Can the Kivy language access inherited layouts and widgets?

Question

Can the kivy language access inherited layouts and widgets? I want to create one basic BoxLayout that contains the styling and title Label for my widget. I want to be able to inherit from this widget and add additional widgets in different positions.

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout

Builder.load_string('''
<SimpleBar>:
    canvas.before:
        Color:
            rgba: 0, 0.5, 0.5, 1
        Rectangle:
            pos: self.pos
            size: self.size
    BoxLayout:
        id: my_layout
        Label:
            text: "hi"

<NewBar>:
    Label:
        text: "2"
''')

class SimpleBar(BoxLayout):
    def log(self, value):
        print(value)

class NewBar(SimpleBar):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        print(dir(self))

class GeneralApp(App):
    def build(self):
        return NewBar()

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

Above is my basic running widget.

I want NewBar's "2" Label to be located before SimpleBar's 'hi' Label like below.

<NewBar>:
     BoxLayout:
         id: my_layout
         Label:
             text: "2"
         Label:
             text: "hi"

I know that - can negate items. However, <-NewBar> removes all of my styling.

Is there any way to do this in the kivy language?

Answer 1

Here's a fun thing: you don't need to specify all classes used in kv lang in the lang itself - you can also add them using Factory.register method later in code. Here's an example:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.factory import Factory

from functools import partial

Builder.load_string('''

<MyWidget>:
    Foo
    Bar
''')

class MyWidget(BoxLayout):
    pass

class MyApp(App):
    def build(self):
        Factory.register('Foo', cls=partial(Label, text='foo'))
        Factory.register('Bar', cls=partial(Label, text='bar'))
        return MyWidget()

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

Let's use it to create a template base widget we later fill with various content. We use a placeholder that we later replace with another widget:

<BaseWidget>:
    orientation: 'vertical'
    Label:
        size_hint: None, 0.1
        text: 'title'
    Placeholder

In the Python code we register a placeholder class in the __init__ method of this base template class.

class BaseWidget(BoxLayout):
    def __init__(self, **args):
        # unregister if already registered...
        Factory.unregister('Placeholder')
        Factory.register('Placeholder', cls=self.placeholder)
        super(BaseWidget, self).__init__(**args)

Now let's define a content class.

<TwoButtonWidget>:
    Button:
        text: 'button 1'
    Button:
        text: 'button 2'

And finally create a customized class that use our base class as a template and replaces its placeholder with a content class. This class don't have its own kivy rules (these are moved into content class) so when we inherite from our base template, no extra widgets are inserted.

# content class
class TwoButtonWidget(BoxLayout):
    pass

# Base class subclass
class CustomizedWidget(BaseWidget):
    placeholder = TwoButtonWidget # set contetnt class

A full example:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
from kivy.factory import Factory

Builder.load_string('''
<BaseWidget>:
    orientation: 'vertical'
    widget_title: widget_title
    placeholder: placeholder
    Label:
        size_hint: None, 0.1
        id: widget_title
    Placeholder
        id: placeholder

<TwoButtonWidget>:
    button1: button1
    Button:
        text: 'button 1'
        id: button1
    Button:
        text: 'button 2'

<ThreeButtonWidget>:
    orientation: 'vertical'
    Button:
        text: 'button a'
    Button:
        text: 'button b'
    Button:
        text: 'button c'
''')

class BaseWidget(BoxLayout):
    def __init__(self, **args):
        # unregister if already registered...
        Factory.unregister('Placeholder')
        Factory.register('Placeholder', cls=self.placeholder)
        super(BaseWidget, self).__init__(**args)

class TwoButtonWidget(BoxLayout):
    pass

class ThreeButtonWidget(BoxLayout):
    pass

class CustomizedWidget1(BaseWidget):
    placeholder = TwoButtonWidget

class CustomizedWidget2(BaseWidget):
    placeholder = ThreeButtonWidget

class MyApp(App):
    def build(self):
        layout = BoxLayout()
        c1 = CustomizedWidget1()
        # we can access base widget...
        c1.widget_title.text = 'First'
        # we can access placeholder
        c1.placeholder.button1.text = 'This was 1 before'

        c2 = CustomizedWidget2()
        c2.widget_title.text = 'Second'

        layout.add_widget(c1)
        layout.add_widget(c2)
        return layout

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

You can easily extend it and for example, have multiple placeholders.

Applying this to your case:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.lang import Builder
from kivy.factory import Factory

from functools import partial

Builder.load_string('''

<SimpleBar>:
    canvas.before:
        Color:
            rgba: 0, 0.5, 0.5, 1
        Rectangle:
            pos: self.pos
            size: self.size
    BoxLayout:
        Placeholder
        Label:
            text: "hi"

<NewBarContent>:
    Label:
        text: "2"
''')

class SimpleBar(BoxLayout):
    def __init__(self, **args):
        # unregister if already registered...
        Factory.unregister('Placeholder')
        Factory.register('Placeholder', cls=self.placeholder)
        super(SimpleBar, self).__init__(**args)

class NewBarContent(BoxLayout):
    pass

class NewBar(SimpleBar):
    placeholder = NewBarContent

class MyApp(App):
    def build(self):
        return NewBar()

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

Answer 2

With simple kv no, because if you put something in the widget (e.g. Label: ...), it'll call <widget>.add_widget() method and when such a method is called without additional parameters, it will by default place the widget after whatever was already placed before it. Therefore you can either search through the kivy/lang/parser.py file and add such a functionality(PR welcome), or do it within python in hm... __init__ or wherever you'll like to add the widget (maybe after some event).

To do that within python you can call <widget (or self)>.add_widget(<child>, index=<where>) according to the docs. For example:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ListProperty

Builder.load_string('''
#:import Factory kivy.factory.Factory
<Ninja@Label>:

<SimpleBar>:
    BoxLayout:
        id: my_layout
        Label:
            text: "hi"

<ChildWithBenefits>:
    placebefore:
        [(Factory.Label(text='I am the first!'), 0),
        (Factory.Ninja(text='No, I am!'), 2)]
''')

class SimpleBar(BoxLayout):
    def log(self, value):
        print(value)

class ChildWithBenefits(SimpleBar):
    placebefore = ListProperty([])
    def __init__(self, *args, **kwargs):
        super(ChildWithBenefits, self).__init__(*args, **kwargs)
        for child, index in self.placebefore:
            print child.text
            print type(child)
            self.add_widget(child, index=index)
        self.log('Hello!')

class GeneralApp(App):
    def build(self):
        return ChildWithBenefits()

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