[Python-ideas] PEP 531: Existence checking operators

[Nick Coghlan]

On 29 October 2016 at 04:08, Mark Dickinson <dickinsm at gmail.com> wrote:
> On Fri, Oct 28, 2016 at 9:30 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> [...] the current practicises of:
>>
>> * obj is not None (many different use cases)
>> * obj is not Ellipsis (in multi-dimensional slicing)
>
> Can you elaborate on this one? I don't think I've ever seen an `is not
> Ellipsis` check in real code.

It's more often checked the other way around: "if Ellipsis is passed
in, then work out the multi-dimensional slice from the underlying
object"

And that reflects the problem Paul and David highlighted: in any
*specific* context, there's typically either only one sentinel we want
to either propagate or else replace with a calculated value, or else
we want to handle different sentinel values differently, which makes
the entire concept of a unifying duck-typing protocol pragmatically
dubious, and hence calls into question the idea of introducing new
syntax for working with it.

On the other hand, if you try to do this as an "only None is special"
kind of syntax, then any of the *other* missing data sentinels (of
which we have 4 in the builtins alone, and 5 when you add the decimal
module) end up being on much the same level as "arbitrary sentinel
objects" in the draft PEP 531, which I would consider to be an
incredibly poor outcome for a change as invasive as adding new syntax:
https://www.python.org/dev/peps/pep-0531/#arbitrary-sentinel-objects

Cheers,
Nick.

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