[Python-ideas] PEP 531: Existence checking operators

[Nick Coghlan]

On 29 October 2016 at 21:44, Steven D'Aprano <steve at pearwood.info> wrote:
> On Fri, Oct 28, 2016 at 06:30:05PM +1000, Nick Coghlan wrote:
>
> [...]
>> 1. Do we collectively agree that "existence checking" is a useful
>> general concept that exists in software development and is distinct
>> from the concept of "truth checking"?
>
> Not speaking for "we", only for myself: of course.
>
>
>> 2. Do we collectively agree that the Python ecosystem would benefit
>> from an existence checking protocol that permits generalisation of
>> algorithms (especially short circuiting ones) across different "data
>> missing" indicators, including those defined in the language
>> definition, the standard library, and custom user code?
>
> Maybe, but probably not.
>
> Checking for "data missing" or other sentinels is clearly an important
> thing to do, but it remains to be seen whether (1) it should be
> generalised and (2) there is a benefit to making it a protocol.
>
> My sense so far is that generalising beyond None is YAGNI. None of the
> other examples you give strike me as common enough to justify special
> syntax, or even a protocol. I'm not *against* the idea, I just remain
> unconvinced.

I considered this the weakest link in the proposal when I wrote it,
and the discussion on the list has persuaded me that it's not just a
weak link, it's a fatal flaw. Accordingly, I've withdrawn the PEP, and
explained why with references back to this discussion:
https://github.com/python/peps/commit/9a70e511ada63b976699bbab9da142379340758c

However, as noted there, I find the possible link back to the rejected
boolean operator overloading proposal in PEP 335 interesting, so I'm
going to invest some time in writing that up to the same level as I
did the existence checking one (i.e. Abstract, Rationale & design
discussion, without a full specification or reference implementation
yet).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia