Change screen with .kv language on button click - python-3.x

Question

I know kivy and kivymd but I don't know the functions yet. I want to change the screen of my app on button click, how can I do that? Thank you.
Below is my code
from kivy.lang import Builder
from kivymd.app import MDApp
KV = '''
Screen:
MDLabel:
text: "Material Design"
font_style: "H3"
theme_text_color: "Secondary"
pos_hint: {"center_y": 0.9}
halign: "center"
MDRaisedButton:
text: "Start"
md_bg_color: 0, 0.26, 0.27, 1
pos_hint: {"center_x": 0.5, "center_y": 0.2}
on_press: root.manager.current = "main"
MDRectangleFlatButton:
text: "Exit"
md_bg_color: rgba(197, 232, 204, 1)
text_color: 0, 0.26, 0.27, 1
pos_hint: {"center_x": 0.5, "center_y": 0.1}
Screen:
name: "main"
MDLabel:
text: "School guru"
font_style: "H3"
pos_hint: {"center_y": 0.8}
halign: "center"
'''
class MaterialDesign(MDApp):
def build(self):
return Builder.load_string(KV)
MaterialDesign().run()
Please try to edit my code and show how to do it.
Once again, thank you.

Answer 1

First, if you are going to switch between Screens, you need a ScreenManager.
Here is a modified version of your code that works:
from kivy.lang import Builder
from kivymd.app import MDApp
KV = '''
ScreenManager:
Screen:
MDLabel:
text: "Material Design"
font_style: "H3"
theme_text_color: "Secondary"
pos_hint: {"center_y": 0.9}
halign: "center"
MDRaisedButton:
text: "Start"
md_bg_color: 0, 0.26, 0.27, 1
pos_hint: {"center_x": 0.5, "center_y": 0.2}
on_press: root.current = "main"
MDRectangleFlatButton:
text: "Exit"
md_bg_color: rgba(197, 232, 204, 1)
text_color: 0, 0.26, 0.27, 1
pos_hint: {"center_x": 0.5, "center_y": 0.1}
Screen:
name: "main"
MDLabel:
text: "School guru"
font_style: "H3"
pos_hint: {"center_y": 0.8}
halign: "center"
'''
class MaterialDesign(MDApp):
def build(self):
return Builder.load_string(KV)
MaterialDesign().run()
So the ScreenManager becomes the root of the App, and its children are the Screens. The on_press property of the MDRaisedButton is now just root.current = "main".