[Python-ideas] Fwd: A better (simpler) approach to PEP 505

David Mertz mertz at gnosis.cx
Tue Jul 24 07:07:48 EDT 2018


I started a new thread where I show a small library I made yesterday that
ALSO matches 505 semantics, as well as more general AttributeError
swallowing. It has two classes with sightly different behavior. So we don't
need syntax for either semantics.

On Tue, Jul 24, 2018, 3:29 AM Grégory Lielens <gregory.lielens at gmail.com>
wrote:

>
>
> On Tuesday, July 24, 2018 at 2:39:19 AM UTC+2, Steven D'Aprano wrote:
>
>> PEP 505 has a section explaining why catching AttributeError
>> is undesirable. I find the reasons it gives are compelling.
>>
>> Can you explain why you reject the PEP's arguments against catching
>> AttributeError?
>>
>>
>
> Yes, and people have done it multiple time already. I will try to make it
> as explicit as possible, with a terminology that some may find useful...
> when u have a object hierarchy linked through attributes, which -seems-
> the use pattern of ?. , the hierarchy is either
> - 1) fully regular (each attribute contains the same type of objects (same
> type=with the same attributes), then normal . access is what you want
> - 2) regular-or-None (contain same type of objects, or None). That's what
> ?. is for, and ?. is only for that.
> - 3) its partially regular (does not contain the same type of objects,
> some objects have more attributes than other, e.g. partial objects).
> - 4) its irregular (objects may be of completely different type (int,
> str,... and do not specailly share attributes).
>
> ?. is intended for 2).
> 2) and 3) can be dealed with swallowing AttributeError
> 4) can also be traversed swallowing AttributeError, but it's probably a
> very bad idea in almost all cases.
>
> I've encountered all cases, from time to time, but none is really typical
> of most of my code. Which is more common? not sure, I would say 3).
> What is sure is that 2) do not stand as hugely more common than the
> others, so introducing a special syntax for 2) do not  please me, so +0...
> Now as I find ?. unpleasant to parse (personal opinion), and having grown
> to share the general operaror / line noise allergy mainstream in the python
> community, i am a firm -1 on this: too small a niche, do not bring much
> compared to current approach, and visual line noise.
> firm -1 on ?? too, it's less polemic, less visually displeasing, but even
> less usefull I think: it does not gain that much compared to a more general
> ternary.
>
> As the pymaybe/this thread cover 2) and 3), and do it without any change
> to python, I find it better. No change > lib change > backward compatible
> object change  - like dict becoming ordered, or None triggering
> NoneAttributeError(AttributeError) > syntax change (expecially a bunch of
> linenoisy operators).
>
> Now I will stop for a while here, arguments have become circular,
> proponents and opponents have exposed their view, but mostly proponents
> have provided very few additional information for a while, especially about
> real use cases, despite a few requests.
> We still do not know what motivated the proposal..We had to guess (JSON).
> Even the PEP do not say, it provide a set of real world example from the
> standard library, which is admirable but different. And personaly, while am
> very thankful for providing those concrete examples, I find them
> unconvicing: the code before is usually fine and improved little by the new
> operators. Sometimes it's worse: only slightly shorter and less clear.
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