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

Guido van Rossum guido at python.org
Thu May 23 02:55:28 CEST 2013

On Wed, May 22, 2013 at 5:14 PM, 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.

This (and your subsequent argument) sounds like a typical case of "perfection is the enemy of the good." Łukasz already pointed out that for dispatch on multiple arguments, consensus has been elusive, and there are some strong statements in opposition. While this does not exclude the possibility that it might be easier to get consensus on dual-argument dispatch, I think the case for dual-argument dispatch is still much weaker than that for single-argument dispatch.

The binary operations, which you use as the primary example, are already special because they correspond to syntactic forms. Python intentionally does not have a generalized syntax to invoke arbitrary binary operations, but only supports a small number of predefined binary operators -- code in other languages (like Haskell) that uses "unconventional" binary operators is usually hard to read except for mathematicians.

Since the language already offers a way to do dual-argument dispatch for the predefined operations, your proposed dual-argument dispatch wouldn't be particularly useful for those. (And retrofitting it would be a very tricky business, given the many subtleties in the existing binary operator dispatch -- for example, did you know that there's a scenario where radd is tried before add?)

For standard function calls, it would be very odd if dual-dispatch were supported but multiple-dispatch weren't. In general, 0, 1 and infinity are fair game for special treatment, but treating 2 special as well usually smells. So I'd say that Łukasz's single-dispatch proposal covers a fairly important patch of new ground, while dual-dispatch is both much harder and less useful. Ergo, Łukasz has made the right trade-off.

-- --Guido van Rossum (python.org/~guido)

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