[Python-Dev] PEP 443 - Single-dispatch generic functions

[Nick Coghlan]

On Thu, May 23, 2013 at 10:14 AM, Glenn Linderman <v+python at g.nevcal.com> wrote:
> Yet about half of the operator overloads would be incomplete if there were
> not corresponding __r*__ methods (__radd__, __rsub__, etc.) because the
> second parameter is as key to the dispatch as the first.
>
> While unary operators, and one argument functions would be fully covered by
> single dispatch, it is clear that single dispatch doesn't cover a large
> collection of useful cases for operator overloading.

The binary operators can be more accurately said to use a complicated
single-dispatch dance rather than supporting native dual-dispatch. As
you say, the PEP would be strengthened by pointing this out as an
argument in favour of staying *away* from a multi-dispatch system
(because it isn't obvious how to build a comprehensible one that would
even support our existing NotImplemented based dual dispatch system
for the binary operators).

Cheers,
Nick.

--
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia