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

Nick Coghlan ncoghlan at gmail.com
Fri May 24 13:41:27 CEST 2013

On Fri, May 24, 2013 at 8:53 PM, Steven D'Aprano <steve at pearwood.info> wrote:

Python built-ins and the standard library already have a standard idiom for specifying multiple values at once. A tuple of types is the One Obvious Way to do this:

@fun.register((float, Decimal))

It's not obvious, it's ambiguous - some third party libraries use that notation for multi-method dispatch, and they always will, no matter what notation we choose for the standard library.

We have three available notations to register the same function for multiple types: stacked decorators, tuple-of-types and multiple arguments.

Of those, the first we cannot avoid supporting, since we want to return the undecorated function regardless for pickle support and ease of testing.

The second two are both used as notations by existing third party multiple dispatch libraries.

Thus, your request is that we add a second way to do it that is known to conflict with existing third party practices. There is no practical gain on offer, it merely aligns with your current sense of aesthetics slightly better than stacked decorators do. While you're entitled to that aesthetic preference, it isn't a valid justification for adding an unneeded alternate spelling.

Furthermore, the proposed registration syntax in the PEP is identical to the syntax which already exists for ABC registration as a class decorator (http://docs.python.org/3/library/abc#abc.ABCMeta.register).

Cheers, Nick.

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

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