How to add Screen Manager to my KIVYMD code, but not in kv file only class method

Question

I'm new here, I have a small problem in kivymd. Is there anyone here who would be able to help me?

this is my main.py file

from kivymd.app import MDApp
from kivymd.theming import ThemeManager
from kivymd.uix.slider import Slider
from kivy.uix.boxlayout import BoxLayout
from kivymd.uix.label import MDLabel
from kivymd.uix.toolbar import MDToolbar
from kivymd.uix.textfield import MDTextField

class rendertime(BoxLayout):
    def init(self, kwargs):
        super().init(kwargs)
        self.padding = "30dp"
        self.orientation = 'vertical'
        self.spacing = "10dp"
        self.top_name = MDToolbar(title = "my app")
        self.how_many_text = MDLabel(text = "my label")
        self.how_many = MDTextField(text = "1")

        self.add_widget(self.top_name)
        self.add_widget(self.how_many_text)
        self.add_widget(self.how_many)

        self.hour_value = MDLabel(text = "0")
        self.add_widget(self.hour_value)


        self.hours_slider = Slider(min = 0, max = 24)
        self.add_widget(self.hours_slider)

        self.hours_slider.bind(value = self.on_value_hours)

    def on_value_hours(self, instance, slider):
        self.hour_value.text = "%d"%slider

class myapp(MDApp):
    title = "my app v"
    def build(self):
        self.theme_cls = ThemeManager()
        self.theme_cls.primary_palette = 'Orange'
        self.theme_cls.theme_style = 'Dark'
        return rendertime()

if name== "main":
    myapp().run()

Now I would like to switch screens for this. Add 2 buttons at the very bottom and they are to switch to the next screen. I don't want to use the .kv file I wanted to do everything in class.

I don't want to use

kv = """
MDTextField:
    id: frame
    hint_text: "text"
"""

Please, if anyone can explain it to me, I would be very grateful. Thank you and have a nice day. Good luck everyone

this is a screenmanager file transition.kv

from kivymd.app import MDApp
from kivy.uix.screenmanager import ScreenManager, Screen, NoTransition
from kivy.lang import Builder

Builder.load_file("transition.kv")

class Screen1(Screen):
    pass

class Screen2(Screen):
    pass

class Screen3(Screen):
    pass

class Screen4(Screen):
    pass

class MainApp(MDApp):
    def build(self):
        self.sm = ScreenManager(transition = NoTransition())
        self.sm.add_widget(Screen1(name = "Screen1"))
        self.sm.add_widget(Screen2(name = "Screen2"))
        self.sm.add_widget(Screen3(name = "Screen3"))
        return self.sm

    def change_screen(self, screen):
        self.sm.current = screen

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

and kv file

<Screen1>:
    BoxLayout:
        orientation: "vertical"
        MDToolbar:
            title: "My App"
        MDRoundFlatButton:
            text: "Go to Screen 2"
            on_release: app.root.current = "Screen2"
        MDRoundFlatButton:
            text: "Go to Screen 3"
            on_release: app.root.current = "Screen3"
        MDLabel:
            text: "Screen 1"

<Screen2>:
    BoxLayout:
        orientation: "vertical"
        MDToolbar:
            title: "Screen 2"
            left_action_items: [["arrow-left",  lambda x: app.change_screen("Screen1")]]
        MDLabel:
            text: "Screen 2"

<Screen3>:
    BoxLayout:
        orientation: "vertical"
        MDToolbar:
            title: "Screen 3"
            left_action_items: [["arrow-left",  lambda x: app.change_screen("Screen1")]]
        MDLabel:
            text: "Screen 3

Just don't know how to connect these code

---

This is a solution only one python file

from kivymd.app import MDApp
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.button import Button
from kivy.uix.image import Image
from kivy.core.window import Window
from kivymd.uix.boxlayout import MDBoxLayout
from kivy.uix.scrollview import ScrollView
from kivymd.uix.list import MDList, OneLineListItem

class Screen1(Screen):
    def __init__(self, **kwargs):
        super(Screen1, self).__init__(**kwargs)
        layout = FloatLayout()
        btn = Button(text = "screen 2", size_hint = (None,None), size = (100,100), pos_hint = {'x':0,'y':0}, on_release = self.screen_switch)
        layout.add_widget(btn) 
        self.add_widget(layout)
    def screen_switch(self, instance):
        self.manager.current = 's2'
        self.manager.transition.direction = 'left'

class Screen2(Screen):
    def __init__(self, **kwargs):
        super(Screen2, self).__init__(**kwargs)
        layout = FloatLayout()
        btn = Button(text = "screen 1", size_hint = (None,None), size = (100,100), pos_hint = {'x':0,'y':0}, on_release = self.screen_switch)
        layout.add_widget(btn)
        self.add_widget(layout)

        myb = MDBoxLayout(padding = 50, orientation = 'vertical')
        mys = ScrollView()
        myl = MDList()

        myb.add_widget(mys)
        mys.add_widget(myl)
        contacts = ["c1", "c2", "c3"]
        for c in contacts:
            myl.add_widget(
                OneLineListItem(
                    text = c 
                    )
                )
        self.add_widget(myb)

    def screen_switch(self, instance):
        self.manager.current = 's1'
        self.manager.transition.direction = 'right'

class myapp(MDApp):
    def build(self):
        self.theme_cls.theme_style = 'Dark'
        sm = ScreenManager()
        sm.add_widget(Screen1(name = 's1'))
        sm.add_widget(Screen2(name = 's2'))
        return sm

if __name__ == "__main__":