How do I access a variable from my main.py in my .kv file

Question

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.core.window import Window
from kivy.uix.label import Label


Balance = 0
Balance_string = str(Balance)

class MyWidget(Widget):
    def ads(self):
        global Balance
        Balance += 0.25
        Balance_string = str(Balance)
        print(Balance_string)
        return Balance_string

class BuzzMoneyApp(App):
    def build(self):
        return MyWidget()

if __name__ == '__main__':
    BuzzMoneyApp().run()

** my .kv file **

<MyWidget>:
    canvas:
        Color:
            rgb: (0, 150,0)
        Rectangle:
            pos: self.pos
            size: self.size

    Button:
        center: self.parent.center
        font_size: 14
        height: 28
        background_color: (1.0, 0.0, 0.0, 1.0)
        text: "Earn money"
        on_press:root.ads()

I want to access the Balance string variable from my main.py in my .kv file, so that I can display it on my screen.

Answer 1

You can easily reference Properties from your python inside your kv. Here is a modified version of your code that does that:

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import NumericProperty, StringProperty
from kivy.uix.widget import Widget

# Balance = 0
# Balance_string = str(Balance)

class MyWidget(Widget):
    Balance = NumericProperty(0)
    Balance_string = StringProperty('0')

    def ads(self):
        self.Balance += 0.25
        print(self.Balance_string)

    def on_Balance(self, *args):
        # update Balance_string whenever Balance changes
        self.Balance_string = str(self.Balance)

class BuzzMoneyApp(App):
    def build(self):
        return MyWidget()

if __name__ == '__main__':
    BuzzMoneyApp().run()

And then you can reference those Properties in the kv:

<MyWidget>:
    canvas:
        Color:
            rgb: (0, 150,0)
        Rectangle:
            pos: self.pos
            size: self.size

    Button:
        center: self.parent.center
        font_size: 14
        height: 28
        background_color: (1.0, 0.0, 0.0, 1.0)
        text: "Earn money"
        on_press:root.ads()
    Label:
        text: root.Balance_string  # root.Balance_string can be replaced with just str(root.Balance)
        size_hint: None, None
        size: self.texture_size
        pos: 300, 200
        color: 0,0,0,1

The Properties must be defined within an EventDispatcher (typically a Widget). The on_Balance() method is automatically triggered whenever the Balance Property changes, and it updates the Balance_string Property. The Balance_string can be used within the kv as shown, and the Label will be updated whenever the Balance_string Property changes. Since the Balance_string is just the string value of Balance, it can be eliminated and replaced with str(root.Balance) in the kv.