when you use a = a + 'world' python sees it as an error because of different type.
But when you use a += 'world' python will change the right into list (because a is a list). So when you're code become: a += 'world' # a += list('world') It really helpfull if you stick to use append instead of += when you operate list Pujo -- http://mail.python.org/mailman/listinfo/python-list