It won't (using your example explicitly) but in general what you've discovered is that:

1. Variables hold values
2. Some of those values are references to shared mutable objects.

Lists fall into the second category. There are ways to copy lists if you want distinct behaviour.

list2 = list1[:]

will perform a "shallow copy". If you have a list of lists, however, the nested lists are still shared references. There is copy.deepcopy available to make a complete clone of something (including all its nested members).