In the doc for itertools, the python implementation of islice() doesn't actually perform the iteration when given an empty slice (e.g. islice(it,3,3)).

This is not the expected behavior for 'consume()' in the recipe given below, i.e. consume(it,3) couldn't actually advance the iterator for 3 steps ahead. Instead, the iterator stays at its starting position.

As a simple fix, in the implementation of islice(), the length of the slice object should be checked first. If it's zero, the iterator should be advanced (slice.start-1) times, and return (See islice_my.py)

The recipe should be modified to consume the initial values. Something like this:

start, stop, step = s.start or 0, s.stop or sys.maxsize, s.step or 1
for i in zip(range(0, start), it):
    pass

The recipe needs to pass these tests:

    it = iter('abcdefghi')
    assert list(islice(it, 4, 4)) == []
    assert list(it) == ['e', 'f', 'g', 'h', 'i']
    assert(list(islice('ABCDEFG', 2)) == ['A', 'B'])
    assert(list(islice('ABCDEFG', 2, 4)) == ['C', 'D'])
    assert(list(islice('ABCDEFG', 2, None)) == ['C', 'D', 'E', 'F', 'G'])
    assert(list(islice('ABCDEFG', 0, None, 2)) == ['A', 'C', 'E', 'G'])

Hi Nofar,

Are you still interested in working on this issue? Thanks!

Cheryl, I think you take this one. It hasn't been touched in a long time and Nofar is focused on another issue.

Thanks, Raymond. I'll take a look.

New changeset da1734c by Raymond Hettinger (Cheryl Sabella) in branch 'master':
bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195)
da1734c

New changeset f328caf by Raymond Hettinger (Miss Islington (bot)) in branch '3.7':
bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195) (#GH-6266)
f328caf

New changeset c8698cf by Raymond Hettinger (Miss Islington (bot)) in branch '3.6':
bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195) (GH-6267)
c8698cf

New changeset 325191b by Raymond Hettinger (Cheryl Sabella) in branch '2.7':
[2.7] bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195) (GH-6339)
325191b

Thanks Cheryl. This was nice work :-)

Thanks! :-)