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Re: Cannot step through asynchronous iterator manually

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  Re: Cannot step through asynchronous iterator manually "Frank Millman" <frank@chagford.com> - 2016-01-30 10:22 +0200

#102287 — Re: Cannot step through asynchronous iterator manually

"Ian Kelly"  wrote in message 
news:CALwzid=sSDSm8hdAN+ORJ54A_jEu9Wc8103iqGKAah8mrj-TXw@mail.gmail.com...

> On Jan 29, 2016 11:04 PM, "Frank Millman" <frank@chagford.com> wrote:
> >
> > Hi all
> >
> > To loop though an iterator one usually uses a higher-level construct 
> > such
as a 'for' loop. However, if you want to step through it manually you can
do so with next(iter).
> >
> > I expected the same functionality with the new 'asynchronous iterator' 
> > in
Python 3.5, but I cannot find it.
> >
> > I can achieve the desired result by calling 'await aiter.__anext__()',
but this is clunky.
> >
> > Am I missing something?
>
> async for x in aiter:
>     pass
>
> Can only be used inside a coroutine, of course.

I know that you can use this to loop through the entire iterator, but I have 
a special case.

There are times when I want to execute a SELECT statement, and test for 
three possibilities -
    - if no rows are returned, the object does not exist
    - if one row is returned, the object does exist
    - if more that one row is returned, raise an exception

We had a recent discussion about the best way to do this, and ChrisA 
suggested the following, which I liked -

    cur.execute('SELECT ...)
    try:
        row = next(cur)
    except StopIteration:
        # row does not exist
    else:
        try:
            next_row = next(cur)
        except StopIteration:
            # row does exist
        else:
            # raise exception

Now that I have gone async, I want to do the same with an asynchronous 
iterator.

Frank

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