repoze.bfg's creator, Chris McDonough, in an informative and corrective comment to my last blog post, among other things disagreed with me about my assertion that "repoze.bfg doesn't provide a model story; it provides a traversal story."
I'm afraid I didn't communicate myself well enough. He might still disagree, and I might still fail to make my points clearly, but I found this the most interesting observation I made, so let me try again.
- I think applying Model-View-Controller to client-side application frameworks makes sense. MVC actually makes sense, with a reasonably clear delineation of responsibilities, when you look at Cocoa, for instance, or even in some of the more recent JS frameworks (Sproutcore, for instance).
- All of the reasonably recent web frameworks out there with which I am currently even vaguely familiar (Zope 3, Django, RoR, bfg, Pylons) have a model and something responsible for rendering. The rendering usually goes out to a template, but not always. It's not really cooked that you have to, and usually that's regarded as an advantage and a flexibility. "Render however you want! Use whatever library you want!" So, the heart of the system is Model-View. The Controller isn't there, and the Template is an implementation detail of the View. "Model-Template-View" or "MTV" just seems silly to me. I'd prefer it if everyone acknowledged that our web frameworks usually just have "MV," and move on.
- bfg doesn't care what it is traversing. Pretty much, give it things that it can use the __getitem__ protocol on, and then when it has consumed the path, it'll adapt to a view class. The __getitem__ bits could be a model...or not! What if your data model didn't jibe with your URL model? That's completely reasonable, and the routes guys have plenty of examples in their apps because of how they think about URLs. What if you like the __getitem__ pattern for your URLs, but your URL story is different than your model. You might build a true MVC system with bfg: pure data-driven models; the bfg traversal system used exclusively over "controller" objects that handle traversal and maybe request (i.e., form) parsing; and views that adapt the controllers to *only* render. Maybe the controllers even optionally have WADL-like contracts based on request inputs.
So, my point was actually not that bfg was cheating in any way--certainly, for instance, nothing like Zope 2. To recap, then.
- I find the "MTV" term to be specious generally, whatever the web framework. That's just a criticism of the term, as web framework marketing has adopted it in the past few years. I'd love for it to retire.
- interestingly, I don't think bfg is truly tied to the "MTV" model. It doesn't care what it traverses. The MTV model works fine, but a story like what I described, in which the models are maintained separately from the URL space, and the traversed objects are traversal "controllers," would also work well. Then thinking about any additional responsibility of the traversed objects is an interesting exercise, especially in light of REST-ian approaches.
So, yes, Chris is right, from one perspective, bfg is as much MTV as anybody else. That's fine. I'm just railing against the term, and saying that bfg can be used for more than just "MTV".