On 2005 Jan 11, at 23:39, Phillip J. Eby wrote:
...
>> cls = type(x)
>>
>> copier = _copy_dispatch.get(cls)
>> if copier:
>> return copier(x)
...
>> this a bug, or a feature of the revised copy/pickle design?
>
> Looks like a bug to me; it breaks the behavior of classic classes,
> since type(classicInstance) returns InstanceType.
It doesn't, because types.InstanceType is a key in _copy_dispatch and
gets a function that implements old-style classe behavior.
> However, it also looks like it might have been introduced to fix the
> possibility that calling '__copy__' on a new-style class with a custom
> metaclass would result in ending up with an unbound method. (Similar
> to the "metaconfusion" issue being recently discussed for PEP 246.)
>
> ISTM the way to fix both issues is to switch to using x.__class__ in
> preference to type(x) to retrieve the __copy__ method from, although
> this still allows for metaconfusion at higher metaclass levels.
What "both issues"? There's only one issue, it seems to me -- one of
metaconfusion.
Besides, getattr(x.__class__, '__copy__') would not give backwards
compatibility if x is an old-style instance -- it would miss the
per-instance x.__copy__ if any. Fortunately, _copy_dispatch deals with
that. So changing from type(x) to x.__class__ seems useless.
> Maybe we need a getclassattr to deal with this issue, since I gather
> from Armin's post that this problem has popped up other places besides
> here and PEP 246.
Apparently we do: a bug in a reference implementation in a draft PEP is
one thing, one that lives so long in a key module of the standard
library is quite another.
> (Follow-up: Guido's checkin comment for the change suggests it was
> actually done as a performance enhancement while adding a related
> feature (copy_reg integration), rather than as a fix for possible
> metaconfusion, even though it also has that effect.)
OK, but if Armin is correct about the code in the reference
implementation of pep 246, and I think he is, this is still a bug in
copy.py (though probably not the specific one that bit /f).
Alex
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