Correct way to iterate twice over a list?

Question

What is the correct way to perform multiple iteration over a container? From python documentation:

Iterator - A container object (such as a list) produces a fresh new iterator each time you pass it to the iter() function or use it in a for loop. Attempting this with an iterator will just return the same exhausted iterator object used in the previous iteration pass, making it appear like an empty container.

The intention of the protocol is that once an iterator’s next() method raises StopIteration, it will continue to do so on subsequent calls. Implementations that do not obey this property are deemed broken. (This constraint was added in Python 2.3; in Python 2.2, various iterators are broken according to this rule.)

If I have this code:

slist = [1,2,3,4]
rlist = reversed(slist)
list(rlist)
#[4,3,2,1]
tuple(rlist)
#()

What would be the easiest and most correct way to iterate over 'rlist' twice?

Answer 1

rlist = list(reversed(slist))

Then iterate as often as you want. This trick applies more generally; whenever you need to iterate over an iterator multiple times, turn it into a list. Here's a code snippet that I keep copy-pasting into different projects for exactly this purpose:

def tosequence(it):
    """Turn iterable into a sequence, avoiding a copy if possible."""
    if not isinstance(it, collections.Sequence):
        it = list(it)
    return it

(Sequence is the abstract type of lists, tuples and many custom list-like objects.)