Is there a way to change kivy api variables (e.g. scroll_distance) inside kv file?

Question

I recently started learning kivy and don't fully understand how the interaction between .kv file and python file works. For example,

class TimelineScr(ScrollView):
    scroll_wheel_distance = 60

If I want to change this variable inside

<TimelineScr>:
    Label:
        canvas.before:
            Color:
                rgb: define.LIGHT_BLUE
            Rectangle:
                pos: self.pos
                size: root.width, self.texture_size[1]
            Color:
                rgb: (0.78, 0.92, 0.91)
            Rectangle:
                size: root.width-40, self.texture_size[1]-40
                pos: 20, 20

        size_hint_y: None
        height: self.texture_size[1]
        padding_y: 40
        #shorter: True
        line_height: 2

        color: define.TEXT_GREEN
        font_size: 20
        font_name: 'src/fonts/static/Inter-Regular.ttf'

        text: str(main.timeline)
        text_size: (root.width - 80, None)

can I do it and how?

Sorry for my English

Answer 1

I am not really a pro with kivy but here is with what i often start: i have a main.py like this:

import # what u need

Builder.load_file('the.kv')
    
class fscreen(Widget):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        


class secscreen(Widget):

    def __init__(self,**kwargs):
        super().__init__(**kwargs)
        pass

class thscreen(Widget):
    def __init__(self,**kwargs):
        super().__init__(**kwargs)
        pass
            
class theapp(App):
    def build(self):
        self.screenm = ScreenManager()

        self.fscreen = fscreen()
        screen = Screen(name = "first screen")
        screen.add_widget(self.fscreen)
        self.screenm.add_widget(screen)

        self.secscreen = secscreen()
        screen  = Screen(name = "secondscreen")
        screen.add_widget(self.secscreen)
        self.screenm.add_widget(screen)

        self.thscreen = thscreen()
        screen  = Screen(name = "thirdscreen")
        screen.add_widget(self.thscreen)
        self.screenm.add_widget(screen)
        return self.screenm

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

and a the.kv like this:

<fscreen>
        
    canvas.before:
        Color:
            rgba: 1, 1, 1, 1
    
        Rectangle:
            size: root.width, root.height
            pos: 0, 0
    
    
    
    ScrollView:
        id: scroll_view
        size: root.width*0.4, root.height*0.4
        pos: root.height*0.3, root.height*0.3
        Label:
            id: output
            color: 0, 0, 0, 1
            text: ''
            size_hint_x: 1.0
            size_hint_y: None
            text_size: self.width, None
            height: self.texture_size[1]

now if i want to change the label thats in the the.kv file there is two ways of changing it:

1. by id: for that we demonstration purpose we have to add a Button in our kv file in our <fscreen> and a function in our main.py like this:

in the.kv file:

Button:
    text: ' change label by id'
    size: root.width*0.4, root.height*0.05
    pos: root.width*0.3, root.height*0.1
    on_press: root.change_label()

in main.py:

def change_label(self):
    self.ids.output.text = 'label has been changed by id'

same if we want to change ScrollView by id:

the ScrollView id takes a lot of attributes same as the label for example lets say we have another Button and another function as shown before we can change anything by id like this:

def change_by_id(self):
    self.ids.scroll_view.size[0] = self.width*0.5   
    self.ids.scroll_view.size[1] = self.height*0.5 

    self.ids.scroll_view.pos[0] = self.width*0.25   
    self.ids.scroll_view.pos[1] = self.height*0.25
    self.ids.scroll_view.#valid attribute = ...

    self.ids.output.text = 'something'
    self.ids.output.font_size = #....
    self.ids.output.color = #....    # and so on

2. by properties:

for this lets change something in the the.kv file:

    ScrollView:
        id: scroll_view
        size: root.width*0.4, root.height*0.4
        pos: root.height*0.3, root.height*0.3
        Label:
            id: output
            color: 0, 0, 0, 1
            text: root.string   ###### this is what we changed
            size_hint_x: 1.0
            size_hint_y: None
            text_size: self.width, None
            height: self.texture_size[1]

        Button:
            text: ' change label by prop'
            size: root.width*0.4, root.height*0.05
            pos: root.width*0.3, root.height*0.1
            on_press: root.change_by_prop()

now the main.py has to have a string property like this:

class fscreen(Widget):
    string = StringProperty()
    def __init__(self, **kwargs):
        super().__init__(**kwargs
        self.string = ''
   
    def change_by_prop(self):
        self.string = 'text has been changed by property'

and ofc there are many properties NumericProperty(), StringProperty() and so on ...

this is how I change things in kv file from python main.