An extension of :func:`itertools.islice` that supports negative values
for *stop*, *start*, and *step*.
>>> iterable = iter('abcdefgh')
>>> list(islice_extended(iterable, -4, -1))
['e', 'f', 'g']
Slices with negative values require some caching of *iterable*, but this
function takes care to minimize the amount of memory required.
For example, you can use a negative step with an infinite iterator:
>>> from itertools import count
>>> list(islice_extended(count(), 110, 99, -2))
[110, 108, 106, 104, 102, 100]
You can also use slice notation directly:
>>> iterable = map(str, count())
>>> it = islice_extended(iterable)[10:20:2]
>>> list(it)
['10', '12', '14', '16', '18']