[Python-3000] An introduction to ABC's (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Apr 17 18🔞38 CEST 2007

At 10:47 AM 4/17/2007 +0100, Jan Grant wrote:

On Tue, 17 Apr 2007, Aurélien Campéas wrote:

> But the unconclusiveness of all the discussions related to ABCs, > interfaces and generic functions only show the immaturity of many on > these topics. Including mine, of course, as in: I lack the experience > required to back up all my claims about the generic function school of > OO. ... (although P. Eby did a great work on this front, he looks a bit > lonely imho). FWIW Phillip's GFs look more and more like being the pythonic way to do things. ABCs and interfaces work in a java world, where arguments to a method arrive with a "preferred" type (ie, the type in the parameter declaraion) and that type is used to interpret method calls when it comes to method overloading. Hence Phillip's comment that interfaces are "receiver-oriented".

Yes... and in Java, it tends to work out that way because a lot of factors conspire to make it work, like the fact that mere method names aren't enough to make an object acceptable, combined with overloading, lack of really useful generic types, etc.

We don't have that in python, which makes me question the utility of creating a complex ABC universe prior to having a mechanism for method dispatch.

Of course, GF implementations do exist which would give the ABCs some utility. It's just a shame that there doesn't appear to be one that's going to be the default way to do it in py3k.

FYI, there's actually one tucked away in the pkgutil module since Python 2.5... single dispatch only, but it was the easiest way to support asking future PEP 302 importers what modules are in a package. It's more primitive than the "simplegeneric" package on the Cheeseshop, but would certainly be functional enough to work with ABC's, if it were documented (at the moment it's technically an implementation detail of pkgutil.

However, if there's interest, I'd be willing to create a small (i.e. about 1000 lines, give or take a few hundred) Py3K/2.6 multi-dispatch/interface/adapters module, written such that it would be extensible for third-party dispatch and interface systems. I'm not sure I could deliver a complete PEP before the deadline, however, and my impression is that Guido feels (or at least felt) that such a thing would be premature. If that has changed, however, I'm willing to write a PEP and take a stab at getting a draft in this week.

Anyway, if we do that, we would then have interfaces, adapters, generic functions, and ABCs in Py3K, so everybody should be happy... except for those who want only one Obvious Way, of course. :)

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