After writing a simplistic implementation of this for itertools.islice, I'm not sure if this is actually useful. While it's nice for symmetry, I have a hard time imagining how to use it -- so I can't write tests for the new feature, and it may not be a useful thing to have.

Also, there are some design decision to be made, like what to do with an islice that needs to skip items. (The prototype just sends them None instead, which may or may not be appropriate for the hypothetical eventual usecase.) Tulip certainly won't be needing this kind of thing, as it doesn't fit the way it uses generators at all. It may be useful for generaors that are used as pipelines, but without an actual code example it's hard to say.

I'm attaching the preliminary send/throw methods for islice, but setting resolution to 'later' until someone comes up with a usecase.