Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Oscar Benjamin <oscar.j.benjamin@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: Cannot step through asynchronous iterator manually
Date: Sat, 30 Jan 2016 13:04:44 +0000
Lines: 41
Message-ID: <mailman.128.1454159539.2338.python-list@python.org>
References: <n8hjmt$b8n$1@ger.gmane.org> <CALwzid=sSDSm8hdAN+ORJ54A_jEu9Wc8103iqGKAah8mrj-TXw@mail.gmail.com> <n8hrs1$orn$1@ger.gmane.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
In-Reply-To: <n8hrs1$orn$1@ger.gmane.org>
Precedence: list
Xref: csiph.com comp.lang.python:102302

On 30 January 2016 at 08:22, Frank Millman <frank@chagford.com> wrote:
> 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

The simplest thing would just be to call list(cur) but I realise that
you don't want to consume more than 2 rows from the database so just
use islice:

rows = list(islice(cur, 2))  # pull at most 2 rows
if not rows:
    # no rows
elif len(rows) > 1:
    # too many rows
row = rows[0]

Depending on your application if you just want to raise any error when
there's not exactly one row then you could just do:

(row,) = list(islice(cur, 2))

--
Oscar