Rule: immutability is on objects, not on variables. Actually, same as other languages.
Rule: An object’s mutability is determined by its type; for instance, numbers, strings and tuples are immutable, while dictionaries and lists are mutable.
Examples of containers are tuples, lists and dictionaries. The value of an immutable container object (like a tuple instance) that contains a reference to a mutable object can change when the latter’s value is changed; however the container is still considered immutable, because the collection of objects it contains cannot be changed. So, immutability is not strictly the same as having an unchangeable content, it is more subtle. See http://docs.python.org/2/reference/datamodel.html
In most cases, when we talk about the value of a container, we imply the values, not the identities of the contained objects; however, when we talk about the mutability of a container, only the identities of the immediately contained objects are implied.