How to manage screens from a different layout - python-3.x

Question

I am trying to have a set of buttons that controls what screen is displayed while still having the buttons displayed.
bobo.py
import kivy
kivy.require("1.9.0")
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.boxlayout import BoxLayout
class ButtSection(BoxLayout):
pass
class Welcome(Screen):
pass
class AccountOne(Screen):
pass
class AccountTwo(Screen):
pass
class ScreenManagement(ScreenManager):
pass
presentation = Builder.load_file("Bobo.kv")
class BoboApp(App):
def build(self):
return presentation
main = BoboApp()
main.run()
Bobo.kv
BoxLayout:
orientation: "horizontal"
BoxLayout:
ButtSection:
orientation: "vertical"
Button:
text: "Account One"
on_press: app.root.current = "a1"
Button:
text: "Account Two"
on_press: app.root.current = "a2"
Button:
text: "Account Three"
on_press: app.root.current = "a3"
ScreenManagement:
Welcome:
name: "wel"
Label:
text: "Welcome To Bobot"
AccountOne:
name: "a1"
Label:
text: "Page: Account One"
AccountTwo:
name: "a2"
Label:
text: "Page: Account One"
When I run the script, the welcome screen is the current screen. When I click a button, is does nothing even thought i included 'on_press: app.root.current = ''.

Answer 1

You have to analyze what app.root means and see if it is the ScreenManager.
app refers to the application that is the BoboApp instance, that is, main. root refers to the object that returns the build method of the application, that is, presentation. And presentation is the root object of the .kv, that is, the BoxLayout. With what we conclude that app.root is not the ScreenManager for which the error is valid.
Instead of using the root as a way to get to the ScreenManager it can be accessed through an id since the id is accessible within the whole tree.
On the other hand I have changed the names of the Screen to match the ones you want to set.
BoxLayout:
orientation: "horizontal"
BoxLayout:
ButtSection:
orientation: "vertical"
Button:
text: "Account One"
on_press: manager.current = "a1" # <---
Button:
text: "Account Two"
on_press: manager.current = "a2" # <---
Button:
text: "Account Three"
on_press: manager.current = "a3" # <---
ScreenManagement:
id: manager # <---
Welcome:
name: "a1" # <---
Label:
text: "Welcome To Bobot"
AccountOne:
name: "a2" # <---
Label:
text: "Page: Account One"
AccountTwo:
name: "a3" # <---
Label:
text: "Page: Account One"