Cannot step through asynchronous iterator manually

[Chris Angelico]

On Sat, Jan 30, 2016 at 11:35 PM, Kevin Conway
<kevinjacobconway at gmail.com> wrote:
> To address the original question, I don't believe a next() equivalent for
> async iterables has been added to the standard library yet. Here's an
> implementation from one of my projects that I use to manually get the next
> value: https://bpaste.net/show/e4bd209fc067. It exposes the same interface
> as the synchronous next(). Usage:
>
>     await anext(some_async_iterator)
>
> Ultimately, it's a fancy wrapper around the original snippet of 'await
> iterator.__anext__()'.

Curious idiom for the one-or-two-arg situation. Any particular reason
not to use the classic sentinel object model?

_SENTINEL = object()
async def anext(iterable, default=_SENTINEL):
    ...
        if default is not _SENTINEL:
            return default

Or if you want to avoid that, at least take iterable as a fixed arg:

async def anext(iterable, *default):
    if len(default) > 1: TypeError
    ...
        if default: return default[0]

Also curious is that you raise a new StopAsyncIteration from the
original one, rather than just reraising the original. I assume
there's a reason for that, but it doesn't have a comment.

ChrisA