It's very powerful and free. While it is positioned as being mainly for mobile development, I found it works fine for desktop applications as well.

Kivy is the best python library that helps you to design a UI in python for a python code.Using kivy you can really develop applications using python with much ease.Kivy helps you to design your application UI just by some simple python codes which are like XML tags(similiar to).You can also add buttons and give them functions to be executed onclick or on hover and so on.Also you can get the input text from the user.You can also set the backgroud color or image and also many more things regarding UI can be done using kivy.

- Documentation and examples provided. - You can build a great UI/UX for your android application using Kivy.