practice_env

https://codeshare.io/
- env to share code and practice. Can be used online by multiple persons in the same time

https://exercism.io/
- code practice and mentorship

http://rosalind.info/about/
- code practice with a twist (focus on bioinformatics, so you will learn also some molecular biology on the way)

https://projecteuler.net/
- math and CS problems to solve

https://codejam.withgoogle.com/codejam/about
https://codejam.withgoogle.com/codejam/past-contests
- algorithmic puzzles

https://pybit.es/
- code challanges

https://www.codingame.com/start
- practice code and play some games in the same time

https://codecombat.com/
- practice code and play some games in the same time

https://checkio.org/
- practice code and play some games in the same time

http://www.cyber-dojo.org/dojo/index/
- code practice, problem solving

https://www.codewars.com/
- code practice, code challenges, problem solving

https://www.hackerrank.com/
- code practice, code challenge, problem solving