When you use the slice, you're making a copy of the most upper list, which is good, but your newly copied list contains references to all the old lists. You need to make copies of those too. I would use copy.deepcopy() for that although you could use slicing again at the next level if this is for school and you're prohibited from using that.
Join the #python-forum IRC channel on irc.freenode.net!
Please do not PM members regarding questions which are meant to be discussed publicly. The point of the forum is so that others can benefit from it. We don't want to help you over PMs or emails.