[Python-Dev] PEP 310 and exceptions

[Nick Coghlan]

Nick Coghlan wrote:
> 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__()
> 

In light of Alex's comments, I'd actually like to suggest the below as a 
potential new definition for PEP 310 (making __exit__ optional, and adding an 
__else__ handler):

     if hasattr(x, '__enter__'):
         x.__enter__()
     try:
         try:
             # Contents of 'with' block
         except:
             if hasattr(x, '__except__'):
                 if not x.__except__(*sys.exc_info()): # [1]
                     raise
             else:
                 raise
         else:
             if hasattr(x, '__else__'):
                 x.__else__()
     finally:
         if hasattr(x, '__exit__'):
             x.__exit__()

[1] A possible tweak to this line would be to have it swallow the exception by 
default (by removing the conditional reraise). I'd prefer to make the silencing 
of the exception explicit, by returning 'True' from the exception handling, and 
have 'falling off the end' of the exception handler cause the exception to 
propagate.

Whichever way that point goes, this definition would allow PEP 310 to handle 
Alex's example of factoring out standardised exception handling, as well as the 
original use case of resource cleanup, and the transaction handling:

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

     def __except__(self, *exc_info):
         abort_transaction()

     def __else__(self):
         commit_transaction()


Cheers,
Nick.

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