[z3-five] Traversal issue [was: 0.2 release]

[Philipp von Weitershausen]

Hi,

I was just reviewing Sidnei's code on the traversal branch and 
discussing it with him on IRC. We went through the problematic Zope code 
again and discussed alternative solutions. He asked me to write a 
summary to the list, so here it is:

ZPublisher's BaseRequest checks for __bobo_traverse__ existance during 
traversal. Here, an exception that is raised by __bobo_traverse__ is 
simply not caught and ends up being displayed like it was raised by any 
other code. Thus, raising NotFound in __bobo_traverse__ brings the 
expected result of a 404 page being served.

In ZPT's Expressions.py, however, the NotFound exception raised in 
__bobo_traverse__ is not caught; here, the method is expected to raise 
AttributeError or KeyError (or several other ones like Unauthorized) to 
indicate that traversal has failed. Five.Traversable/Viewable returned 
None here in case it couldn't traverse. That was not correct because it 
made not existing traversal paths look like they existed. It also broke 
ZPublisher because ZPublisher thought None had no docstring, hence the 
docstring problem.

The code in Sidnei's branch raises NotFoundError now except when it 
thinks it's being called from ZPT pathexpressions; then it returns None. 
As explained above, this is not correct behaviour. I suggested to solve 
the issue by implementing the following:

- __bobo_traverse__ always raises AttributeError when it cannot 
traverse. This seems to be the overall expected behaviour.

- for the ZPublisher, we don't want AttributeError showing but NotFound. 
To achieve this, we wrap __bobo_traverse__ in a descriptor that does the 
funky frame introspection and converts AttributeErrors into NotFound. 
That way the magic is at least contained in that descriptor.

Objections?

Philipp