KivyMD：“超级”对象在基于条件的屏幕转换期间没有属性“__getattr__”

Question

I want to switch from login screen to menu screen based on a successful authentication, but this is the best I could do after a long search on condition-based screen transitions. Most sites say that screen-transitions in kivymd should be done using 'on-release' in .kv file, but I don't think that it would work in my code.

I've marked on the code the problematic line, which is raising the exception.

Teste.py

from kivy.lang import Builder
from kivymd.app import MDApp
from kivymd.uix.dialog import MDDialog
from kivymd.uix.button import MDFlatButton
from kivy.uix.screenmanager import ScreenManager, Screen


class login(Screen):
    pass
class menu(Screen):
    pass

Builder.load_file('lteste.kv')
class LoginApp(MDApp):
    dialog = None
    def build(self): #método construtor da parte visual do aplicativo
        
        self.theme_cls.theme_style = "Light"
        self.theme_cls.primary_palette = "Indigo"
        self.theme_cls.accent_palette = "Blue"

        self.sm = ScreenManager()
        self.sm.add_widget(login(name="login"))
        self.sm.add_widget(menu(name="menu"))
        self.sm.current = "menu"
        return self.sm
    
    def dialog_box(self):
        if not self.dialog:
            self.dialog = MDDialog(
                title="Log In",
                text=f"Welcome {self.root.ids.user.text}!",
                buttons=[MDFlatButton(text="Ok", text_color=self.theme_cls.primary_color,
                                      on_release=self.close),],)
        return self.dialog.open()
    
    def login(self):
        if self.root.ids.user.text=='1' and self.root.ids.password.text=='1':
            self.sm.current = "menu" #<- problem
            self.dialog_box()
            return True
        else:
            return False
    
    def close(self, instance):
        self.dialog.dismiss()
        

LoginApp().run()

lteste.kv

<login>:
    id: login
    name: "login"
    MDCard:
        size_hint: None, None
        size: 300, 600
        pos_hint: {"center_x": 0.5, "center_y": 0.5}
        elevation: 10
        padding: 65
        spacing: 35
        orientation: 'vertical'
        MDIcon:
            icon: 'account'
            icon_color: 0, 0, 0, 0
            halign: 'center'
            font_size: 180
        MDTextFieldRound:
            id: user
            icon_left: "account-check"
            hint_text: "Usuário"
            foreground_color: 1, 0, 1, 1
            size_hint_x: None
            width: 220
            font_size: 20
            pos_hint: {"center_x": 0.5}
        MDTextFieldRound:
            id: password
            icon_left: "key-variant"
            hint_text: "Senha"
            foreground_color: 1, 0, 1, 1
            size_hint_x: None
            height: 1
            width: 220
            font_size: 20            
            pos_hint: {"center_x": 0.5}
            password: True
        MDFillRoundFlatButton:
            text: "ENTRAR"
            font_size: 15
            pos_hint: {"center_x": 0.5}
            on_press: app.login()
        MDFillRoundFlatButton:
            text: "REGISTRAR-SE"
            font_size: 15
            pos_hint: {"center_x": 0.5}
<menu>
    name: "menu"
    id: menu
    MDCard:
        size_hint: None, None
        size: 300, 600
        pos_hint: {"center_x": 0.5, "center_y": 0.5}
        elevation: 10
        padding: 65
        spacing: 35
        orientation: 'vertical'
        MDRaisedButton:
            text: "Test"

Answer 1

The application root does not contain the ids you want. They are in the login screen widget because you defined them under it in kivy language. It would help to store a reference on the login screen to access them. Replace

        self.sm.add_widget(login(name="login"))

with

        self.login_screen = login(name="login")
        self.sm.add_widget(self.login_screen)

Then, you can access the widgets like so,

                text=f"Welcome {self.login_screen.ids.user.text}!",

and

        if self.login_screen.ids.user.text=='1' \
                and self.login_screen.ids.password.text=='1':

There is an example in the Kivy Documentation on kivy.uix.widget.Widget.ids and also Accessing Widgets defined inside Kv lang in your Python code .