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

Giovanni Bajo rasky at develer.com
Mon Apr 30 14:43:26 CEST 2007

On 30/04/2007 3.40, Collin Winter wrote:

The mechanism I'm most familiar with for solving this problem (which, unless I've missed something, is, "how do I make sure this object does what I expect?") is Perl 6's roles system; if you know about Squeak Smalltalk's "traits" system, you're on the same page.

+1. I like those, specifically the fact that they can be dynamic (you can "add" a role to a class or an instance of a class and "remove" it later). This is basically mixins they way they should be done.

The idea is that we leave the traditional class system to manage an object's implementation and use a more-or-less orthogonal system to manage that same object's behavior.

Yes. In other words, traditional inheritance is very good for reusing an implementation, but it's not very flexible for defining behaviour. It works for basic cases of course, but gets limited fast when your object network gets more complex.

2. I gave a strawman syntax proposal for runtime role composition above (modulo the method name): "Ring.doneby(int)" or "int.does(Ring)". (Perl 6 speaks in terms of "an object/class does a role".) Perl 6 also allows individual instances to do different roles than their class does, but I'm not sure we want to go that far.

Well, I do! I know there have been times I have needed it badly, and when you're there there's little you can do. If you want to champion a "role" proposal for Python, please don't forget about this.

Giovanni Bajo

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