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 da1734c58d2f97387ccc9676074717d38b044128 by Raymond Hettinger (Cheryl Sabella) in branch 'master':
bpo-27212: Modify islice recipe to consume initial values preceding start (GH-6195)
https://github.com/python/cpython/commit/da1734c58d2f97387ccc9676074717d38b044128

New changeset f328caf4caafd4521c356af8cb8a299f27603c90 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)
https://github.com/python/cpython/commit/f328caf4caafd4521c356af8cb8a299f27603c90

New changeset c8698cff7ccc8dc730c58523c7ef4113ea8d3049 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)
https://github.com/python/cpython/commit/c8698cff7ccc8dc730c58523c7ef4113ea8d3049

New changeset 325191bd6b3a2d5e3012adee868f19baf3e2c3ab 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)
https://github.com/python/cpython/commit/325191bd6b3a2d5e3012adee868f19baf3e2c3ab

Thanks Cheryl.  This was nice work :-)

Thanks!  :-)