I've had this idea for a while (it's been on the Breve to-do page as a speculative feature), but I also saw that Genshi supports such a feature and that there's a new template engine "Art" (not released yet) that will also have such a feature.

At first I thought of it as a cool, but not really important feature. I mean, you could do things like embed your last.fm playlist easily but other than that it seemed like fluff, so it was low on my priority list.

However, on the Pylons list, the author of Art mentioned how he planned to leverage this concept to easily support plugins. Duh! It's brilliantly simple (just how I like it). I was thinking in terms of pulling in XML data from other sites, but actually it could be used to pull a fragment from another controller on the same server. This would allow plugins to be nicely encapsulated and easily allow a page to be built from fragments generated by multiple controllers (something not currently possible with Pylons that I'm aware of), all from within the template.

I quickly hacked up an xinclude tag in Breve (mostly a thin wrapper around urllib2.urlopen() and it worked fine for pulling from remote sites. However, when I pointed it to another controller on the same Pylons app, things fell apart. The rendered template was incomplete. Further testing revealed that it only broke if the controller rendered another template (i.e. returning a simple string worked fine). My initial guess (without further testing) is that somehow the subrequest is happening withing the same context as the outer request or the template subsystem has an ugly global somewhere (i.e. the subrequest tries to reuse the existing Template instance). There's also the possibility that it's a bug in Breve, but this seems unlikely at this point (Breve is pretty simple).

My next phase is to narrow down where it's happening with the following tests:

1. Have a Pylons controller directly call another Pylons controller using urlopen and see what we get. Note that it must utilize the template subsystem since simple strings already appear to work.
2. If this works without issue, then work up a simple Breve app (outside of Pylons) and try to replicate the issue there.
3. If that works, then file a bug report on Pylons since I suspect it will be outside my abilities to track down.

Anyway, this feature seems so killer as to be well-worth the effort. I was casting about for ways to support plugins for this blog and this is clearly it.

Follow up:

Lo and behold, the bug was in Breve, and it was a pretty serious one. breve.Template was using register_flattener on a method named __slot which I had expected to not collide with other Template instances but it didn't work. What did work was making it an internal class, but what worked even better was avoiding register_flattener altogether and simply adding a __str__ method to the slot class. 1.0.32 is now the recommended release as it addresses this bug, a memory-leak issue (related to the first bug) and provides an initial (read: testing only) implementation of xinclude (although I think I'll rename it at some point).