[Web-SIG] Re: Just lost another one to Rails

[Martijn Faassen]

A.M. Kuchling wrote:
> On Tue, Apr 19, 2005 at 05:31:59PM +0200, Martijn Faassen wrote:
> 
>>How would you deal with the flexibility requirements that these systems 
>>have, though?
>  
> I have no idea what these requirements are.  

Someone builds some web app or framework/app, for instance, a CMS.

Now someone else wants to extend or change the behavior of this app. For 
instance, someone wants to add a new page to document objects in a CMS, 
that contains some textual summary information (Word count, say). Other 
people do not want or need this functionality however, so they don't 
want to see this page.

A more low level scenario could be modifications to the way URI 
traversal works (you hook in another adapter that knows how to look up a 
subobject), without having to change the core framework.

> However, I do think that,
> if your implementation requires infrastructure that builds a whole new
> object model on top of Python, that may be a good clue that the
> implementation should be done in some other language than Python.

What new object model are you talking about? I mean, adapters and views 
(they're very similar, views provide user interfaces, and adapters 
developer interfaces). They're well established design patterns. Looking 
up a view or adapter in some registry is also fairly common. I see this 
scattered all across the Zope-based CMS I helped build, Silva, wherever 
we needed the flexibility (which was typically customer project driven, 
very real world all). I see them everywhere; views for content objects, 
metadata set mapping to content objects, XML importer/exporter to 
content objects, different behavior for binary content based on the mime 
type (such as showing different icons), and I'm sure there's more I'm 
forgetting..

All Zope 3 (which I know a lot better than Twisted) does with its 
adapter system is provide a uniform, flexible way to register these 
adapters and views and then later look them up, instead of having 
hand-written registries and different lookup APIs growing all across the 
system.

Are you suggesting I switch to a language with, say, multi-methods 
instead, or is there another pattern to accomplish this kind of 
extensibility in Python that you favor?

Regards,

Martijn