[Python-3000] ABC PEP isinstance issue Was: PEP 31XX: A Type Hierarchy for Numbers (and other algebraic entities)

Jean-Paul Calderone exarkun at divmod.com
Thu Apr 26 04:06:05 CEST 2007

On Wed, 25 Apr 2007 20:40:14 -0400, "Phillip J. Eby" <pje at telecommunity.com> wrote:
>At 06:40 PM 4/25/2007 -0400, Jean-Paul Calderone wrote:
>>On Wed, 25 Apr 2007 18:10:23 -0400, Jim Jewett <jimjjewett at gmail.com> 
>> >The current ABC proposal is to use isinstance as the test; Jeffrey
>> >Yaskin's numbers PEP highlighted the weakness there with a concrete
>> >example.
>> >
>> >If you need to an abstraction less powerful than an existing ABC,
>> >you're out of luck; you can't just assert that the existing class is
>> >already sufficient, nor can you expect everyone else to use multiple
>> >annotations.
>>I'm sure everyone is already aware of the behavior of the classImplements
>>and directlyProvides functions available in zope.interface, which exactly
>>satisfy this use-case in the interface world.
>I'm either misunderstanding Jim or you, because I don't see the relationship 
>here.  If I understand Jim correctly, he's actually asking for something 
>like PyProtocols' "protocolIsSubsetOf" declaration -- something that would 
>be like dynamically adding a superclass to an existing interface in 
>zope.interface.  Whereas the features you're talking about sound like 
>declaring that a class object itself implements an interface -- something 
>apparently unrelated to the question at hand.

Ugh, you're at least half right.  I didn't read the implementation of
ABC.meets carefully enough to notice that it's actually an adapter
registry, not just a predicate (meets seems like a funny name for that

classImplements would have satisfied the adapterless use-case by letting
Jim declare that someone else's class definition actually implements the
feature set that some unit of code he wants to use that class with is

With the adapter registry, I don't see why this isn't just PEP 246 adapt()
(if I am reading one of your later mails correctly though, you think it is

I'm not particularly familiar with protocolIsSubsetOf, but it wouldn't
surprise me at all if it could also satisfy this use-case.  There are
lots of ways to do this, many of them fairly simple.  Using inheritance
is the part that makes it hard, since inheritance is controlled by the
wrong party in most cases and comes with unrelated features that are,
at best, irrelevant to the particular use case and at worst actively

I'm sure a way around this can be invented, I just don't see why it
should be.


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