Error When Trying to get Python Variable into Kivy (Python)

Question

I'm currently trying to get a python variable (called prior) into kivy and dynamicaly change the text of a label when I do. I'm still new to kivy and I'm not sure why I'm getting this error kivy.lang.parser.ParserException: Parser: File "/Users/sam/Downloads/App/answer.kv", line 35:

...
     33:                color: 0,1,0,1
     34:                font_size: 75
>>   35:            if app.prior == "yes":
     36:                print("Hello")
     37:                text: "Correct"
...
Invalid class name

Here is what I currently have Python:

import kivy
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import ObjectProperty
from kivy.lang import Builder 
from kivy.uix.image import Image
import random
import os
from kivy.core.window import Window
import numpy as np
from kivy.properties import StringProperty
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput

class MyLayout(Widget):
    pass    

        

            
        
        
        

class RootLayout(FloatLayout):
    pass

        

    

class colorApp(App):


    flagfile = StringProperty("0")
    
    path = "/Users/sam/Downloads/App/w320"
    files = os.listdir(path)
    flagfile= path + "/" + random.choice(files)
    
    prior = StringProperty()
    def button_press(self):
        self.prior = self.root.ids.user_input.text
        return RootLayout()

        
        

    def build(self):
 

        return RootLayout()



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

Kivy

<RootLayout>
    MyLayout:  
        BoxLayout:
                   
        
            orientation: "vertical"
            size: root.width, root.height

            padding: 50
            spacing: 20
            Image:
                id: cardimage
                #source: app.flagfile
                source: "w320/in.png"
            
            TextInput:
                multiline: False
                text: "Enter Guess Here"
                on_focus: self.text = "" if args[1] else self.text
                id: user_input
                
            Button:
                text: "Submit"
                font_size: 32
                on_press: app.button_press()
            
        
        
        FloatLayout:    
            Label:
                text:''
                pos: 1200, 910
                color: 0,1,0,1
                font_size: 75
            if app.prior == "yes":
                print("Hello")
                text: "Correct"
                
                

Answer 1

In your kv, the unindentation of the if app.prior == "yes": line makes kivy parser expect a new child of the FloatLayout instance. When that line is interpreted as a class, you see that error. Since you are trying to set the text property of the Label, that logic should be used in the text property definition. Here is one way to do that:

        Label:
            pos: 1200, 910
            color: 0,1,0,1
            font_size: 75
            text: "Correct" if app.prior == "yes" else ''