Using add_widget in the kv file

Question

I'm having trouble dynamically adding widgets in my kv file, everything I've seen so far has been around adding widgets in the py file. The reason I'd like to use the kv file for this is that I've got a modified Button widget I'd like to use that's written in the kv file and I don't know how to create it in the py file.

So I guess there are two possible questions:

1) How do I use add_widget correctly in the kv file. In the below code I get the NameError: SmoothButton is not defined.

Or

2) How do I create my SmoothButton in the py file?

kv file:

WindowManager:
    ChatPage:

<ChatPage>:
    name: "chat_page"

    NavigationLayout:
        id: nav_layout
        MDNavigationDrawer:
            NavigationDrawerIconButton:
                text: "Test"
                on_release: app.root.current = "login"

        FloatLayout:
            MDToolbar:
                pos_hint: {'top': 1}
                md_bg_color: 0.2, 0.6, 1, 1
                left_action_items: [['menu', lambda x: root.ids.nav_layout.toggle_nav_drawer()]]

            Button:
                text: "Create button"
                pos_hint: {"x": 0.65, "top": 0.15}
                size_hint: 0.35, 0.15
                on_release: root.add_widget(SmoothButton)


<SmoothButton@Button>:
    background_color: 0,0,0,0
    background_normal: ""
    back_color: 1,0,1,1
    border_radius: [6]
    canvas.before:
        Color:
            rgba: self.back_color
        RoundedRectangle:
            size: self.size
            pos: self.pos
            radius: self.border_radius

py file:

import kivy
from kivy.app import App
from kivy.properties import ObjectProperty, StringProperty, NumericProperty, ListProperty
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.lang import Builder
from kivy.uix.popup import Popup
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.core.window import Window
from kivymd.theming import ThemeManager
import mysql.connector
from kivymd.uix.picker import MDDatePicker

class ChatPage(Screen):
    pass


class WindowManager(ScreenManager):
    pass


class MyApp(App):
    theme_cls = ThemeManager()

    def build(self):
        kv = Builder.load_file("kivy.kv")
        sm = WindowManager()

        screens = [ChatPage(name="chat_page")]
        for screen in screens:
            sm.add_widget(screen)

        sm.current = "chat_page"
        return sm


if __name__ == '__main__':
    MyApp().run()
Thanks

Answer 1

If you want to add modified widgets in the .kv it doesn't mean you have to add it there. The solution is to expose the modified widget in the .kv to the .py, and create a method in "ChatPage" where you add the widget:

*.py

# ...
class SmoothButton(Button):
    pass


class ChatPage(Screen):
    def on_released(self):
        self.add_widget(SmoothButton())

# ...

*.kv

# ...
<ChatPage>:
    name: "chat_page"

    NavigationLayout:
        id: nav_layout
        MDNavigationDrawer:
            NavigationDrawerIconButton:
                text: "Test"
                on_release: app.root.current = "login"

        FloatLayout:
            MDToolbar:
                pos_hint: {'top': 1}
                md_bg_color: 0.2, 0.6, 1, 1
                left_action_items: [['menu', lambda x: root.ids.nav_layout.toggle_nav_drawer()]]

            Button:
                text: "Create button"
                pos_hint: {"x": 0.65, "top": 0.15}
                size_hint: 0.35, 0.15
                on_release: root.on_released() # <---


<SmoothButton>: # <---
    background_color: 0,0,0,0
    # ...