How to set a variable in kivy file with classes that are made in kivy file

I'm working on a project and I'm designing the setting screen. I want to use it so many time so I made a class with variable such as content_cls My Question Is How To Set This Variable With Classes That Are Made In Kivy File

main.py

from kivymd.app import MDApp
from kivymd.uix.screenmanager import MDScreenManager
from kivymd.uix.card import MDCard

from kivy.proeprties import ObjectProperty , StringProperty
from kivy.lang import Builder

Builder.load_file("mainfiles\\kivymd2.kv")
class SettingCard(MDCard):
    right_text = StringProeprty("")
    content_cls = ObjectProperty()

class WidgetsManager(MDScreenManager):
     pass

class MainApp(MDApp):
     def build(self):
         return WidgetsManager()
    def on_start(self):
        SettingCard.add_widget(Setting_card.content_cls)

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

kivymd2.kv

<Nav_drawer_header@MDLabel>:
    size_hint : None , None
    adaptive_size : True
    font_style : "H5"
    theme_text_color : "Primary"
    font_name : "mainfiles\\B-NAZANIN.ttf"

<Setting_card>:
    _no_ripple_behavior : True
    size_hint : None , None
    adaptive_height : True
    radius : dp(20) , dp(20) , dp(20) , dp(20)
    md_bg_color : "lightgrey"
    width : self.parent.width


    MDLabel:
        bold : True
        font_size : 35
        text : app.farsi_font(root.right_text)
        size_hint : None, None
        adaptive_size : True
        pos_hint : {"center_y" : .5  , "right" : .9}
        font_name : "mainfiles\\B-NAZANIN.ttf"

<WidgetManager>:
    
    MDScreen:
        name : "setting screen"


        FitImage:
            size_hint : 1 , .9
            source : "images\\download (8).jpg"
            pos_hint : {"top" : .9}


        MDScrollView:
            do_scroll_x : False
            do_scroll_y : True
            id : setting_scroll
            width : root.width
            height : root.height/10*9


            MDGridLayout:
                size_hint : None , None
                adaptive_height : True
                width : self.parent.width
                cols : 1


                Setting_card:
                    id : fast_setting
                    right_text : "شروع سریع"
                    content_cls : Nav_drawer_header

I Try to have a clean code and because of that I made that class And I Except to call classes in kivy file and put it in content_*cls but when I did this it said that Nav_*drawer_header is not defined

Pay Attention That I made an example for Nav_drawer_header and I won't never use it as content_cls

Solution

You can use the kivy.factory for this. At the top of your kv file, add the line:

#:import Factory kivy.factory.Factory

Then, change the content_cls attribute in your kv file to:

content_cls : Factory.get('Nav_drawer_header')

This sets the content_cls attribute to the Nav_drawer_header class object. You can then create an instance of Nav_drawer_header by using setting_card.content_cls(), where setting_card is that instance of Setting_card. Note that you cannot use Setting_card.content_cls() because Setting_card is not an instance.

So your on_start() method can be:

 def on_start(self):
     setting_card = self.root.ids.fast_setting
     setting_card.add_widget(setting_card.content_cls())

There are other errors in your code. Check your spelling of Property.