[Python-Dev] GeneratorExit inheriting from Exception

[Nick Coghlan]

Nick Coghlan wrote:
> Should GeneratorExit inherit from Exception or BaseException?
> 
> Currently, a generator that catches Exception and continues on to yield 
> another value can't be closed properly (you get a runtime error pointing out 
> that the generator ignored GeneratorExit).
> 
> The only decent reference I could find to it in the old PEP 348/352 
> discussions is Guido writing [1]:
> 
>> when GeneratorExit or StopIteration
>> reach the outer level of an app, it's a bug like all the others that
>> bare 'except:' WANTS to catch.
> 
> (at that point in the conversation, I believe bare except was considered the 
> equivalent of "except Exception:")
> 
> While I agree with what Guido says about GeneratorExit being a bug if it 
> reaches the outer level of an app, it seems like a bit of a trap that a 
> correctly written generator can't write "except Exception:" without preceding 
> it with an "except GeneratorExit:" that reraises the exception. Isn't that 
> exactly the idiom we're trying to get rid of for SystemExit and KeyboardInterrupt?

The last comment I heard from Guido on this topic was that he was still 
thinking about it.

However, I now have an additional data point - if GeneratorExit inherits 
directly from BaseException, it makes it much easier to write exception 
handling code in generators that does the right thing on both Python 2.4 and 2.5.

In 2.4, PEP 342 hasn't happened, so "except Exception:" can't misbehave in 
response to GeneratorExit (the latter doesn't exist, and nor does generator 
finalisation). If GeneratorExit inherits directly from BaseException, the code 
still does the right thing since the exception isn't caught.

OTOH, if GeneratorExit inherits from Exception (as in current SVN), then two 
things will be needed to make the generator work correctly:

1. add a preceding exception clause to fix Python 2.5 behaviour:
   except GeneratorExit:
       raise
   except Exception:
       # whatever

2. add header code to the module to make it work again on Python 2.4:

   try:
       GeneratorExit
   except NameError:
       class GeneratorExit(Exception): pass

IMO, that would be an ugly bit of backwards incompatibility (even though I 
wouldn't expect such broad exception handling in generators to be at all common).

Cheers,
Nick.

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