[Zope3-dev] Re: [Zope-dev] Re: tal:on-error is a bare except...

[Tres Seaver]

Jim Fulton wrote:
> Tres Seaver wrote:
>> I don't believe the bare except is *ever* good form in 
>> production-quality code.  *No* application-level code has enough zen 
>> to swallow some kinds of exceptions, which is why the Java folks have 
>> a separate RuntimeError hierarchy.
> 
> This isn't application level code.

I guess I don't know what you mean by I "application level":  the code 
path occurs because some application developer put '<div tal:on-error>' 
into a template to "prettify" the error messages.  That hardly feels 
like "infrastructure" coding to me.

Perhaps where we agree is in calling a bug the fact that the 
implementation of the on-error handler (like 'hasattr'!) uses a bare except.

>> The fact that newer ZODB's *appear* to prevent later commits does not 
>> exculpate the use of the bare except in 'tal:on-error' (emphasis added 
>> to indicate a non-trivial amount of suspicion against the code paths 
>> in question;  we have claimed to have solved that problem before).
> 
> 
> Right, we have to do something different here. Adding a dependency on
> ZODB isn't an option.  There needs to be some framework for deciding
> which exceptions to catch and which not to.  It's hard though, because
> we do want to catch conflict errors. We just don't want to catch them
> anywhere but the publisher.
> 
> I'm open to suggestions that don't involve sprinling references to
> ConflictError all over the place.

We could introduce a new base exception, not dependent on ZODB, and then 
sprinkle references to *that* everywhere:

   # zExceptions.py -- I don't recall where this belongs in Zope3
   class FrameworkError:
       """ Base for exceptions not intended to be caught by default.
       """

   # ZODB3/POSException.py -- likewise
   class POSError(FrameworkError):
       """Persistent object system error.
       """

Ripping out bare 'except:' is *not* easy (which is why I think it is the 
single biggest wart in Python).

Tres.
-- 
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Tres Seaver                                tseaver at zope.com
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