[Zope3-dev] Please, no bare 'except:' clauses!

[Steve Alexander]

> try:
>
> except DatabaseError: # All ZODB/transactional errors (conflicts, etc)
>
> except ZopeError: # All Zope framework exceptions
>
> except AppError: # All application defined exceptions
>
> except Exception: # All Python exceptions

This poses a problem when you want to use ordinary python code with the 
ZODB. If the code wants to catch exceptions, it has either to be very 
specific about what exceptions it catches, or to be aware of the 
DatabaseError type that it should not catch.

I guess the designers of Java tried to draw this kind of distinction 
when making the broad divisions of

    Throwable
     |      |
   Error    Exception
             |
            RuntimeException

(Do I have that right? It's been a while...)

All exceptions are Throwable.
Error-derived exceptions are things that the VM raises, for example, 
MemoryErrors.
Exceptions are part of the compile-time type-checking system, unless 
they are RuntimeExceptions.

When writing regular Java code, you shouldn't catch Throwables or Errors 
unless you really know what you are doing.

In our case, ZODB errors should be the equivalent of Errors. These are 
not the responsibility of applications, but are the responsibility of 
the "framework"... the plumbing... etc. of which the VM is the lowest level.


--
Steve Alexander