[Python-ideas] Add single() to itertools

[Stéfane Fermigier]

IIUC, this would be similar to "first" ( https://pypi.python.org/pypi/first/
) but would raise exception in case the iterable returns more than one (or
less than one) element.

Would also be similar to one() in SQLAlchemy queries (
http://docs.sqlalchemy.org/en/latest/orm/query.html#sqlalchemy.orm.query.Query.one
).

Regards,

  S.

On Mon, Oct 30, 2017 at 5:51 AM, Steven D'Aprano <steve at pearwood.info>
wrote:

> On Mon, Oct 30, 2017 at 07:14:10AM +0300, Ivan Pozdeev via Python-ideas
> wrote:
>
> > The eponymous C#'s LINQ method, I found very useful in the following,
> > quite recurring use-case:
>
> If I have understood your use-case, you have a function that returns a
> list of results (or possibly an iterator, or a tuple, or some other
> sequence):
>
>     print(search(haystack, needle))
>     # prints ['bronze needle', 'gold needle', 'silver needle']
>
> There are times you expect there to be a single result, and if there are
> multiple results, that is considered an error. Am I correct so far?
>
> If so, then sequence unpacking is your friend:
>
>     result, = search(haystack, needle)
>
> Note the comma after the variable name on the left-hand side of the
> assignment. That's a special case of Python's more general sequence
> unpacking:
>
>     a, b, c = [100, 200, 300]
>
> assigns a = 100, b = 200, c == 300. Using a single item is valid:
>
> py> result, = [100]
> py> print(result)
> 100
>
>
> but if the right-hand side has more than one item, you get an exception:
>
> py> result, = [100, 200, 300]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: too many values to unpack (expected 1)
>
>
> I *think* this will solve your problem.
>
> If not, can you please explain what "single()" is supposed to do, why it
> belongs in itertools, and show an example of how it will work.
>
>
> --
> Steve
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>



-- 
Stefane Fermigier - http://fermigier.com/ - http://twitter.com/sfermigier -
http://linkedin.com/in/sfermigier
Founder & CEO, Abilian - Enterprise Social Software -
http://www.abilian.com/
Chairman, Free&OSS Group / Systematic Cluster -
http://www.gt-logiciel-libre.org/
Co-Chairman, National Council for Free & Open Source Software (CNLL) -
http://cnll.fr/
Founder & Organiser, PyData Paris - http://pydata.fr/
---
“You never change things by fighting the existing reality. To change
something, build a new model that makes the existing model obsolete.” — R.
Buckminster Fuller
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20171030/867ba3a7/attachment.html>