Kv language

Concept behind the language

As your application grow more complex, it’s common that the construction of widget trees and explicit declaration of bindings, becomes verbose and hard to maintain. The KV Language is a attempt to overcome these short-comings.

The KV language (sometimes called kvlang, or kivy language), allows you to create your widget tree in a declarative way and to bind widget properties to each other or to callbacks in a natural manner. It allows for very fast prototyping and agile changes to your UI. It also facilitates a good separation between the logic of your application and it’s User Interface.

How to load KV

There are two ways to load Kv code into your application: - By name convention:

Kivy looks if there is a Kv file with the same name as your App class in lowercase, minus “App” if it ends with ‘App’. E.g:

MyApp -> my.kv.

If this file defines a Root Widget it will be attached to the App’s root attribute and used as the base of the application widget tree.

  • Builder: you can tell kivy to directly load a string or a file. If this string or file defines a root widget, it will be returned by the method:

    Builder.load_file('path/to/file.kv')
    

    or:

    Builder.load_string(kv_string)
    

Rule context

A Kv source constitutes of rules, which are used to describe the content of a Widget, you can have one root rule, and any number of class or template rules.

The root rule is declared by declaring the class of your root widget, without any indentation, followed by : and will be set as the root attribute of the App instance:

Widget:

A class rule, which defines how any instance of that widget class will be graphically represented is declared by declaring the name of the class, between < >, followed by ::

<MyWidget>:

Rules use indentation for delimitation, as python, indentation should be of four spaces per level, like the python good practice recommendations.

There are three keywords specific to Kv language:

  • app: always refers to the instance of your application.
  • root: refers to the base widget/template in the current rule
  • self: always refer to the current widget

Special syntaxes

There are two special syntax to define values for the whole Kv context:

To import something from python:

#:import name x.y.z

Is equivalent to:

from x.y import z as name

in python.

To set a global value:

#:set name value

Is equivalent to:

name = value

in python.

Instantiate children

To declare the widget has a child widget, instance of some class, just declare this child inside the rule:

MyRootWidget:
    BoxLayout:
        Button:
        Button:

The example above defines that our root widget, an instance of MyRootWidget has a child; an instance of the BoxLayout which has two children, instances of the Button class.

A python equivalent of this code could be:

root = MyRootWidget()
box = BoxLayout()
box.add_widget(Button())
box.add_widget(Button())
root.add_widget(box)

Which you way find maybe less nice, both to read and to write.

Of course, in python, you can pass keyword arguments to your widgets at creation, to specify their behaviour, for example, to set the number of columns of a gridlayout, we would do:

grid = GridLayout(cols=3)

To do the same thing in kv, you can set properties of the child widget directly in the rule:

GridLayout:
    cols: 3

The value is evaluated as a python expression, and all the properties used in the expression will be observed, that means that if you had something like this in python (this assume self is a widget with a data ListProperty):

grid = GridLayout(cols=len(self.data))
self.bind(data=grid.setter('cols'))

To have your display updated when your data change, you can now have just:

GridLayout:
    cols: len(root.data)

Event Bindings

You can bind to events in Kv using the ”:” syntax, that is, associating a callback to an event:

Widget:
    on_size: my_callback()

You can pass the values dispatched by the signal using the args keyword:

TextInput:
    on_text: app.search(args[1])

Extend canvas

Kv lang can be used to define the canvas instructions of your widget too:

MyWidget:
    canvas:
        Color:
            rgba: 1, .3, .8, .5
        Line:
            points: zip(self.data.x, self.data.y)

And yes, they get updated too if properties values change.

Of course you can use canvas.before and canvas.after.

Referencing Widgets

In a widget tree there is often a need to access/reference other widgets. Kv Language provides a way to do this using id’s. Think of them as class level variables that can only be used in the Kv language. Consider the following:

<MyFirstWidget>:
    Button:
        id: f_but
    TextInput:
        text: f_but.state

<MySecondWidget>:
    Button:
        id:s_but
    TextInput:
        text: s_but.state

id’s are limited in scope to the rule they are declared in so, in the code above s_but can not be accessed outside the <MySecondWidget> rule.

Accessing Widgets defined inside Kv lang in your python code

Consider the code below in my.kv:

<MyFirstWidget>:
    # both these variable don't have to be the same name and this doesn't
    # lead to a issue with uniqueness as id's are accessible only in kv.
    txt_inpt: txt_inpt
    Button:
        id: f_but
    TextInput:
        id: txt_inpt
        text: f_but.state
        on_text: root.check_status(f_but)

myapp.py:

...
class MyFirstWidget(BoxLayout):

    txt_inpt = ObjectProperty(None)

    def check_status(self, btn):
        print ('button state is: {state}'.format(state=btn.state))
        print ('text input text is: {txt}'.format(txt=self.txt_inpt))
...

txt_inpt is defined as a ObjectProperty initialized to None inside the Class.:

txt_inpt = ObjectProperty(None)

At this point self.txt_inpt is None. In Kv lang this property is updated to hold the instance of the TextInput referenced by the id txt_inpt.:

txt_inpt: txt_inpt

Thus; self.txt_inpt from this point onwards holds the instance to the widget referenced by the id txt_input and can be used anywhere in the class like in the function check_status. In contrast to this method you could also just pass the id to the function that needs to use it, like in case of f_but in the code above.

Templates

Consider the code below:

<MyWidget>:
    Button:
        text:
            "Hello world, watch this text wrap inside the button"
        text_size: self.size
        font_size: '25sp'
        markup: True
    Button:
        text:
            "Even absolute is relative to itself"
        text_size: self.size
        font_size: '25sp'
        markup: True
    Button:
        text:
            "repeating the same thing over and over in a comp = fail"
        text_size: self.size
        font_size: '25sp'
        markup: True
    Button:

Instead of having to repeat the same values for every button, we can just use a template instead, like so:

[Mbut@Button]:
    text: ctx.text if hasattr(ctx, 'text') else ''
    text_size: self.size
    font_size: '25sp'
    markup: True

<MyWidget>:
    MButton:
        text: "Hello world, watch this text wrap inside the button"
    MButton:
        text: "Even absolute is relative to itself"
    MButton:
        text: "repeating the same thing over and over in a comp = fail"
    MButton:

ctx is a keyword inside a template that can be used to access the individual attributes of each instance of this template.