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

Nick Coghlan ncoghlan at gmail.com
Sat May 25 16:59:09 CEST 2013

On Sun, May 26, 2013 at 12:08 AM, PJ Eby <pje at telecommunity.com> wrote:

On Sat, May 25, 2013 at 8:08 AM, Łukasz Langa <lukasz at langa.pl> wrote:

The most important change in this version is that I introduced ABC support and completed a reference implementation. Excellent! A couple of thoughts on the implementation... While the dispatch() method allows you to look up what implementation would be selected for a target type, it does not let you figure out whether a particular method has been registered for a type. That is, if I have a class MyInt that subclasses int, I can't use dispatch() to check whether a MyInt implementation has been registered, because I might get back an implementation registered for int or object. ISTM there should be some way to get at the raw registration info, perhaps by exposing a dictproxy for the registry.

I like that idea - types.MappingProxyType makes it straightforward to expose a read-only view of the dispatch registry.

Second, it should be possible to memoize dispatch() using a weak key dictionary that is cleared if new ABC implementations have been registered or when a call to register() is made. The way to detect ABC registrations is via the ABCMeta.abcinvalidationcounter attribute: if its value is different than the previous value saved with the cache, the cache must be cleared, and the new value stored.

(Unfortunately, this is a private attribute at the moment; it might be a good idea to make it public, however, because it's needed for any sort of type dispatching mechanism, not just this one particular generic function implementation.)

I think I added an issue on the tracker for that somewhere... yup: http://bugs.python.org/issue16832

Given the global nature of the cache invalidation, it may be better as a module level abc.get_cache_token() function.

Cheers, Nick.

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

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