Cannot step through asynchronous iterator manually

[Frank Millman]

"Chris Angelico"  wrote in message 
news:CAPTjJmoAmVNTCKq7QYaDRNQ67Gcg9TxSXYXCrY==S9Djjna_rA at mail.gmail.com...
>
> On Sat, Jan 30, 2016 at 7:22 PM, Frank Millman <frank at chagford.com> wrote:
> > 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.
>
> Here's a crazy option. (Assuming that a row can't be None. If not, use
> a unique sentinel object.)
>
> cur.execute(whatever)
> have_row = None
> async for row in cur:
>     if have_row is not None:
>         raise TooManyRows
>     have_row = row
> if have_row is None:
>     raise NoRowFound
>

Not so crazy :-) If Ian doesn’t come up with a better idea I will run with 
it.

Here is a slight variation - just variable name changes really, but I think 
it is slightly easier to read -

found = False
async for row in cur:
    if found:
        raise TooManyRows
    found = True
if found:
    process row
else:
    no rows found

Frank