Bug? If not, how to work around it?

Gonçalo Rodrigues op73418 at mail.telepac.pt
Thu Aug 7 19:38:07 CEST 2003


On Thu, 7 Aug 2003 12:39:06 -0400, "Terry Reedy" <tjreedy at udel.edu>
wrote:

>
>"Gonçalo Rodrigues" <op73418 at mail.telepac.pt> wrote in message
>news:md94jvccu02b9dv5890k34629rkot79roj at 4ax.com...
>> On Thu, 07 Aug 2003 00:54:03 +0100, Gonçalo Rodrigues
>> <op73418 at mail.telepac.pt> wrote:
>>
>> >On Wed, 6 Aug 2003 19:46:39 -0400, "Terry Reedy" <tjreedy at udel.edu>
>> >>If you don't want to do what works,
>> >>why ask us to bother answering?
>
>> >Because it may not work. That is, the use case I have in mind
>serves
>> >as proxy for an object that may or may not have __iter__. IOW if I
>> >stick __iter__ I have to code a second class, etc... etc... Oh
>well...
>
>> Just to make things clearer, having iter(self.__obj) as in
>>
>> def __iter__(self):
>>     return iter(self.__obj)
>>
>> Would mean that my proxy class would become an iterable. And that is
>> not what I want because *other* parts in my code check that an
>object
>> is in iterable by looking for __iter__.
>
>As I understand your clarification, you want the proxy to actually be
>iterable (via delegation) when given as an arg to iter(), but to not
>look iterable to other code (call it test_iterable()).  And your
>test_iterable() *currently* looks only for the presence/absence of the
>very thing needed to make iter() work.
>

To look an iterable by delegation *if* the underlying object is, yes.

>It is definitely not a Python bug that this does not work.
>

It finally dawned on me why this is so: it's the way new classes work.
In a sloppy language: a __getattr__ hook is like an instance
attribute, but the iter machinery goes after the class not the
instance so it misses __getattr__.

So my test_iterable (in your nomenclature) is wrongly coded because it
gives false positives.

[Ideas snipped]

Thanks for the input. But since something has to change anyway, the
best idea seems to be to stick __iter__ in the proxy class after all
and change the other code that was counting on testing iterableness by
looking __iter__. This will involve some changes, but it's the "least
worse of all evils."

With my best regards,
G. Rodrigues




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