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

[Nicholas Chammas]

On Wed, Jul 25, 2018 at 10:12 PM David Mertz <mertz at gnosis.cx> wrote:

> On Wed, Jul 25, 2018 at 9:47 PM Nicholas Chammas <
> nicholas.chammas at gmail.com> 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.
>>>
>>
>
>>     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.
>>
>
> No, you illustrate it perfectly! I had to stare at your translation for a
> while to decide if it was really identical to the proposed
> `spam?.eggs?.bacon`.  The fact I have to think so hard makes the syntax
> feel non-obvious.
>
> Plus, there's the fact that your best effort at translating the proposed
> syntax is WRONG.  Even a strong proponent cannot explain the behavior on a
> first try.  And indeed, it behaves subtly different from plain attribute
> access in where it raises AttributeError.
>
> >>> spam = SimpleNamespace()
> >>> spam.eggs = None
> >>> spam.eggs.bacon
> AttributeError: 'NoneType' object has no attribute 'bacon'
>
> >>> # spam?.eggs?.bacon
> >>> # Should be: None
>
> >>> "Translation" does something different
> >>> food = spam
> >>> if spam is not None and spam.eggs is not None:
> ...     food = spam.eggs.bacon
> >>> food
> namespace(eggs=None)
>

Indeed. Thanks for the counter-example. I think the correct translation is
as follows:

    food = spam?.eggs?.bacon

Becomes:

    food = None
    if spam is not None and spam.eggs is not None:
        food = spam.eggs.bacon

Did I get it right now? :)

What misled me was this example from the PEP
<https://www.python.org/dev/peps/pep-0505/#the-maybe-dot-and-maybe-subscript-operators>
showing
how atoms are evaluated. The breakdown begins with `_v = a`, so I copied
that pattern incorrectly when trying to explain how an example assignment
would work, instead of translating directly from what I understood the PEP
505 variant should do.

So, shame on me. I think this particular mistake reflects more on me than
on PEP 505, but I see how this kind of mistake reflects badly on the folks
advocating for the PEP (or at least, playing devil's advocate).
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