[Python-ideas] PEP 531: Existence checking operators

[David Mertz]

On Fri, Oct 28, 2016 at 11:17 AM, Paul Moore <p.f.moore at gmail.com> wrote:

> On 28 October 2016 at 15:40, Nick Coghlan <ncoghlan at gmail.com> wrote:
> > I also don't think the idea is sufficiently general to be worthy of
> > dedicated syntax if it's limited specifically to "is not None" checks
> > - None's definitely special, but it's not *that* special. Unifying
> > None, NaN, NotImplemented and Ellipsis into a meta-category of objects
> > that indicate the absence of information rather than a specific value,
> > though?
>

First thing is that I definitely DO NOT want new SYNTAX to do this.  I
wouldn't mind having a new built-in function for this purpose if we could
get the call signature right.  Maybe it would be called 'exists()', but
actually something like 'valued()' feels like a better fit.

For the unusual case where the "null-coalescing" operation is what I'd
want, I definitely wouldn't mind something like Barry's proposal of
processing a string version of the expression.  Sure, *some* code editors
might not highlight it as much, but it's a corner case at most, to my
mind.  For that, I can type 'valued("x.y.z[w]", coalesce=ALL)' or whatever.


> However, I'm not convinced by your proposal that we can unify None, NaN,
> NotImplemented and Ellipsis in the way you suggest. I wouldn't expect
> a[1, None, 2] to mean the same as a[1, ..., 2], so why should an
> operator that tested for "Ellipsis or None" be useful?


I *especially* think None and nan have very different meanings.  A list of
[1.1, nan, 3.3] means that I have several floating point numbers, but one
came from a calculation that escaped the real domain.  A list with [1.1,
None, 3.3] means that I have already calculated three values, but am
marking the fact I need later to perform a calculation to figure out the
middle one.  These are both valid and important use cases, but they are
completely different from each other.

Yours, David...

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