join lists?

edit

You can simply add them:

sage: a = [1,2,3]
sage: b = [4,5,6]
sage: z = a+b
sage: z
[1, 2, 3, 4, 5, 6]

Note that you can't subtract them, though:

sage: a-b
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for -: 'list' and 'list'

This is basically because it's not obvious what to do with [1,2,3]-[4,5], and a fundamental Python principle is to resist the temptation to guess in the face of ambiguity.

It might be helpful to work through a Python tutorial.