Ian Bicking: the old part of his blog

Events and sqlobject comment 000

I had thought about generic functions along the way, but I guess I was too nervous to try them for what seemed like a nominal benefit (over something more conventional like PyDispatcher). There's issues like the fact it introduces C code into a system that has no C code (except, I guess, the database drivers -- but those are commonly packaged). But even more I'm worried that I'd be a drag on an 0.8 release -- as it is, I feel I was a drag on 0.7. So I don't want to introduce new things that I don't entirely understand, unless they are at least limited. So PyDispatcher's more limited rules are an advantage there. Events are also a less complicated concept for other contributors to understand, and since I really want this feature to help open up SQLObject to other developers the comprehensibility is important.

If I was going to implement an adaptation system, I would think very seriously about generic functions. In part because conventional adapation systems like Zope Interfaces already use C code. But also because I think adaptation is not easy as it is, and so generic functions aren't such a new and fancy notion. Or, at least, they are no more new and fancy than conventional adaptation -- a concept which has gained little traction outside of Zope 3 (which actually surprises me). TurboGears people are planning to implement a JSON adaptation process... maybe I should suggest something there. Relatedly, there's some good room for adaptation-like processes with FormEncode (and with SQLObject and web framework integration thereof), and that might be an interesting place for experimentation. All the more because I think it's a hard problem that needs new ideas and new programming techniques, so generic functions may very well make it successful where traditional techniques would not. When going down a road strewn with the corpses of previous projects, one must consider why the next project will be different. Forms are a place where exceptions are common, and generic functions are uniquely capable of handling exceptional behavior.

Comment on Re: Events and SQLObject
by Ian Bicking

Comments:

"""it introduces C code into a system that has no C code"""

Not true; RuleDispatch can be built --without-speedups if you want. It does make some difference to the performance, sure, but if I recall correctly the difference is measured in a handful of microseconds for most things, mainly because I haven't written a lot of C code for it yet anyway. And for Windows -- the main platform where lack of a compiler is usually an issue -- I distribute a binary egg.

"""Events are also a less complicated concept for other contributors to understand"""

What's so complicated about that? It's true that you have to make some policy decisions about what kinds of subscribers you'll have - i.e., before/after/around/custom - but once you've made them that's that.

Further, nothing says you have to expose the full power of the system. You can always wrap it in a shortcut API that implements common use cases. Just say that the generic functions are event objects, and have subscribe function(s) that take an event, a callback, and a filter. Voila! It's an event system, and nobody needs to know any part of RuleDispatch that you don't want to expose.

# Phillip J. Eby