How to access id/widget of different class from a kivy file (.kv)?

What I want to know?

1. If button with id: button_b (Get_Boys class) is released, then Label with id: label_g (Get_Girls class) must change.
2. If Button with id: button_b (Get_Boys class) is pressed, then Label with id: root_lbl (Get_People class) must change.
3. If Button with id: root_btn (Get_People class) is released, then Label with id: label_b (Get_Boys class) must change.

It is explained (little) in this link, but not from the beginner's point of view.

I have 2 files

1. test.py
2. dates_test.kv

test.py

class Get_People(BoxLayout):
    pass

class Get_Boys(BoxLayout):
    pass

class Get_Girls(BoxLayout):
    pass

class TestApp(App):
    def build(self):
        self.load_kv('dates_test.kv')
        return Get_People()

dates_test.kv file

<Get_People>:
    orientation: 'vertical'
    Button:
        name: root_btn
        id: root_btn
        text: "I am Root Button"
        on_release: change_label_b
    Label:
        id: root_lbl
        text: "I am Root Label"
    Get_Boys:
    Get_Girls:

<Get_Boys>:
    Button:
        id: button_b
        text: "Button for boys"
        on_press: change_label_root
        on_release: change_label_g
    Label:
        id: label_b
        text: "Label for boys"

<Get_Girls>:
    Button:
        id: button_g
        text: "Button for girls"
    Label:
        id: label_g
        text:"Label for girls"

Well!, looks like I myself found the answer and I would like to share it.

First of all let us give "id" in dates_test.kv file. So that you can access them in python code or in .kv file.

<Get_People>:
    stuff_p: root_lbl
    ...
    Get_Boys:
        id: gb
    Get_Girls:
        id: gg

<Get_Boys>:
    stuff_b: label_b

<Get_Girls>:
    stuff_c: label_g

you might wonder what is stuff_p,stuff_b and stuff_c???

They are ObjectProperty defined in their own classes. The changes you make in stuff_b in your python code makes changes in label_b as you have linked in kivy file.

class Get_People(BoxLayout):
    stuff_p = ObjectProperty(None)
    ...

class Get_Boys(BoxLayout):
    stuff_b = ObjectProperty(None)
    ...

class Get_Girls(BoxLayout):
    stuff_c = ObjectProperty(None)
    ...

For Part 1 and Part 2

1. If button with id: button_b (Get_Boys class) is released, then Label with id: label_g (Get_Girls class) must change.

2. If Button with id: button_b (Get_Boys class) is pressed, then Label with id: root_lbl (Get_People class) must change.

In the Get_Boys class (test.py) define these methods.

def change_girl(self):

    self.parent.ids.gg.stuff_c.text = "Boys changed Girls!"
    #self.stuff_b.text = "i changed myself!"

def change_people(self):
    self.parent.ids.root_lbl.text = "Boys changed people!"

let's see what happened here...

self.parent.ids.gg.stuff_c.text = "Boys changed Girls!"

1. self.parent refers to Get_Parent class.
2. .ids.gg refers to the id that we created above for Get_Girls.
3. .stuff_c is used to refer label_g (above) in Get_Girls class.
4. .text is used to change the text in the label.

and in the .kv file

<Get_Boys>:
    stuff_b: label_b
    Button:
        id: button_b
        text: "button 1"
        on_release: root.change_girl()
        on_press: root. change_people()

For Part 3

3. If Button with id: root_btn (Get_People class) is released, then Label with id: label_b (Get_Boys class) must change.

in the Get_People class (test.py) define a method.

def rooted(self):
    self.ids.gb.stuff_b.text = "people changed boys!"

and in .kv file

Button:
    id: root_btn
    text: "I am Root"
    on_release: root.rooted()