[Python-ideas] PEP 505: None-aware operators

[Chris Angelico]

On Thu, Jul 26, 2018 at 11:45 AM, Nicholas Chammas
<nicholas.chammas at gmail.com> wrote:
> On Wed, Jul 25, 2018 at 9:20 PM Chris Angelico <rosuav at gmail.com> wrote:
>>
>> On Thu, Jul 26, 2018 at 11:02 AM, David Mertz <mertz at gnosis.cx> wrote:
>> > That is disingenuous, I think.  Can this raise an AttributeError?
>> >
>> >     spam?.eggs?.bacon
>> >
>> > Of course it can! And this is exactly the pattern used in many examples
>> > in
>> > the PEP and the discussion. So the PEP would create a situation where
>> > code
>> > will raise AttributeError in a slightly—and subtly—different set of
>> > circumstances than plain attribute access will.
>>
>> I don't understand. If it were to raise AttributeError, it would be
>> because spam (or spam.eggs) isn't None, but doesn't have an attribute
>> eggs (or bacon). Exactly the same as regular attribute access. How is
>> it slightly different? Have I missed something?
>
>
> That was my reaction, too.
>
>     food = spam?.eggs?.bacon
>
> Can be rewritten as:
>
>     food = spam
>     if spam is not None and spam.eggs is not None:
>         food = spam.eggs.bacon
>
> They both behave identically, no? Maybe I missed the point David was trying
> to make.

Aside from questions of repeated evaluation/assignment, yes. The broad
semantics should be the same.

(If you want to get technical, "spam" gets evaluated exactly once,
"spam.eggs" a maximum of once, and "food" gets assigned exactly once.
Your equivalent may evaluate and assign multiple times.)

ChrisA