[Zope3-dev] Re: TALES PathExpr doesn't call old style classes

[Florent Guillaume]

On 25 May 2006, at 00:38, Philipp von Weitershausen wrote:
> just to get this straight: I'm not going to discuss in this thread  
> what
> changes we might do to TALES in the future. I'm strictly concerned  
> about
> this release cycle, not a future one. That doesn't I won't be  
> interested
> in discussing these matters in the future...
>
>> Are there really usecases where something can go wrong
>> if we use the proposed PEP?
>
> Calling something that may or may not be callable can yield at  
> least two
> different exceptions: TypeError or AttributeError (on __call__). That
> means we'd already have to catch two pretty common exceptions and
> therefore "eat" exceptions that might occur inside the called routine.
> This would make debugging very hard. It also isn't mentioned in the  
> spec
> currently that TypeErrors and AttributeErrors would be eaten silently,
> so we'd really be changing the behaviour.
>
> There *is* a way to check whether the error resulted from the current
> frame or a lower frame. Introspections like these have the tendency to
> be or become hacks, but then again, we use frame inspections in other
> places, too.

Yes, I was about to suggest using the standard PJE trick used  
elsewhere in Zope 3 (to check for deep ImportErrors), namely:

try:
   ob()
except (TypeError, AttributeError):
   if sys.exce_info()[2].tb_next is not None:
     raise
   # here we know the exception was not "deep"
   # so the object isn't callable

Florent

>
>
> If no one objects, I will fix Zope 3's PathExpr to check for a  
> __call__
> method using getattr() (as documented in the collector issue,
> http://www.zope.org/Collectors/Zope3-dev/635) and additionally check
> whether the object might be an old style class, as suggested by  
> Florent.
> We will have to result to different measures in Zope 2 anyways, for  
> BBB
> reasons.
>
> Philipp

-- 
Florent Guillaume, Nuxeo (Paris, France)   Director of R&D
+33 1 40 33 71 59   http://nuxeo.com   fg at nuxeo.com