[Python-ideas] Null coalescing operator

[Nick Coghlan]

On 1 November 2016 at 01:51, Mark E. Haase <mehaase at gmail.com> wrote:
> Stephen J. Turnbull wrote:
>>
>> I gather you think you have a deadlock here.  The way to break it is
>> to just do it.  Pick a syntax and do the rewriting.  My memory of some
>> past instances is that many of the senior devs (especially Guido) will
>> "see through the syntax" to evaluate the benefits of the proposal,
>> even if they've said they don't particularly like the initially-
>> proposed syntax.
>
> I don't feel deadlocked, but I think you're right about committing to a
> syntax. So I updated the PEP, summarized here:
>
> Spelling a new operator as a keyword is difficult due to backward
> compatibility. It can be done (see PEP-308 and PEP-492) but requires extreme
> care.
> A keyword operator is considered less ugly than punctuation, but it makes
> assignment shortcut syntax very ugly. Assume the coalesce operator is "foo",
> then the assignment shortcut would be "x foo= y". This is unacceptable.
> If eliminate the possibility of a keyword and focus on punctuation, we find
> that most people think "??" — the syntax that exists in several other
> mainstream languages — is ugly and not Pythonic.
> However, any other punctuation spelling will be at least as ugly and will
> not have the benefit of being familiar to programmers who have seen null
> coalescing in other languages.
> Therefore, the most reasonable spelling is to borrow the same spelling that
> other languages use, e.g. "??", "?.", and "?[".
>
> I did go down the road of trying to create a new keyword, trying some
> mundane ideas ("foo else bar") and some more exotic ideas ("try foo then
> bar"), but I don't know if those syntaxes are even parseable, and as I
> worked through a bunch of examples, I realized that all of the keywords I
> was trying were very awkward in practical use, especially when combined with
> other expressions.

I do think it would be worth covering the symbol+keyword option
discussed in PEP 531 (i.e. "?else" instead of "??", but keeping "?.",
and "?[")

Cheers,
Nick.

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