[Python-ideas] Null coalescing operators

Chris Angelico rosuav at gmail.com
Mon Sep 21 06:05:15 CEST 2015

On Mon, Sep 21, 2015 at 1:50 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sun, Sep 20, 2015 at 07:38:18PM +1000, Chris Angelico wrote:
>> On Sun, Sep 20, 2015 at 5:31 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>> > Technically, x.y x[y] and x(y) aren't operators, but for the sake of
>> > convenience I'll call them such. Even though these are binary operators,
>> > the ? only shortcuts according to the x, not the y. So we can call
>> > these ?. ?[] ?() operators "pseudo-unary" operators rather than binary
>> > operators.
>> That's how all Python's short-circuiting works - based on the value of
>> what's on the left, decide whether or not to evaluate what's on the
>> right. (Well, nearly all - if/else evaluates the middle first, but
>> same difference.) This is another form of short-circuiting; "x[y]"
>> evaluates x, then if that's None, doesn't bother evaluating y because
>> it can't affect the result.
> I think you are mistaken about x[y]:
> py> None[print("side effect")]
> side effect
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: 'NoneType' object is not subscriptable
> That's why x?[y] is a proposal.

Oops, that was a typo in my statement. I meant "x?[y]" should behave
that way - once it's discovered that x is None, the evaluation of y
can't affect the result, and so it doesn't get evaluated (as per the
normal short-circuiting rules). Yes, x[y] has to evaluate both x and y
(after all, the value of y is passed to __getitem__). Sorry for the


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