Sometimes an in situ sort or reversal is not what is required. For example, it may cause a function argument to be modified, giving the function confusing side effects. For out-of-place equivalents of the reverse() and sort() list methods you can use the reversed() and sorted() build-in functions which return a reverse iterator and a new sorted list respectively. For example:

>>> x = [4, 9, 2, 1]
>>> y = sorted(x)
>>> y
[1, 2, 4, 9]
>>> x
[4, 9, 2, 1]

We can also use the reversed() function:

>>> p = [9, 3, 1, 0]
>>> q = reversed(p)
>>> q
<list_reverseiterator object at 0x1007bf290>
>>> list(q)
[0, 1, 3, 9]

Notice how we used a list constructor to evaluate the result of reversed(). This is because reversed() returns an iterator, a topic which we'll cover in much more detail later.

These functions have the advantage that they'll work on any finite iterable source object.