We're going to practice with something Python does really well: processing & parsing --- by building a little program that makes a protein from DNA.
While the data you end up working with in your ML projects might not be strings of DNA or RNA, this coding challenge will give you some good challenges with a common pre processing task for many data sets -- cleaning, munging, and transforming various types of text.
But if this just isn't for you -- you can always get your Python practice below through exercism.io instead. Or even supplement this homework with exercism.io.
For enven more practice, go to exercism.io (you can use GitHub to login), and work your way through the first 5 or so exercises. You'll get some good practice in basic Python, have some automated tests of your code .... and get valuable feedback from volunteer mentors. It's really a great place to learn...and who knows? You might want to buckle down and do even more exercises when you're through.