[Python-Dev] PEP 443 - Single-dispatch generic functions (original) (raw)

Łukasz Langa lukasz at langa.pl
Thu May 23 13:25:47 CEST 2013

On 23 maj 2013, at 01:16, Terry Jan Reedy <tjreedy at udel.edu> wrote:

I like the general idea. Does you have any specific stdlib use cases in mind?

I thought of pprint, which at some point dispatches on dict versus set/sequence, but overall it seems more complicated than mere arg type dispatch.

I want to make pprint extensible for 3.4 and PEP 443 started out as an idea to introduce a uniform API for the boilerplate I'm going to need anyway. It turned out the idea has been around for years.

Unittest.TestCase.assertEqual mostly (but not completely) uses first arg dispatch based on an instance-specific dict, and it has an custom instance registration method addTypeEqualityFunc. (Since each testxxx runs in a new instance, a registration for multiple methods has to be done either in a setup method or repeated in each testmethod.)

If a registration mechanism is already in place, it will probably need to stay (backwards compatibility). The feasability of refactoring to @singledispatch will have to be considered on a case-by-case basis.

On a more general note, I'm sure that @singledispatch won't cover every use case. Still, PJE implemented both pkgutil.simplegeneric and PEAK-Rules because the former is the proverbial 20% that gets you 80% there. For those use cases the simplicity and transparency provided by a basic solution are a virtue. This is what PEP 443 targets.

If @singledispatch turns out so successful that we'll find ourselves longing for multiple dispatch or predicate-based dispatch in the future, I'm sure there's still going to be enough PEP numbers free. The @singledispatch name has been chosen to ensure there's no name clash in that case (thanks Nick for suggesting that!).

-- Best regards, Łukasz Langa

WWW: http://lukasz.langa.pl/ Twitter: @llanga IRC: ambv on #python-dev

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