A free program for Android, by David Phillips.

App Inventor 2 Tutorials is an online course that teaches you how to build and code apps with Python. You will learn to program in Python using the appinventor 2 interface. 

This course is the most comprehensive one that is free to use. There are 5 lessons to complete and a final exam to pass. The lessons are made up of several program sections, so that you will not be overwhelmed. The appinventor 2 program is very similar to the Python language.

The course is made up of several lessons and an accompanying final exam. Each lesson has several tasks to complete, and if you get all of them done, you will pass the final exam.