:
                Label:
                    text:'Main'
              
        TabbedPanelItem:
            text:"First"  
            on_release: root.manager.current= 'first'
            
       
        TabbedPanelItem:
            text:"Second" 
            on_release:root.manager.current= 'second'
           

<Firstscreen>:
    id: first
    TabbedPanel:
        do_default_tab: False

        TabbedPanelItem:
            text:"Main"
            on_state: if self.state == "down":root.manager.current= 'main'

        TabbedPanelItem:
            text:"First"
            BoxLayout:
                Label:
                    text:"first"

        TabbedPanelItem:
            text:"Second"
            on_state: if self.state == "down":root.manager.current= 'second'


<Secondscreen>:
    id: second
    TabbedPanel:
        do_default_tab: False

        TabbedPanelItem:
            text:"Main"
            on_state: if self.state == "down":root.manager.current= 'main'

        TabbedPanelItem:
            text:"First"
            on_state: if self.state == "down":root.manager.current= 'first'

        TabbedPanelItem:
            text:"Second"
            BoxLayout:
                Label:
                    text:'second'
          

Objective: When I press a tab the program shall switch to the corresponding screen and illuminate the corresponding tab, immediately.
Do you have any idea what kind of event might work or is there better solution?

You can use some ids and the on_enter() method of each Screen to do this.

First, assign an id to the TabbedPanel and another to the default tab that you want to be opened when you enter that Screen. For example:

<MainScreen>:
    id: main

    TabbedPanel:
        id: tab
        do_default_tab: False
        
        TabbedPanelItem:
            id: home_tab
            text:"Main"
            BoxLayout:
                Label:
                    text:'Main'
              
        TabbedPanelItem:
            text:"First"  
            on_release: root.manager.current='first'
       
        TabbedPanelItem:
            text:"Second" 
            on_release:root.manager.current= 'second'

Then add an on_enter() method to that Screen:

class MainScreen(Screen):
    def on_enter(self, *args):
        tab = self.ids.tab
        home_tab = self.ids.home_tab
        Clock.schedule_once(lambda *arges: tab.switch_to(home_tab))

The same scheme of ids can be added to each Screen, and the identical on_enter() method can be added to each Screen.

main.kv

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen, NoTransition 
from kivy.uix.tabbedpanel import TabbedPanel
from kivy.uix.floatlayout import FloatLayout
from kivy.clock import Clock

class TabbedTest(TabbedPanel):
    pass

class MainScreen(Screen):
    def on_enter(self, *args):
        tab= self.ids.tabbedpanel
        home_tabX = self.ids.home_tab
        Clock.schedule_once(lambda *args: tab.switch_to(home_tabX))
    #pass

class Firstscreen(Screen):
    def on_enter(self, *args):
        tab= self.ids.tabbedpanel
        first_tabX = self.ids.first_tab
        Clock.schedule_once(lambda *args: tab.switch_to(first_tabX))

class Secondscreen(Screen):
    def on_enter(self, *args):
        tab= self.ids.tabbedpanel
        second_tabX = self.ids.second_tab
        Clock.schedule_once(lambda *args: tab.switch_to(second_tabX))

class TabTest_with_Screenmanager_V2(App):

    def build(self):
        sm = ScreenManager(size_hint_y=0.99, 
        pos_hint={'y': 0},transition=NoTransition())
        sm.add_widget(MainScreen(name='main'))
        sm.add_widget(Firstscreen(name='first'))
        sm.add_widget(Secondscreen(name='second'))
        root.add_widget(sm)

        return root

if __name__ == '__main__':
    TabTest_with_Screenmanager_V2().run()

TabTest_with_Screenmanager_V2.kv:

<MainScreen>:
    id: main
    TabbedPanel:
        id: tabbedpanel
        do_default_tab: False
        TabbedPanelItem:
            id: home_tab
            text:"Main"

            BoxLayout:
                Label:
                    text:'Main'

        TabbedPanelItem:
            id: first_tab
            text:"First"  
            on_release: root.manager.current= 'first'



        TabbedPanelItem:
            id: second_tab
            text:"Second" 
            on_release:root.manager.current= 'second'


<Firstscreen>:
    id: first
    TabbedPanel:
        id: tabbedpanel
        do_default_tab: False
        TabbedPanelItem:
            id: home_tab
            text:"Main"
            on_state: if self.state == "down":root.manager.current= 'main'
        TabbedPanelItem:
            id: first_tab
            text:"First"
            BoxLayout:
                Label:
                    text:"first"

        TabbedPanelItem:
            id: second_tab
            text:"Second"
            on_state: if self.state == "down":root.manager.current= 'second'

<Secondscreen>:
    id: second
    TabbedPanel:
        id: tabbedpanel
        do_default_tab: False
        TabbedPanelItem:
            id: home_tab
            text:"Main"
            on_state: if self.state == "down":root.manager.current= 'main'
        TabbedPanelItem:
            id: first_tab
            text:"First"
            on_state: if self.state == "down":root.manager.current= 'first'
        TabbedPanelItem:
            id: second_tab
            text:"Second"
            BoxLayout:
                Label:
                    text:'second'