Kivy is a modern cross-platform Python GUI toolkit that runs on mobile devices (Android and IPhone) and supports modern input events (multitouch, accelerometor). I was really skeptical at first, but I spent the weekend developing an app in Kivy, and now I’m hooked.
In the past, I have tried most of the available Python GUI toolkits (even Swing, through jython). In recent history, I’ve mostly done HTML based interfaces, partially because that’s where the money has been, but mostly because pyQT, pyGTK,wxpython, and tk are all painful to work with.
In Kivy, interfaces are defined in a language called (aptly) the Kivy Language. The best way to describe the Kivy Language is “what HTML5 wishes it was.” It is mostly just a layout language, but it also supports binding properties and events using inline Python snippets. It is indentation defined, like Python, and utilizes a minimal amount of syntax with a maximal amount of readability. It is vaguely reminiscent of CoffeeKup but cleaner, and doesn’t suffer from having to be compiled to HTML. I love how it mimics Python syntax, but also clearly separates presentation from control code.
My favourite feature of Kivy is that properties are events. You can easily hook a property on one widget to the property on another. For example, I wrote a simple demo that connects the value of a slider to the value of a progressbar:
on_value: bar.value = self.value / 10
When the value property on the slider changes (whether because the user moved the slider, or because it was adjusted programmatically in response to some other event), the on_value event is fired. This is connected to the value of the progress bar using a Python snippet.
Only a few lines of Python code are required to to get a Kivy application running once its interface has been defined in Kivy language. The Kivy Hello World example is actually overly complicated because it doesn’t actually use the Kivy language. The bare essentials, if the above file is named
from kivy.app import App
class SampleApp(App): pass
Just three lines of code. Kivy introspects the class name (convention over configuration) and automatically loads sample.kv as the interface for a class named SampleApp.
The Kivy API is very simple. In fact, my earliest impression was that it was almost amateurish. It felt like if I wanted to do anything serious with it, I’d be disappointed. It’s the kind of API I would teach a child. However, the few features of the API (basically properties, events, and graphics instructions) are extremely well thought out and well-defined. They can be combined like Lego bricks to create widgets as complicated as one could possibly wish. The fact that the library includes a ReStructured Text rendering widget is testament to that fact!
Kivy is extremely well documented. I actually found their pong tutorial exciting. Seeing how simple it was to do each step actually made me giggle out loud. The programmer’s guide and API are well presented as well. Further, if you get stuck, the developers on IRC are terrific. I felt like I was an accepted member of the team after asking only one question.
It took a bit of fiddling to get Kivy to install correctly under Arch Linux. I suppose if I’d followed the instructions, all would have been well. However, I like to do all my development in a virtualenv, and there is no trivial way to track down and install all Kivy’s dependencies. This is especially compounded by the fact that pygame currently needs to be patched under Arch in order to compile.
There are instructions for packaging your Kivy app onto Android, but I wasn’t able to pull it off, yet. I’ll have to revisit it when I have more time.
I tried some sample Kivy apps in the Android market. They work just fine once they are loaded, but it takes each app several seconds to initialize on my Galaxy Nexus. Hopefully the Kivy team can reduce this load time. I don’t know if Kivy supports displaying a splash screen while stuff is loading; that would certainly enhance the usability.
Most of these issues are solvable. I’m sure the team is actively working on some of them. Kivy is a quite young project, but it is remarkably mature.
My weekend project was the Kivy Catalog, an interactive showcase of Kivy widgets. It allows you to view the Kivy language code used to define the widgets and to edit it and see what effects your changes have. I think it will be a tremendous aid to users looking to get started with the Kivy language. Developing it certainly was for me!
I am expecting to have this code included with the Kivy distribution in the examples directory in a future release.