How to unzip an iterator?

Question

Given a list of pairs xys, the Python idiom to unzip it into two lists is:

xs, ys = zip(*xys)

If xys is an iterator, how can I unzip it into two iterators, without storing everything in memory?

Answer 1

If you want to consume one iterator independently from the other, there's no way to avoid pulling stuff into memory, since one of the iterators will progress while the other does not (and hence has to buffer).

Something like this allows you to iterate over both the 'left items' and the 'right items' of the pairs:

 import itertools
 import operator

 it1, it2 = itertools.tee(xys)
 xs = map(operator.itemgetter(0), it1))
 ys = map(operator.itemgetter(1), it2))

 print(next(xs))
 print(next(ys))

...but keep in mind that if you consume only one iterator, the other will buffer items in memory until you start consuming them.

(Btw, assuming Python 3. In Python 2 you need to use itertools.imap(), not map().)