Hello: I am the author of the CVSFile product, available at http://www.zope.org/Members/arielpartners/CVSFile I am thinking about implementing a new design, and wanted to get some feedback from the list first (perhaps others have tried this approach?) Here is a description of our problem: ExternalFile is a Zope product that behaves like a DTMLDocument/method/etc but points to a file outside of ZODB for its content (sort of like a symbolic link) CVSFile inherits from ExternalFile, where the content file is stored in a CVS sandbox. It features buttons to do updates, commits, etc. So far, so good. The problem is that we are working on a third Zope product whose instances represent XML documents that automatically render themselves to HTML (or whatever) when referred to. You edit the XML content using ZMI or whatever, but when you navigate there, you see the resulting web page/PDF/etc. We are going to call it something along the lines of XSLTFile because it uses XSLT to do the conversion. Here's the issue: the XML file that is the source of the transformation may or may not reside inside the ZODB. For example, we store most of our XML documents in CVS. But if revision control is not important (such as for one-off or throw-away documents) we keep them in ZODB. How did we do this? Well we had XSLTFile inherit from CVSFile. This is bad. What happens when we switch to some other revision control tool (like clearcase, subversion, sourcesafe)? Either you have multiple classes like "CVS-XSLTFile, Clearcase-XSLTFile, etc." or a single class that knows about every possible version control system. yuk. Clearly, inheritance and mixins are not the answer. Instead we want delegation. How to do this? One idea is to register a custom protocol handler with python, using urllib2. See: http://www.python.org/doc/current/lib/module-urllib2.html install_opener() I am imagining the following. a new XSLTFile instance is created, pointing to a URL for its content. Here are some examples of URLs: cvs://myserver/myproject/my/file#VERSION-10 zope://foobar http://www.foo.bar/myfile In this way, the XSLTFile instance knows nothing about CVS or any other revision control system. Instead, it only knows about a URL, and the CVS protocol handler would know about how to retrieve files from CVS. We could create our own scheme where a query parameter would specify the sandbox name or something. The Zope protocol handler idea is interesting, and I haven't thought this all the way through yet. Could <dtml-var blat> be thought of as referring to "zope://blat" where the Zope protocol is the default and therefore omitted? The idea of unifying all object references through URIs is intriguing to me. (for more info, see REpresentational State Transfer: http://conveyor.com/RESTwiki/moin.cgi ) Comments? --Craeg