[Python-3000] Traits/roles instead of ABCs (original) (raw)

Talin talin at acm.org
Mon Apr 30 18:27:29 CEST 2007

Collin Winter wrote:

The key part of traits/roles is that, because the system is separate from classes, you can do runtime role composition without a) mucking with bases, or b) making isinstance() and issubclass() squishy and ill-defined. By "runtime role composition", I mean it would be possible to do something like this at runtime:

py> int.implements(Ring) or (depending on your spelling preferences) py> Ring.implementedby(int) That is, it would be possible to distribute Jeffery's numeric kinds as a third-party library and still have them affect the built-in numeric types.

The question I have about all this runtime composition stuff is, what happens if different modules have different ideas about how things should be composed? How are conflicts resolved?

With ABCs, we have a limitation that is also a strength: The mapping between ABC and its bases is either built-in to the ABC or built-in to the class, meaning that these decisions have to all be made up front - which forces the ABC implementer to pre-decide the outcome of all such conflicts before they happen.

-- Talin

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