I have a book for learning python that teaches it with python3 in mind. I also have been using codecademy.com to learn in a more structured way. The thing is that codecademy teaches python from when the most recent form of python was 2.7.3. Their reasoning behind this is that python3 isn't very backwards compatible so they're teaching 2.7.3 for now. If you were learning python for physics and mathematics programming, which version would you learn?