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[Python-Dev] PEP 443 - Single-dispatch generic functions (including ABC support)

Łukasz Langa lukasz at langa.pl
Sat May 25 16:53:59 CEST 2013 
On 25 maj 2013, at 16:08, PJ Eby <pje at telecommunity.com> wrote:

> ISTM there should be some way to get at the raw
> registration info, perhaps by exposing a dictproxy for the registry.

Is that really useful? Just today Antoine asked about changing
behaviour of __subclasses__(), suspecting it isn't used in real world
code anyway. What you're proposing is the functional equivalent of
__subclasses__().

If you need direct access to the registry, do you think the ability to
specify your own registry container isn't enough?


> The way to detect ABC registrations is via the
> ABCMeta._abc_invalidation_counter attribute: if its value is different
> than the previous value saved with the cache, the cache must be
> cleared, and the new value stored.

Wow, I was looking at it just today morning and somehow missed how it
can be used. Great idea, I'll play around with it later today.


> (Unfortunately, this is a private attribute at the moment; it might be
> a good idea to make it public, however, because it's needed for any
> sort of type dispatching mechanism, not just this one particular
> generic function implementation.)

I think we can safely use it within the standard library, anyway. As for
making it public, it's an idea for a separate discussion.


> This would basically eliminate doing any extra (Python) function calls
> in the common case, and might actually be faster than my current
> simplegeneric implementation on PyPI (which doesn't even do ABCs at
> the moment).

Yes, that sounds neat.

Thanks for feedback!


-- 
Best regards,
Łukasz Langa

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Twitter: @llanga
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