How to use .kv file in kivy

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by vsasikalabe dot icon Updated: Aug 29, 2023

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Kivy has the KV design language. It is thought of as a language like HTML and CSS. It handles styling and adding elements to the display but does not handle any logic. 


The KV language is sometimes called kvlang or the kivy language. It allows you to create your widget tree. You can use it to connect widget properties or callbacks and make statements. However, it depends on the development environment. You can pick a Kivy installation that works with your development environment. Developers use different widgets and canvas instructions to build Kivy widgets.   


You can create many kv files. Also, you can include it in one place (root file). You can distribute/branch your code Kivy documentation. A kv file will contain only one root widget; most means only one kv main file. Whenever we define the id attribute in a KV file, it will not be available on the widget instance. Instead, it will be on weakref to the widget. A Kivy rule will apply to a specific widget.  


The KV file changes the widget. It specifies how it behaves or adds a visual representation. The KV language also supports importing Python modules in KV, creating dynamic classes. You can also parse and load Kv language in a multi-line string in your Python file. By default, Kivy loads the kv file that will match the prefix of the app class defined in the Python script.  


A Kivy dynamic class allows you to create new widgets or rules. You create this without making any Python declaration. Root widgets are the building blocks of GUIs in Kivy, and a KV file can contain only one root widget. Use the args keyword to access the arguments in the on_<action> callback methods.   


Because BoxLayout is a sub-class of Layout and Widget. It inherits many other methods and keyword arguments. If the Root Widget defines this file, it will attach to the App's root attribute. The application widget tree uses it as the base. The App class calls the build method and returns either a widget or an instance of the Builder class.