[Python-Dev] PEP 310 and exceptions

[Nick Coghlan]

holger krekel wrote:
> Moreover, i think that there are more than the "transactional"
> use cases mentioned in the PEP.  For example, a handler 
> may want to log exceptions to some tracing utility 
> or it may want to swallow certain exceptions when
> its block does IO operations that are ok to fail. 

With the current PEP 310 definition, these can be manually handled using 
sys.exc_info() in the __exit__ method. Cleaning up my earlier transaction 
handler example:

class transaction(object):
     def __enter__(self):
         begin_transaction()

     def __exit__(self):
         ex = sys.exc_info()
         if ex[0] is not None:
             abort_transaction()
         else:
             commit_transaction()

Alternately, PEP 310 could be defined as equivalent to:

     if hasattr(x, '__enter__'):
         x.__enter__()
     try:
         try:
             ...
         except:
             if hasattr(x, '__except__'):
                 x.__except__(*sys.exc_info())
             else:
                 raise
     finally:
         x.__exit__()

Then the transaction handler would look like:

class transaction(object):
     def __enter__(self):
         self.aborted = False
         begin_transaction()

     def __except__(self, *exc_info):
         self.aborted = True
         abort_transaction()

     def __exit__(self):
         if not self.aborted:
             commit_transaction()


Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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