[Zope-Coders] Lazy items in REQUEST

[Matthew T. Kromer]

When doing an automatic traversal hook for the Session Tracking product 
in Zope 2.5; the traversal hook goes to call a Session Data Manager to 
get the current session, then stuffs this result object into 
REQUEST.SESSION.

It occured to Chris and I that this is a wee bit expensive, since it 
happens on EVERY REQUEST; you'd rather defer the penalty of actually 
retrieving the session data until it is used.

To that end, I extended the REQUEST object to take a fifth class of 
element, lazy items.

When REQUEST.set_lazy(name, callable) is called, it registers a lazy 
callable with the key "name" into the REQUEST object.  Should the 
REQUEST object be asked for this name, and it not be found during the 
normal search path, the lazy objects are checked.  Then, if the name 
exists in the lazy object area, its value is checked to see if it is 
callable.  If it is callable, it's invoked -- but regardless, the result 
value is then stored in the request normally, and the lazy key is deleted.

In this way, for instance

    <dtml-var REQUEST>

does not incur the penalty of looking up the session, but

    <dtml-with REQUEST>
    <dtml-var SESSION>
    </dtml-with>

does.

However, I only modified the __getitem__, and keys methods, and added 
the set_lazy method -- looks like everything else is working (items() 
and values() use keys() and get()).

I'm soliciting feedback...