[Zope] overloading getattr on a product

[Dieter Maurer]

Christopher N. Deckard writes:
 > I'm writing a product which stores the equivalent
 > of Folder properties in XML.  Now I want to be
 > able to catalog objects of this type, but looking
 > at how that's done by the ZCatalog shows that
 > getattr(object, index) is called for all indexes.  
 > 
 > How do I overload getattr to parse my XML and
 > return the value of the "property".
This sounds very expensive...

 > My product is subclassed from ParsedXML and uses
 > the DOM to go through the tree and find my
 > "properties".  What seems to be happening is that
 > when I define a new __getattr__, implicit getattr
 > calls are made to the DOM object and it ends up
 > calling my newly defined getattr method.  This
 > sort of send it into a recursive neverending loop.
You can find a similar problem ("Something wrong with METAL")
in the ZPT mailing list archives. I made a similar error to that
you do.

  It is very easy to create "__getattr__" loops.

  You must make sure that every attribute reference in
  your "__getattr__" is defined.

  My error was that I used "return getattr(self.attr,key)"
  where "attr" was a (defined) attribute from
  a class derived by "Acquisition.Implicit".
  Instead of raising an "AttributeError" when "attr" did not
  define "key", it ask "self" (--> recursive loop).


Dieter