[Zope-dev] AttributeError validate using the Visitor pattern

[R. David Murray]

On Mon, 19 Feb 2001, Itai Tavor wrote:
> You posted your questions 3 days ago, so maybe you found a solution 
> by now... but this might still be useful to you.

I just unwrapped the visitor pattern into an psuedo-case statement <grin>.

> - I'm passing _ explicitly instead of relying on binding. I vaguely 
> recall reading something about a problem binding the namespace. It 
> might be fixed in the latest Python Scripts, I haven't tried the CVS 
> version yet.

This appeared to be the key.  I changed my 'accept(me)' call to
read 'accept(me,_=_)', and everything started working.  Thanks
very much!

> - visitLineItem is called in the context of an OrderLineItem, so no 
> <dtml-with item> is needed.

I had actually tried that and gotten an even less intelligable error
message.  So now I've reverted to that form and it works great.

> - If you wanted the visitor to implement looping over the items 
> (which is how the GoF do it) you could easily make something like:
> 
>      Order.displayItemsVisitor.displayItems (DTML Method):
>          <dtml-in line_items>
>            <tr>
>              <dtml-var "_['sequence-item'].accept(container, _)">
>            </tr>
>          </dtml-in>

Yeah, that's more or less what I do.  My display dtml-method gets passed the
list of objects, and it does the loop (implementing the batching logic
for dtml-in) calling accept on the items in the list.  I need to rename
my folder to make it's purpose clearer, though <grin>.

> BTW, I never thought of using a visitor for this until you brought it 
> up, so thanks! What I really like about it is that the same 'accept' 
> method can be used by multiple visitors, each one implementing a 
> different view of the object. So I can have displayItemsVisitor, 
> displayItemsCompactVisitor, and displayItemsEditableVisitor, all 

Yeah, exactly.  displayItemsEditableVisitor is why I chose to
implement the visitor pattern.  I've wound up doing it another
way (checking the auth of the logged in user and putting in a button),
but I suspect I'll have another Visitor before I'm through with the
project.

Thanks again for your help.

--RDM