Kivy
I wanted to add to what @JohnMudd has written about Kivy. It has been years since the situation he described, and Kivy has evolved substantially.
The biggest selling point of Kivy, in my opinion, is its cross-platform compatibility. You can code and test everything using any desktop environment (Windows/*nix etc.), then package your app for a range of different platforms, including Android, iOS, MacOS and Windows (though apps often lack the native look and feel).
With Kivy's own KV language, you can code and build the GUI interface easily (it's just like Java XML, but rather than TextView etc., KV has its own ui.widgets for a similar translation), which is in my opinion quite easy to adopt.
Currently Buildozer and python-for-android are the most recommended tools to build and package your apps. I have tried them both and can firmly say that they make building Android apps with Python a breeze. Their guides are well documented too.
iOS is another big selling point of Kivy. You can use the same code base with few changes required via kivy-ios Homebrew tools, although Xcode is required for the build, before running on their devices (AFAIK the iOS Simulator in Xcode currently doesn't work for the x86-architecture build). There are also some dependency issues which must be manually compiled and fiddled around with in Xcode to have a successful build, but they wouldn't be too difficult to resolve and people in Kivy Google Group are really helpful too.
With all that being said, users with good Python knowledge should have no problem picking up the basics quickly.
If you are using Kivy for more serious projects, you may find existing modules unsatisfactory. There are some workable solutions though. With the (work in progress) pyjnius for Android, and pyobjus, users can now access Java/Objective-C classes to control some of the native APIs.
