[Python-ideas] Null coalescing operators

[Paul Moore]

On 21 September 2015 at 15:41, Steven D'Aprano <steve at pearwood.info> wrote:
> An actual real-life example where we work around this by using a
> temporary name that otherwise isn't actually used for anything:
>
> mo = re.match(needle, haystack)
> if mo:
>     substr = mo.group()
> else:
>     substr = None
>
>
> I think it is perfectly reasonable to ask for syntactic sugar to avoid
> having to write code like the above:
>
> substr = re.match(needle, haystack)?.group()

Well, (1) Mark had focused on the "coalesce" operator ??, not the ?.
variant, and that is less obviously useful here, and (2) I find the
former version more readable. YMMV on readability of course - which is
why I added the proviso that if someone comes up with an "obviously
right" syntax, I may well change my mind. But the options suggested so
far are all far less readable than a simple multi-line if (maybe with
a temporary variable) to me, at least.

By the way, in your example you're passing on the "none or useful"
property by making substr be either the matched value or None. In real
life, I'd probably do something more like

mo = re.match(needle, haystack)
if mo:
    process(mo.group())
else:
    no_needle()

possibly with inline code if process or no_needle were simple. But it
is of course easy to pick apart examples - real code isn't always that
tractable. For example we get (what seems to me like) a *lot* of bug
reports about "None has not attribute foo" style errors in pip. My
comments here are based on my inclinations about how I would fix them
in pip - I'd always go back to *why* we got a None, and try to avoid
getting the None in the first place. But that's not always easy to do.
Again, of course, YMMV.

Paul